python-cheatsheet/README.md

Comprehensive Python Cheatsheet
===============================
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![Monty Python](web/image_888.jpeg)


Contents
--------
**&nbsp;&nbsp;&nbsp;** **1. Collections:** **&nbsp;** **[`List`](#list)**__,__ **[`Dictionary`](#dictionary)**__,__ **[`Set`](#set)**__,__ **[`Tuple`](#tuple)**__,__ **[`Range`](#range)**__,__ **[`Enumerate`](#enumerate)**__,__ **[`Iterator`](#iterator)**__,__ **[`Generator`](#generator)**__.__  
**&nbsp;&nbsp;&nbsp;** **2. Types:** **&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;**  **[`Type`](#type)**__,__ **[`String`](#string)**__,__ **[`Regular_Exp`](#regex)**__,__ **[`Format`](#format)**__,__ **[`Numbers`](#numbers)**__,__ **[`Combinatorics`](#combinatorics)**__,__ **[`Datetime`](#datetime)**__.__  
**&nbsp;&nbsp;&nbsp;** **3. Syntax:** **&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;**  **[`Args`](#arguments)**__,__ **[`Inline`](#inline)**__,__ **[`Closure`](#closure)**__,__ **[`Decorator`](#decorator)**__,__ **[`Class`](#class)**__,__ **[`Duck_Types`](#duck-types)**__,__ **[`Enum`](#enum)**__,__ **[`Exceptions`](#exceptions)**__.__  
**&nbsp;&nbsp;&nbsp;** **4. System:** **&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;**  **[`Print`](#print)**__,__ **[`Input`](#input)**__,__ **[`Command_Line_Arguments`](#command-line-arguments)**__,__ **[`Open`](#open)**__,__ **[`Path`](#path)**__,__ **[`Command_Execution`](#oscommands)**__.__  
**&nbsp;&nbsp;&nbsp;** **5. Data:** **&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;**  **[`JSON`](#json)**__,__ **[`Pickle`](#pickle)**__,__ **[`CSV`](#csv)**__,__ **[`SQLite`](#sqlite)**__,__ **[`Bytes`](#bytes)**__,__ **[`Struct`](#struct)**__,__ **[`Array`](#array)**__,__ **[`MemoryView`](#memory-view)**__,__ **[`Deque`](#deque)**__.__  
**&nbsp;&nbsp;&nbsp;** **6. Advanced:** **&nbsp;&nbsp;&nbsp;**  **[`Threading`](#threading)**__,__ **[`Operator`](#operator)**__,__ **[`Introspection`](#introspection)**__,__ **[`Metaprograming`](#metaprograming)**__,__ **[`Eval`](#eval)**__,__ **[`Coroutine`](#coroutine)**__.__  
**&nbsp;&nbsp;&nbsp;** **7. Libraries:** **&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;**  **[`Progress_Bar`](#progress-bar)**__,__ **[`Plot`](#plot)**__,__ **[`Table`](#table)**__,__ **[`Curses`](#curses)**__,__ **[`Logging`](#logging)**__,__ **[`Scraping`](#scraping)**__,__ **[`Web`](#web)**__,__ **[`Profile`](#profiling)**__,__  
**&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;** **[`NumPy`](#numpy)**__,__ **[`Image`](#image)**__,__ **[`Animation`](#animation)**__,__ **[`Audio`](#audio)**__.__


Main
----
```python
if __name__ == '__main__':     # Runs main() if file wasn't imported.
    main()
```


List
----
```python
<list> = <list>[from_inclusive : to_exclusive : ±step_size]
```

```python
<list>.append(<el>)            # Or: <list> += [<el>]
<list>.extend(<collection>)    # Or: <list> += <collection>
```

```python
<list>.sort()
<list>.reverse()
<list> = sorted(<collection>)
<iter> = reversed(<list>)
```

```python
sum_of_elements  = sum(<collection>)
elementwise_sum  = [sum(pair) for pair in zip(list_a, list_b)]
sorted_by_second = sorted(<collection>, key=lambda el: el[1])
sorted_by_both   = sorted(<collection>, key=lambda el: (el[1], el[0]))
flatter_list     = list(itertools.chain.from_iterable(<list>))
product_of_elems = functools.reduce(lambda out, x: out * x, <collection>)
list_of_chars    = list(<str>)
```

```python
<int> = <list>.count(<el>)     # Returns number of occurrences. Also works on strings.
index = <list>.index(<el>)     # Returns index of first occurrence or raises ValueError.
<list>.insert(index, <el>)     # Inserts item at index and moves the rest to the right.
<el> = <list>.pop([index])     # Removes and returns item at index or from the end.
<list>.remove(<el>)            # Removes first occurrence of item or raises ValueError.
<list>.clear()                 # Removes all items. Also works on dictionary and set.
```


Dictionary
----------
```python
<view> = <dict>.keys()                          # Coll. of keys that reflects changes.
<view> = <dict>.values()                        # Coll. of values that reflects changes.
<view> = <dict>.items()                         # Coll. of key-value tuples that reflects chgs.
```

```python
value  = <dict>.get(key, default=None)          # Returns default if key is missing.
value  = <dict>.setdefault(key, default=None)   # Returns and writes default if key is missing.
<dict> = collections.defaultdict(<type>)        # Creates a dict with default value of type.
<dict> = collections.defaultdict(lambda: 1)     # Creates a dict with default value 1.
```

```python
<dict> = dict(<collection>)                     # Creates a dict from coll. of key-value pairs.
<dict> = dict(zip(keys, values))                # Creates a dict from two collections.
<dict> = dict.fromkeys(keys [, value])          # Creates a dict from collection of keys.
```

```python
<dict>.update(<dict>)                           # Adds items. Replaces ones with matching keys.
value = <dict>.pop(key)                         # Removes item or raises KeyError.
{k for k, v in <dict>.items() if v == value}    # Returns set of keys that point to the value.
{k: v for k, v in <dict>.items() if k in keys}  # Returns dictionary filtered by keys.
```

### Counter
```python
>>> from collections import Counter
>>> colors = ['blue', 'red', 'blue', 'red', 'blue']
>>> counter = Counter(colors)
>>> counter['yellow'] += 1
Counter({'blue': 3, 'red': 2, 'yellow': 1})
>>> counter.most_common()[0]
('blue', 3)
```


Set
---
```python
<set> = set()
```

```python
<set>.add(<el>)                                 # Or: <set> |= {<el>}
<set>.update(<collection>)                      # Or: <set> |= <set>
```

```python
<set>  = <set>.union(<coll.>)                   # Or: <set> | <set>
<set>  = <set>.intersection(<coll.>)            # Or: <set> & <set>
<set>  = <set>.difference(<coll.>)              # Or: <set> - <set>
<set>  = <set>.symmetric_difference(<coll.>)    # Or: <set> ^ <set>
<bool> = <set>.issubset(<coll.>)                # Or: <set> <= <set>
<bool> = <set>.issuperset(<coll.>)              # Or: <set> >= <set>
```

```python
<el> = <set>.pop()                              # Raises KeyError if empty.
<set>.remove(<el>)                              # Raises KeyError if missing.
<set>.discard(<el>)                             # Doesn't raise an error.
```

### Frozen Set
* **Is immutable and hashable.**
* **That means it can be used as a key in a dictionary or as an element in a set.**
```python
<frozenset> = frozenset(<collection>)
```


Tuple
-----
**Tuple is an immutable and hashable list.**
```python
<tuple> = ()
<tuple> = (<el>, )
<tuple> = (<el_1>, <el_2> [, ...])
```

### Named Tuple
**Tuple's subclass with named elements.**

```python
>>> from collections import namedtuple
>>> Point = namedtuple('Point', 'x y')
>>> p = Point(1, y=2)
Point(x=1, y=2)
>>> p[0]
1
>>> p.x
1
>>> getattr(p, 'y')
2
>>> p._fields  # Or: Point._fields
('x', 'y')
```


Range
-----
```python
<range> = range(to_exclusive)
<range> = range(from_inclusive, to_exclusive)
<range> = range(from_inclusive, to_exclusive, ±step_size)
```

```python
from_inclusive = <range>.start
to_exclusive   = <range>.stop
```


Enumerate
---------
```python
for i, el in enumerate(<collection> [, i_start]):
    ...
```


Iterator
--------
```python
<iter> = iter(<collection>)                 # `iter(<iter>)` returns unmodified iterator.
<iter> = iter(<function>, to_exclusive)     # A sequence of return values until 'to_exclusive'.
<el>   = next(<iter> [, default])           # Raises StopIteration or returns 'default' on end.
<list> = list(<iter>)                       # Returns a list of iterator's remaining elements.
```


### Itertools
```python
from itertools import count, repeat, cycle, chain, islice
```

```python
<iter> = count(start=0, step=1)             # Returns incremented value endlessly.
<iter> = repeat(<el> [, times])             # Returns element endlessly or 'times' times.
<iter> = cycle(<collection>)                # Repeats the sequence endlessly.
```

```python
<iter> = chain(<coll_1>, <coll_2> [, ...])  # Empties collections in order.
<iter> = chain.from_iterable(<collection>)  # Empties collections inside a collection in order.
```

```python
<iter> = islice(<collection>, to_exclusive)
<iter> = islice(<collection>, from_inclusive, to_exclusive [, +step_size])
```


Generator
---------
* **Any function that contains a yield statement returns a generator.**
* **Generators and iterators are interchangeable.**

```python
def count(start, step):
    while True:
        yield start
        start += step
```

```python
>>> counter = count(10, 2)
>>> next(counter), next(counter), next(counter)
(10, 12, 14)
```


Type
----
* **Everything is an object.**
* **Every object has a type.**
* **Type and class are synonymous.**

```python
<type> = type(<el>)                          # Or: <el>.__class__
<bool> = isinstance(<el>, <type>)            # Or: issubclass(type(<el>), <type>)
```

```python
>>> type('a'), 'a'.__class__, str
(<class 'str'>, <class 'str'>, <class 'str'>)
```

#### Some types do not have built-in names, so they must be imported:
```python
from types import FunctionType, MethodType, LambdaType, GeneratorType
```

### ABC
**An abstract base class introduces virtual subclasses, that don’t inherit from it but are still recognized by isinstance() and issubclass().**

```python
>>> from collections.abc import Sequence, Collection, Iterable
>>> isinstance([1, 2, 3], Iterable)
True
```

```text
+------------------+------------+------------+------------+
|                  |  Sequence  | Collection |  Iterable  |
+------------------+------------+------------+------------+
| list, range, str |    yes     |    yes     |    yes     |
| dict, set        |            |    yes     |    yes     |
| iter             |            |            |    yes     |
+------------------+------------+------------+------------+
```

```python
>>> from numbers import Integral, Rational, Real, Complex, Number
>>> isinstance(123, Number)
True
```

```text
+--------------------+----------+----------+----------+----------+----------+
|                    | Integral | Rational |   Real   | Complex  |  Number  |
+--------------------+----------+----------+----------+----------+----------+
| int                |   yes    |   yes    |   yes    |   yes    |   yes    |
| fractions.Fraction |          |   yes    |   yes    |   yes    |   yes    |
| float              |          |          |   yes    |   yes    |   yes    |
| complex            |          |          |          |   yes    |   yes    |
| decimal.Decimal    |          |          |          |          |   yes    |
+--------------------+----------+----------+----------+----------+----------+
```


String
------
```python
<str>  = <str>.strip()                       # Strips all whitespace characters from both ends.
<str>  = <str>.strip('<chars>')              # Strips all passed characters from both ends.
```

```python
<list> = <str>.split()                       # Splits on one or more whitespace characters.
<list> = <str>.split(sep=None, maxsplit=-1)  # Splits on 'sep' str at most 'maxsplit' times.
<list> = <str>.splitlines(keepends=False)    # Splits on line breaks. Keeps them if 'keepends'.
<str>  = <str>.join(<coll_of_strings>)       # Joins elements using string as separator.
```

```python
<bool> = <sub_str> in <str>                  # Checks if string contains a substring.
<bool> = <str>.startswith(<sub_str>)         # Pass tuple of strings for multiple options.
<bool> = <str>.endswith(<sub_str>)           # Pass tuple of strings for multiple options.
<int>  = <str>.find(<sub_str>)               # Returns start index of first match or -1.
<int>  = <str>.index(<sub_str>)              # Same but raises ValueError if missing.
```

```python
<str>  = <str>.replace(old, new [, count])   # Replaces 'old' with 'new' at most 'count' times.
<str>  = <str>.translate(<table>)            # Use `str.maketrans(<dict>)` to generate table.
```

```python
<str>  = chr(<int>)                          # Converts int to unicode char.
<int>  = ord(<str>)                          # Converts unicode char to int.
```
* **Also: `'lstrip()'`, `'rstrip()'`.**
* **Also: `'lower()'`, `'upper()'`, `'capitalize()'` and `'title()'`.**

### Property Methods
```text
+---------------+----------+----------+----------+----------+----------+
|               | [ !#$%…] | [a-zA-Z] |  [¼½¾…]  |  [²³¹…]  |  [0-9]   |
+---------------+----------+----------+----------+----------+----------+
| isprintable() |   yes    |   yes    |   yes    |   yes    |   yes    |
| isalnum()     |          |   yes    |   yes    |   yes    |   yes    |
| isnumeric()   |          |          |   yes    |   yes    |   yes    |
| isdigit()     |          |          |          |   yes    |   yes    |
| isdecimal()   |          |          |          |          |   yes    |
+---------------+----------+----------+----------+----------+----------+
```
* **Also: `'isspace()'` checks for `'[ \t\n\r…]'`.**


Regex
-----
```python
import re
<str>   = re.sub(<regex>, new, text, count=0)  # Substitutes all occurrences with 'new'.
<list>  = re.findall(<regex>, text)            # Returns all occurrences as strings.
<list>  = re.split(<regex>, text, maxsplit=0)  # Use brackets in regex to keep the matches.
<Match> = re.search(<regex>, text)             # Searches for first occurrence of the pattern.
<Match> = re.match(<regex>, text)              # Searches only at the beginning of the text.
<iter>  = re.finditer(<regex>, text)           # Returns all occurrences as match objects.
```

* **Search() and match() return None if they can't find a match.**
* **Argument `'flags=re.IGNORECASE'` can be used with all functions.**
* **Argument `'flags=re.MULTILINE'` makes `'^'` and `'$'` match the start/end of each line.**
* **Argument `'flags=re.DOTALL'` makes dot also accept newline.**
* **Use `r'\1'` or `'\\1'` for backreference.**
* **Add `'?'` after an operator to make it non-greedy.**

### Match Object
```python
<str>   = <Match>.group()                      # Returns whole match. Also group(0).
<str>   = <Match>.group(1)                     # Returns part in first bracket.
<tuple> = <Match>.groups()                     # Returns all bracketed parts.
<int>   = <Match>.start()                      # Returns start index of a match.
<int>   = <Match>.end()                        # Returns exclusive end index of a match.
```

### Special Sequences
* **By default digits, whitespaces and alphanumerics from all alphabets are matched, unless `'flags=re.ASCII'` argument is used.**
* **Use capital letter for negation.**
```python
'\d' == '[0-9]'                                # Matches any digit.
'\s' == '[ \t\n\r\f\v]'                        # Matches any whitespace.
'\w' == '[a-zA-Z0-9_]'                         # Matches any alphanumeric.
```


Format
------
```python
<str> = f'{<el_1>}, {<el_2>}'
<str> = '{}, {}'.format(<el_1>, <el_2>)
```

### Attributes
```python
>>> from collections import namedtuple
>>> Person = namedtuple('Person', 'name height')
>>> person = Person('Jean-Luc', 187)
>>> f'{person.height}'
'187'
>>> '{p.height}'.format(p=person)
'187'
```

### General Options
```python
{<el>:<10}                                     # '<el>      '
{<el>:^10}                                     # '   <el>   '
{<el>:>10}                                     # '      <el>'
```

```python
{<el>:.<10}                                    # '<el>......'
{<el>:<0}                                      # '<el>'
```

### Strings
**`'!r'` calls object's [repr()](#class) method, instead of [str()](#class), to get a string.**
```python
{'abcde'!r:<10}                                # "'abcde'   "
{'abcde':.3}                                   # 'abc'
{'abcde':10.3}                                 # 'abc       '
```

### Numbers
```python
{ 123456:10,}                                  # '   123,456'
{ 123456:10_}                                  # '   123_456'
{ 123456:+10}                                  # '   +123456'
{-123456:=10}                                  # '-   123456'
{ 123456: }                                    # ' 123456'
{-123456: }                                    # '-123456'
```

### Floats
```python
{1.23456:10.3}                                 # '      1.23'
{1.23456:10.3f}                                # '     1.235'
{1.23456:10.3e}                                # ' 1.235e+00'
{1.23456:10.3%}                                # '  123.456%'
```

#### Comparison of float presentation types:
```text
+---------------+-----------------+-----------------+-----------------+-----------------+
|               |    {<float>}    |   {<float>:f}   |   {<float>:e}   |   {<float>:%}   |
+---------------+-----------------+-----------------+-----------------+-----------------+
|   0.000056789 |    '5.6789e-05' |     '0.000057'  |  '5.678900e-05' |     '0.005679%' |
|   0.00056789  |    '0.00056789' |     '0.000568'  |  '5.678900e-04' |     '0.056789%' |
|   0.0056789   |    '0.0056789'  |     '0.005679'  |  '5.678900e-03' |     '0.567890%' |
|   0.056789    |    '0.056789'   |     '0.056789'  |  '5.678900e-02' |     '5.678900%' |
|   0.56789     |    '0.56789'    |     '0.567890'  |  '5.678900e-01' |    '56.789000%' |
|   5.6789      |    '5.6789'     |     '5.678900'  |  '5.678900e+00' |   '567.890000%' |
|  56.789       |   '56.789'      |    '56.789000'  |  '5.678900e+01' |  '5678.900000%' |
| 567.89        |  '567.89'       |   '567.890000'  |  '5.678900e+02' | '56789.000000%' |
+---------------+-----------------+-----------------+-----------------+-----------------+
```
```text
+---------------+-----------------+-----------------+-----------------+-----------------+
|               |   {<float>:.2}  |  {<float>:.2f}  |  {<float>:.2e}  |  {<float>:.2%}  |
+---------------+-----------------+-----------------+-----------------+-----------------+
|   0.000056789 |    '5.7e-05'    |       '0.00'    |    '5.68e-05'   |       '0.01%'   |
|   0.00056789  |    '0.00057'    |       '0.00'    |    '5.68e-04'   |       '0.06%'   |
|   0.0056789   |    '0.0057'     |       '0.01'    |    '5.68e-03'   |       '0.57%'   |
|   0.056789    |    '0.057'      |       '0.06'    |    '5.68e-02'   |       '5.68%'   |
|   0.56789     |    '0.57'       |       '0.57'    |    '5.68e-01'   |      '56.79%'   |
|   5.6789      |    '5.7'        |       '5.68'    |    '5.68e+00'   |     '567.89%'   |
|  56.789       |    '5.7e+01'    |      '56.79'    |    '5.68e+01'   |    '5678.90%'   |
| 567.89        |    '5.7e+02'    |     '567.89'    |    '5.68e+02'   |   '56789.00%'   |
+---------------+-----------------+-----------------+-----------------+-----------------+
```

### Ints
```python
{90:c}                                # 'Z'
{90:b}                                # '1011010'
{90:X}                                # '5A'
```


Numbers
-------
### Types
```python
<int>      = int(<float/str/bool>)    # Or: math.floor(<float>)
<float>    = float(<int/str/bool>)    # Or: <real>e±<int>
<complex>  = complex(real=0, imag=0)  # Or: <real> ± <real>j
<Fraction> = fractions.Fraction(numerator=0, denominator=1)
<Decimal>  = decimal.Decimal(<str/int/float>)
```
* **`'int(<str>)'` and `'float(<str>)'` raise ValueError on malformed strings.**
* **Decimal numbers can be represented exactly, unlike floats where `'1.1 + 2.2 != 3.3'`.**
* **Precision of decimal operations is set with: `'decimal.getcontext().prec = <int>'`.**

### Basic Functions
```python
<num> = pow(<num>, <num>)             # Or: <num> ** <num>
<num> = abs(<num>)                    # <float> = abs(<complex>)
<num> = round(<num> [, ±ndigits])     # `round(126, -1) == 130`
```

### Math
```python
from math import e, pi, inf, nan
from math import cos, acos, sin, asin, tan, atan, degrees, radians
from math import log, log10, log2
```

### Statistics
```python
from statistics import mean, median, variance, pvariance, pstdev
```

### Random
```python
from random import random, randint, choice, shuffle
<float> = random()
<int>   = randint(from_inclusive, to_inclusive)
<el>    = choice(<list>)
shuffle(<list>)
```

### Bin, Hex
```python
<int>     = 0b<bin>                   # Or: 0x<hex>
<int>     = int('<bin>', 2)           # Or: int('<hex>', 16)
<int>     = int('0b<bin>', 0)         # Or: int('0x<hex>', 0)
'0b<bin>' = bin(<int>)                # Or: '0x<hex>' = hex(<int>)
```

### Bitwise Operators
```python
<int>     = <int> & <int>             # And
<int>     = <int> | <int>             # Or
<int>     = <int> ^ <int>             # Xor (0 if both bits equal)
<int>     = <int> << n_bits           # Shift left
<int>     = <int> >> n_bits           # Shift right
<int>     = ~<int>                    # Compliment (flips bits)
```


Combinatorics
-------------
* **Every function returns an iterator.**
* **If you want to print the iterator, you need to pass it to the list() function!**

```python
from itertools import product, combinations, combinations_with_replacement, permutations
```

```python
>>> product([0, 1], repeat=3)
[(0, 0, 0), (0, 0, 1), (0, 1, 0), (0, 1, 1),
 (1, 0, 0), (1, 0, 1), (1, 1, 0), (1, 1, 1)]
```

```python
>>> product('ab', '12')
[('a', '1'), ('a', '2'),
 ('b', '1'), ('b', '2')]
```

```python
>>> combinations('abc', 2)
[('a', 'b'), ('a', 'c'), ('b', 'c')]
```

```python
>>> combinations_with_replacement('abc', 2)
[('a', 'a'), ('a', 'b'), ('a', 'c'),
 ('b', 'b'), ('b', 'c'),
 ('c', 'c')]
```

```python
>>> permutations('abc', 2)
[('a', 'b'), ('a', 'c'),
 ('b', 'a'), ('b', 'c'),
 ('c', 'a'), ('c', 'b')]
```


Datetime
--------
* **Module 'datetime' provides 'date' `<D>`, 'time' `<T>`, 'datetime' `<DT>` and 'timedelta' `<TD>` classes. All are immutable and hashable.**
* **Time and datetime can be 'aware' `<a>`, meaning they have defined timezone, or 'naive' `<n>`, meaning they don't.**
* **If object is naive, it is presumed to be in the system's timezone.**

```python
from datetime import date, time, datetime, timedelta
from dateutil.tz import UTC, tzlocal, gettz, resolve_imaginary
```

### Constructors
```python
<D>  = date(year, month, day)
<T>  = time(hour=0, minute=0, second=0, microsecond=0, tzinfo=None, fold=0)
<DT> = datetime(year, month, day, hour=0, minute=0, second=0, ...)
<TD> = timedelta(days=0, seconds=0, microseconds=0, milliseconds=0,
                 minutes=0, hours=0, weeks=0)
```
* **Use `'<D/DT>.weekday()'` to get the day of the week (Mon == 0).**
* **`'fold=1'` means second pass in case of time jumping back for one hour.**
* **`'<DTa> = resolve_imaginary(<DTa>)'` fixes DTs that fall into missing hour.**

### Now
```python
<D/DTn>  = D/DT.today()                     # Current local date or naive datetime.
<DTn>    = DT.utcnow()                      # Naive datetime from current UTC time.
<DTa>    = DT.now(<tzinfo>)                 # Aware datetime from current tz time.
```
* **To extract time use `'<DTn>.time()'`, `'<DTa>.time()'` or `'<DTa>.timetz()'`.**

### Timezone
```python
<tzinfo> = UTC                              # UTC timezone. London without DST.
<tzinfo> = tzlocal()                        # Local timezone. Also gettz().
<tzinfo> = gettz('<Cont.>/<City>')          # 'Continent/City_Name' timezone or None.
<DTa>    = <DT>.astimezone(<tzinfo>)        # Datetime, converted to passed timezone.
<Ta/DTa> = <T/DT>.replace(tzinfo=<tzinfo>)  # Unconverted object with new timezone.
```

### Encode
```python
<D/T/DT> = D/T/DT.fromisoformat('<iso>')    # Object from ISO string. Raises ValueError.
<DT>     = DT.strptime(<str>, '<format>')   # Datetime from str, according to format.
<D/DTn>  = D/DT.fromordinal(<int>)          # D/DTn from days since Christ, at midnight.
<DTn>    = DT.fromtimestamp(<real>)         # Local time DTn from seconds since Epoch.
<DTa>    = DT.fromtimestamp(<real>, <tz.>)  # Aware datetime from seconds since Epoch.
```
* **ISO strings come in following forms: `'YYYY-MM-DD'`, `'HH:MM:SS.ffffff[±<offset>]'`, or both separated by a space or a `'T'`. Offset is formatted as: `'HH:MM'`.**
* **On Unix systems Epoch is `'1970-01-01 00:00 UTC'`, `'1970-01-01 01:00 CET'`, ...**

### Decode
```python
<str>    = <D/T/DT>.isoformat(sep='T')      # Also timespec='auto/hours/minutes/seconds'.
<str>    = <D/T/DT>.strftime('<format>')    # Custom string representation.
<int>    = <D/DT>.toordinal()               # Days since Christ, ignoring time and tz.
<float>  = <DTn>.timestamp()                # Seconds since Epoch from DTn in local time.
<float>  = <DTa>.timestamp()                # Seconds since Epoch from DTa.
```

### Format
```python
>>> from datetime import datetime
>>> dt = datetime.strptime('2015-05-14 23:39:00.00 +0200', '%Y-%m-%d %H:%M:%S.%f %z')
>>> dt.strftime("%A, %dth of %B '%y, %I:%M%p %Z")
"Thursday, 14th of May '15, 11:39PM UTC+02:00"
```
* **When parsing, `'%z'` also accepts `'±HH:MM'`.**
* **For abbreviated weekday and month use `'%a'` and `'%b'`.**

### Arithmetics
```python
<D/DT>   = <D/DT> ± <TD>                    # Result can fall into missing hour.
<TD>     = <D/DT> - <D/DT>                  # Ignores time jumps if they share non tzlocal tz.
<TD>     = <DT_UTC> - <DT_UTC>              # Convert DTs to UTC to get the actual delta.
```


Arguments
---------
### Inside Function Call
```python
<function>(<positional_args>)                  # f(0, 0)
<function>(<keyword_args>)                     # f(x=0, y=0)
<function>(<positional_args>, <keyword_args>)  # f(0, y=0)
```

### Inside Function Definition
```python
def f(<nondefault_args>):                      # def f(x, y):
def f(<default_args>):                         # def f(x=0, y=0):
def f(<nondefault_args>, <default_args>):      # def f(x, y=0):
```


Splat Operator
--------------
### Inside Function Call
**Splat expands a collection into positional arguments, while splatty-splat expands a dictionary into keyword arguments.**
```python
args   = (1, 2)
kwargs = {'x': 3, 'y': 4, 'z': 5}
func(*args, **kwargs)
```

#### Is the same as:
```python
func(1, 2, x=3, y=4, z=5)
```

### Inside Function Definition
**Splat combines zero or more positional arguments into a tuple, while splatty-splat combines zero or more keyword arguments into a dictionary.**
```python
def add(*a):
    return sum(a)
```

```python
>>> add(1, 2, 3)
6
```

#### Legal argument combinations:
```python
def f(x, y, z):                # f(x=1, y=2, z=3) | f(1, y=2, z=3) | f(1, 2, z=3) | f(1, 2, 3)
def f(*, x, y, z):             # f(x=1, y=2, z=3)
def f(x, *, y, z):             # f(x=1, y=2, z=3) | f(1, y=2, z=3)
def f(x, y, *, z):             # f(x=1, y=2, z=3) | f(1, y=2, z=3) | f(1, 2, z=3)
```

```python
def f(*args):                  # f(1, 2, 3)
def f(x, *args):               # f(1, 2, 3)
def f(*args, z):               # f(1, 2, z=3)
def f(x, *args, z):            # f(1, 2, z=3)
```

```python
def f(**kwargs):               # f(x=1, y=2, z=3)
def f(x, **kwargs):            # f(x=1, y=2, z=3) | f(1, y=2, z=3)
def f(*, x, **kwargs):         # f(x=1, y=2, z=3)
```

```python
def f(*args, **kwargs):        # f(x=1, y=2, z=3) | f(1, y=2, z=3) | f(1, 2, z=3) | f(1, 2, 3)
def f(x, *args, **kwargs):     # f(x=1, y=2, z=3) | f(1, y=2, z=3) | f(1, 2, z=3) | f(1, 2, 3)
def f(*args, y, **kwargs):     # f(x=1, y=2, z=3) | f(1, y=2, z=3)
def f(x, *args, z, **kwargs):  # f(x=1, y=2, z=3) | f(1, y=2, z=3) | f(1, 2, z=3)
```

### Other Uses
```python
<list>  = [*<collection> [, ...]]
<set>   = {*<collection> [, ...]}
<tuple> = (*<collection>, [...])
<dict>  = {**<dict> [, ...]}
```

```python
head, *body, tail = <collection>
```


Inline
------
### Lambda
```python
<function> = lambda: <return_value>
<function> = lambda <argument_1>, <argument_2>: <return_value>
```

### Comprehension
```python
<list> = [i+1 for i in range(10)]                   # [1, 2, ..., 10]
<set>  = {i for i in range(10) if i > 5}            # {6, 7, 8, 9}
<iter> = (i+5 for i in range(10))                   # (5, 6, ..., 14)
<dict> = {i: i*2 for i in range(10)}                # {0: 0, 1: 2, ..., 9: 18}
```

```python
out = [i+j for i in range(10) for j in range(10)]
```

#### Is the same as:
```python
out = []
for i in range(10):
    for j in range(10):
        out.append(i+j)
```

### Map, Filter, Reduce
```python
from functools import reduce
<iter> = map(lambda x: x + 1, range(10))            # (1, 2, ..., 10)
<iter> = filter(lambda x: x > 5, range(10))         # (6, 7, 8, 9)
<obj>  = reduce(lambda out, x: out + x, range(10))  # 45
```

### Any, All
```python
<bool> = any(<collection>)                          # False if empty.
<bool> = all(el[1] for el in <collection>)          # True if empty.
```

### If - Else
```python
<obj> = <expression_if_true> if <condition> else <expression_if_false>
```

```python
>>> [a if a else 'zero' for a in (0, 1, 2, 3)]
['zero', 1, 2, 3]
```

### Namedtuple, Enum, Dataclass
```python
from collections import namedtuple
Point     = namedtuple('Point', 'x y')
point     = Point(0, 0)
```

```python
from enum import Enum
Direction = Enum('Direction', 'n e s w')
direction = Direction.n
```

```python
from dataclasses import make_dataclass
Creature  = make_dataclass('Creature', ['location', 'direction'])
creature  = Creature(Point(0, 0), Direction.n)
```


Closure
-------
**We have a closure in Python when:**
* **A nested function references a value of its enclosing function and then**
* **the enclosing function returns the nested function.**

```python
def get_multiplier(a):
    def out(b):
        return a * b
    return out
```

```python
>>> multiply_by_3 = get_multiplier(3)
>>> multiply_by_3(10)
30
```

* **If multiple nested functions within enclosing function reference the same value, that value gets shared.**
* **To dynamically access function's first free variable use `'<function>.__closure__[0].cell_contents'`.**

### Partial
```python
from functools import partial
<function> = partial(<function> [, <arg_1>, <arg_2>, ...])
```

```python
>>> import operator as op
>>> multiply_by_3 = partial(op.mul, 3)
>>> multiply_by_3(10)
30
```
* **Partial is also useful in cases when function needs to be passed as an argument, because it enables us to set its arguments beforehand.**
* **A few examples being: `'defaultdict(<function>)'`, `'iter(<function>, to_exclusive)'` and dataclass's `'field(default_factory=<function>)'`.**

### Nonlocal
**If variable is being assigned to anywhere in the scope, it is regarded as a local variable, unless it is declared as a 'global' or a 'nonlocal'.**

```python
def get_counter():
    i = 0
    def out():
        nonlocal i
        i += 1
        return i
    return out
```

```python
>>> counter = get_counter()
>>> counter(), counter(), counter()
(1, 2, 3)
```


Decorator
---------
**A decorator takes a function, adds some functionality and returns it.**

```python
@decorator_name
def function_that_gets_passed_to_decorator():
    ...
```

### Debugger Example
**Decorator that prints function's name every time it gets called.**

```python
from functools import wraps

def debug(func):
    @wraps(func)
    def out(*args, **kwargs):
        print(func.__name__)
        return func(*args, **kwargs)
    return out

@debug
def add(x, y):
    return x + y
```
* **Wraps is a helper decorator that copies the metadata of the passed function (func) to the function it is wrapping (out).**
* **Without it `'add.__name__'` would return `'out'`.**

### LRU Cache
**Decorator that caches function's return values. All function's arguments must be hashable.**

```python
from functools import lru_cache

@lru_cache(maxsize=None)
def fib(n):
    return n if n < 2 else fib(n-2) + fib(n-1)
```
* **CPython interpreter limits recursion depth to 1000 by default. To increase it use `'sys.setrecursionlimit(<depth>)'`.**

### Parametrized Decorator
**A decorator that accepts arguments and returns a normal decorator that accepts a function.**
```python
from functools import wraps

def debug(print_result=False):
    def decorator(func):
        @wraps(func)
        def out(*args, **kwargs):
            result = func(*args, **kwargs)
            print(func.__name__, result if print_result else '')
            return result
        return out
    return decorator

@debug(print_result=True)
def add(x, y):
    return x + y
```


Class
-----
```python
class <name>:
    def __init__(self, a):
        self.a = a
    def __repr__(self):
        class_name = self.__class__.__name__
        return f'{class_name}({self.a!r})'
    def __str__(self):
        return str(self.a)

    @classmethod
    def get_class_name(cls):
        return cls.__name__
```
* **Return value of repr() should be unambiguous and of str() readable.**
* **If only repr() is defined, it will also be used for str().**

#### Str() use cases:
```python
print(<el>)
print(f'{<el>}')
raise Exception(<el>)
loguru.logger.debug(<el>)
csv.writer(<file>).writerow([<el>])
```

#### Repr() use cases:
```python
print([<el>])
print(f'{<el>!r}')
>>> <el>
loguru.logger.exception()
Z = dataclasses.make_dataclass('Z', ['a']); print(Z(<el>))
```

### Constructor Overloading
```python
class <name>:
    def __init__(self, a=None):
        self.a = a
```

### Inheritance
```python
class Person:
    def __init__(self, name, age):
        self.name = name
        self.age  = age

class Employee(Person):
    def __init__(self, name, age, staff_num):
        super().__init__(name, age)
        self.staff_num = staff_num
```

### Multiple Inheritance
```python
class A: pass
class B: pass
class C(A, B): pass
```

**MRO determines the order in which parent classes are traversed when searching for a method:**
```python
>>> C.mro()
[<class 'C'>, <class 'A'>, <class 'B'>, <class 'object'>]
```

### Property
**Pythonic way of implementing getters and setters.**
```python
class MyClass:
    @property
    def a(self):
        return self._a

    @a.setter
    def a(self, value):
        self._a = value
```

```python
>>> el = MyClass()
>>> el.a = 123
>>> el.a
123
```

### Dataclass
**Decorator that automatically generates init(), repr() and eq() special methods.**
```python
from dataclasses import dataclass, field

@dataclass(order=False, frozen=False)
class <class_name>:
    <attr_name_1>: <type>
    <attr_name_2>: <type> = <default_value>
    <attr_name_3>: list/dict/set = field(default_factory=list/dict/set)
```
* **Objects can be made sortable with `'order=True'` and/or immutable and hashable with `'frozen=True'`.**
* **Function field() is needed because `'<attr_name>: list = []'` would make a list that is shared among all instances.**
* **Default_factory can be any [callable](#callable).**

#### Inline:
```python
from dataclasses import make_dataclass
<class> = make_dataclass('<class_name>', <coll_of_attribute_names>)
<class> = make_dataclass('<class_name>', <coll_of_tuples>)
<tuple> = ('<attr_name>', <type> [, <default_value>])
```

### Slots
**Mechanism that restricts objects to attributes listed in 'slots' and significantly reduces their memory footprint.**

```python
class MyClassWithSlots:
    __slots__ = ['a']
    def __init__(self):
        self.a = 1
```

### Copy
```python
from copy import copy, deepcopy
<object> = copy(<object>)
<object> = deepcopy(<object>)
```


Duck Types
----------
**A duck type is an implicit type that prescribes a set of special methods. Any object that has those methods defined is considered a member of that duck type.**

### Comparable
* **If eq() method is not overridden, it returns `'id(self) == id(other)'`, which is the same as `'self is other'`.**
* **That means all objects compare not equal by default.**
* **Only the left side object has eq() method called, unless it returns NotImplemented, in which case the right object is consulted.**

```python
class MyComparable:
    def __init__(self, a):
        self.a = a
    def __eq__(self, other):
        if isinstance(other, type(self)):
            return self.a == other.a
        return NotImplemented
```

### Hashable
* **Hashable object needs both hash() and eq() methods and its hash value should never change.**
* **Hashable objects that compare equal must have the same hash value, meaning default hash() that returns `'id(self)'` will not do.**
* **That is why Python automatically makes classes unhashable if you only implement eq().**

```python
class MyHashable:
    def __init__(self, a):
        self._a = copy.deepcopy(a)
    @property
    def a(self):
        return self._a
    def __eq__(self, other):
        if isinstance(other, type(self)):
            return self.a == other.a
        return NotImplemented
    def __hash__(self):
        return hash(self.a)
```

### Sortable
* **With total_ordering decorator you only need to provide eq() and one of lt(), gt(), le() or ge() special methods.**
```python
from functools import total_ordering

@total_ordering
class MySortable:
    def __init__(self, a):
        self.a = a
    def __eq__(self, other):
        if isinstance(other, type(self)):
            return self.a == other.a
        return NotImplemented
    def __lt__(self, other):
        if isinstance(other, type(self)):
            return self.a < other.a
        return NotImplemented
```

### Iterator
* **Any object that defines methods next() and iter() is an iterator.**
* **Next() should return next item or raise StopIteration.**
* **Iter() should return 'self'.**
```python
class Counter:
    def __init__(self):
        self.i = 0
    def __next__(self):
        self.i += 1
        return self.i
    def __iter__(self):
        return self
```

```python
>>> counter = Counter()
>>> next(counter), next(counter), next(counter)
(1, 2, 3)
```

#### Python has many different iterator objects:
* **Iterators returned by the [iter()](#iterator) function, such as list\_iterator and set\_iterator.**
* **Objects returned by the [itertools](#itertools) module, such as count, repeat and cycle.**
* **Generators returned by the [generator functions](#generator) and [generator expressions](#comprehension).**
* **File objects returned by the [open()](#open) function, etc.**

### Callable
* **All functions and classes have a call() method, hence are callable.**
* **When this cheatsheet uses `'<function>'` for an argument, it actually means `'<callable>'`.**
```python
class Counter:
    def __init__(self):
        self.i = 0
    def __call__(self):
        self.i += 1
        return self.i
```

```python
>>> counter = Counter()
>>> counter(), counter(), counter()
(1, 2, 3)
```

### Context Manager
* **Enter() should lock the resources and return an object.**
* **Exit() should release the resources.**
```python
class MyOpen():
    def __init__(self, filename):
        self.filename = filename
    def __enter__(self):
        self.file = open(self.filename)
        return self.file
    def __exit__(self, *args):
        self.file.close()
```

```python
>>> with open('test.txt', 'w') as file:
...     file.write('Hello World!')
>>> with MyOpen('test.txt') as file:
...     print(file.read())
Hello World!
```


Iterable Duck Types
-------------------

### Iterable
* **Only required method is iter(). It should return an iterator of object's items.**
* **Contains() automatically works on any object that has iter() defined.**
```python
class MyIterable:
    def __init__(self, a):
        self.a = a
    def __iter__(self):
        for el in self.a:
            yield el
```

```python
>>> z = MyIterable([1, 2, 3])
>>> iter(z)
<generator object MyIterable.__iter__>
>>> 1 in z
True
```

### Collection
* **Only required methods are iter() and len().**
* **This cheatsheet actually means `'<iterable>'` when it uses `'<collection>'`.**
* **I chose not to use the name 'iterable' because it sounds scarier and more vague than 'collection'.**
```python
class MyCollection:
    def __init__(self, a):
        self.a = a
    def __iter__(self):
        return iter(self.a)
    def __contains__(self, el):
        return el in self.a
    def __len__(self):
        return len(self.a)
```

### Sequence
* **Only required methods are len() and getitem().**
* **Getitem() should return an item at index or raise IndexError.**
* **Iter() and contains() automatically work on any object that has getitem() defined.**
* **Reversed() automatically works on any object that has getitem() and len() defined.**
```python
class MySequence:
    def __init__(self, a):
        self.a = a
    def __iter__(self):
        return iter(self.a)
    def __contains__(self, el):
        return el in self.a
    def __len__(self):
        return len(self.a)
    def __getitem__(self, i):
        return self.a[i]
    def __reversed__(self):
        return reversed(self.a)
```

### Collections.abc.Sequence
* **It's a richer interface than the basic sequence.**
* **Extending it generates iter(), contains(), reversed(), index(), and count().**
* **Unlike `'abc.Iterable'` and `'abc.Collection'`, it is not a duck type. That is why `'issubclass(MySequence, collections.abc.Sequence)'` would return False even if MySequence had all the methods defined.**

```python
class MyAbcSequence(collections.abc.Sequence):
    def __init__(self, a):
        self.a = a
    def __len__(self):
        return len(self.a)
    def __getitem__(self, i):
        return self.a[i]
```

#### Table of required and automatically available special methods:
```text
+------------+------------+------------+------------+--------------+
|            |  Iterable  | Collection |  Sequence  | abc.Sequence |
+------------+------------+------------+------------+--------------+
| iter()     |    REQ     |    REQ     |    yes     |     yes      |
| contains() |    yes     |    yes     |    yes     |     yes      |
| len()      |            |    REQ     |    REQ     |     REQ      |
| getitem()  |            |            |    REQ     |     REQ      |
| reversed() |            |            |    yes     |     yes      |
| index()    |            |            |            |     yes      |
| count()    |            |            |            |     yes      |
+------------+------------+------------+------------+--------------+
```
* **Other ABCs that generate missing methods are: MutableSequence, Set, MutableSet, Mapping and MutableMapping.**
* **Names of their required methods are stored in `'<abc>.__abstractmethods__'`.**


Enum
----
```python
from enum import Enum, auto

class <enum_name>(Enum):
    <member_name_1> = <value_1>
    <member_name_2> = <value_2_a>, <value_2_b>
    <member_name_3> = auto()
```
* **If there are no numeric values before auto(), it returns 1.**
* **Otherwise it returns an increment of the last numeric value.**

```python
<member> = <enum>.<member_name>                # Returns a member.
<member> = <enum>['<member_name>']             # Returns a member or raises KeyError.
<member> = <enum>(<value>)                     # Returns a member or raises ValueError.
<str>    = <member>.name                       # Returns member's name.
<obj>    = <member>.value                      # Returns member's value.
```

```python
list_of_members = list(<enum>)
member_names    = [a.name for a in <enum>]
member_values   = [a.value for a in <enum>]
random_member   = random.choice(list(<enum>))
```

```python
def get_next_member(member):
    members = list(member.__class__)
    index   = (members.index(member) + 1) % len(members)
    return members[index]
```

### Inline
```python
Cutlery = Enum('Cutlery', ['fork', 'knife', 'spoon'])
Cutlery = Enum('Cutlery', 'fork knife spoon')
Cutlery = Enum('Cutlery', {'fork': 1, 'knife': 2, 'spoon': 3})
```

#### Functions can not be values, so they must be wrapped:
```python
from functools import partial
LogicOp = Enum('LogicOp', {'AND': partial(lambda l, r: l and r),
                           'OR' : partial(lambda l, r: l or r)})
```
* **Another solution in this particular case, is to use `'and_'` and `'or_'` functions from module [operator](#operator).**


Exceptions
----------

### Basic Example
```python
try:
    <code>
except <exception>:
    <code>
```

### Complex Example
```python
try:
    <code_1>
except <exception_a>:
    <code_2_a>
except <exception_b>:
    <code_2_b>
else:
    <code_2_c>
finally:
    <code_3>
```

### Catching Exceptions
```python
except <exception>:
except <exception> as <name>:
except (<exception>, ...):
except (<exception>, ...) as <name>:
```
* **Also catches subclasses of the exception.**

### Raising Exceptions
```python
raise <exception>
raise <exception>()
raise <exception>(<el>)
raise <exception>(<el>, ...)
```

#### Re-raising caught exception:
```python
except <exception>:
    <code>
    raise
```

#### Useful built-in exceptions:
```python
raise ValueError('Argument is of right type but inappropriate value!')
raise TypeError('Argument is of wrong type!')
raise RuntimeError('None of above!')
```

### Common Built-in Exceptions
```text
BaseException
 +-- SystemExit                   # Raised by the sys.exit() function.
 +-- KeyboardInterrupt            # Raised when the user hits the interrupt key (ctrl-c).
 +-- Exception                    # User-defined exceptions should be derived from this class.
      +-- StopIteration           # Raised by next() when run on an empty iterator.
      +-- ArithmeticError         # Base class for arithmetic errors.
      |    +-- ZeroDivisionError  # Raised when dividing by zero.
      +-- AttributeError          # Raised when an attribute is missing.
      +-- EOFError                # Raised by input() when it hits end-of-file condition.
      +-- LookupError             # Raised when a look-up on a sequence or dict fails.
      |    +-- IndexError         # Raised when a sequence index is out of range.
      |    +-- KeyError           # Raised when a dictionary key is not found.
      +-- NameError               # Raised when a variable name is not found.
      +-- OSError                 # Failures such as “file not found” or “disk full”.
      |    +-- FileNotFoundError  # When a file or directory is requested but doesn't exist.
      +-- RuntimeError            # Raised by errors that don't fall in other categories.
      |    +-- RecursionError     # Raised when the the maximum recursion depth is exceeded.
      +-- TypeError               # Raised when an argument is of wrong type.
      +-- ValueError              # When an argument is of right type but inappropriate value.
           +-- UnicodeError       # Raised when encoding/decoding strings from/to bytes fails.
```

#### Collections and their exceptions:
```text
+-----------+------------+------------+------------+
|           |    list    |    dict    |    set     |
+-----------+------------+------------+------------+
| getitem() | IndexError |  KeyError  |            |
| pop()     | IndexError |  KeyError  |  KeyError  |
| remove()  | ValueError |            |  KeyError  |
| index()   | ValueError |            |            |
+-----------+------------+------------+------------+
```

### User-defined Exceptions
```python
class MyError(Exception):
    pass

class MyInputError(MyError):
    pass
```


Print
-----
```python
print(<el_1>, ..., sep=' ', end='\n', file=sys.stdout, flush=False)
```

* **Use `'file=sys.stderr'` for errors.**
* **Use `'flush=True'` to forcibly flush the stream.**

### Pretty Print
```python
from pprint import pprint
pprint(<collection>, width=80, depth=None)
```
* **Levels deeper than 'depth' get replaced by '...'.**


Input
-----
**Reads a line from user input or pipe if present.**
```python
<str> = input(prompt=None)
```
* **Trailing newline gets stripped.**
* **Prompt string is printed to the standard output before reading input.**
* **Raises EOFError when user hits EOF or input stream gets exhausted.**


Command Line Arguments
----------------------
```python
import sys
script_name = sys.argv[0]
arguments   = sys.argv[1:]
```

### Argparse
```python
from argparse import ArgumentParser, FileType
p = ArgumentParser(description=<str>)
p.add_argument('-<short_name>', '--<name>', action='store_true')  # Flag
p.add_argument('-<short_name>', '--<name>', type=<type>)          # Option
p.add_argument('<name>', type=<type>, nargs=1)                    # First argument
p.add_argument('<name>', type=<type>, nargs='+')                  # Remaining arguments
p.add_argument('<name>', type=<type>, nargs='*')                  # Optional arguments
args  = p.parse_args()                                            # Exits on error.
value = args.<name>
```

* **Use `'help=<str>'` to set argument description.**
* **Use `'default=<el>'` to set the default value.**
* **Use `'type=FileType(<mode>)'` for files.**


Open
----
**Opens the file and returns a corresponding file object.**

```python
<file> = open('<path>', mode='r', encoding=None, newline=None)
```
* **`'encoding=None'` means that the default encoding is used, which is platform dependent. Best practice is to use `'encoding="utf-8"'` whenever possible.**
* **`'newline=None'` means all different end of line combinations are converted to '\n' on read, while on write all '\n' characters are converted to system's default line separator.**
* **`'newline=""'` means no conversions take place, but input is still broken into chunks by readline() and readlines() on either '\n', '\r' or '\r\n'.**

### Modes
* **`'r'`  - Read (default).**
* **`'w'`  - Write (truncate).**
* **`'x'`  - Write or fail if the file already exists.**
* **`'a'`  - Append.**
* **`'w+'` - Read and write (truncate).**
* **`'r+'` - Read and write from the start.**
* **`'a+'` - Read and write from the end.**
* **`'t'`  - Text mode (default).**
* **`'b'`  - Binary mode.**

### Exceptions
* **`'FileNotFoundError'` can be risen when reading with `'r'` or `'r+'`.**
* **`'FileExistsError'` can be risen when writing with `'x'`.**
* **`'IsADirectoryError'` and `'PermissionError'` can be risen by any.**
* **`'OSError'` is the parent class of all listed exceptions.**

### File Object
```python
<file>.seek(0)                      # Moves to the start of the file.
<file>.seek(offset)                 # Moves 'offset' chars/bytes from the start.
<file>.seek(0, 2)                   # Moves to the end of the file.
<bin_file>.seek(±offset, <anchor>)  # Anchor: 0 start, 1 current pos., 2 end.
```

```python
<str/bytes> = <file>.read(size=-1)  # Reads 'size' chars/bytes or until EOF.
<str/bytes> = <file>.readline()     # Returns a line or empty string on EOF.
<list>      = <file>.readlines()    # Returns a list of remaining lines.
<str/bytes> = next(<file>)          # Returns a line using buffer. Do not mix.
```

```python
<file>.write(<str/bytes>)           # Writes a string or bytes object.
<file>.writelines(<collection>)     # Writes a coll. of strings or bytes objects.
<file>.flush()                      # Flushes write buffer.
```
* **Methods do not add or strip trailing newlines, even writelines().**

### Read Text from File
```python
def read_file(filename):
    with open(filename, encoding='utf-8') as file:
        return file.readlines()
```

### Write Text to File
```python
def write_to_file(filename, text):
    with open(filename, 'w', encoding='utf-8') as file:
        file.write(text)
```


Path
----
```python
from os import getcwd, path, listdir, scandir
from glob import glob
```

```python
<str>  = getcwd()                   # Returns the current working directory.
<str>  = path.join(<path>, ...)     # Joins two or more pathname components.
<str>  = path.abspath(<path>)       # Return an absolute path.
```

```python
<str>  = path.basename(<path>)      # Returns final component.
<str>  = path.dirname(<path>)       # Returns path without final component.
<tup.> = path.splitext(<path>)      # Splits on last period of final component.
```

```python
<list> = listdir(path='.')          # Returns filenames located at path.
<list> = glob('<pattern>')          # Returns paths matching the wildcard pattern.
```

```python
<bool> = path.exists(<path>)        # Or: <Path>.exists()
<bool> = path.isfile(<path>)        # Or: <DirEntry/Path>.is_file()
<bool> = path.isdir(<path>)         # Or: <DirEntry/Path>.is_dir()
```

### DirEntry
**Using scandir() instead of listdir() can significantly increase the performance of code that also needs file type information.**

```python
<iter> = scandir(path='.')          # Returns DirEntry objects located at path.
```

```python
<str>  = <DirEntry>.path            # Returns path as a string.
<str>  = <DirEntry>.name            # Returns final component as a string.
<file> = open(<DirEntry>)           # Opens the file and returns a file object.
```

### Path Object
```python
from pathlib import Path
```

```python
<Path> = Path(<path> [, ...])       # Accepts strings, Paths and DirEntry objects.
<Path> = <path> / <path> [/ ...]    # One of the paths must be a Path object.
```

```python
<Path> = Path()                     # Returns relative cwd. Also Path('.').
<Path> = Path.cwd()                 # Returns absolute cwd. Also Path().resolve().
<Path> = <Path>.resolve()           # Returns absolute Path without symlinks.
```

```python
<Path> = <Path>.parent              # Returns Path without final component.
<str>  = <Path>.name                # Returns final component as a string.
<str>  = <Path>.stem                # Returns final component without extension.
<str>  = <Path>.suffix              # Returns final component's extension.
<tup.> = <Path>.parts               # Returns all components as strings.
```

```python
<iter> = <Path>.iterdir()           # Returns dir contents as Path objects.
<iter> = <Path>.glob('<pattern>')   # Returns Paths matching the wildcard pattern.
```

```python
<str>  = str(<Path>)                # Returns path as a string.
<file> = open(<Path>)               # Opens the file and returns a file object.
```


OS Commands
-----------
### Files and Directories
* **Paths can be either strings, Paths, or DirEntry objects.**
* **Functions report OS related errors by raising either OSError or one of its [subclasses](#exceptions-1).**

```python
import os, shutil
```

```python
os.chdir(<path>)                    # Changes current working directory.
os.mkdir(<path>, mode=0o777)        # Creates a directory. Mode is in octal.
```

```python
shutil.copy(from, to)               # Copies the file.
shutil.copytree(from, to)           # Copies the directory.
```

```python
os.rename(from, to)                 # Renames the file or directory.
os.replace(from, to)                # Same, but overwrites 'to' if it exists.
```

```python
os.remove(<path>)                   # Deletes the file.
os.rmdir(<path>)                    # Deletes empty directory.
shutil.rmtree(<path>)               # Deletes non-empty directory.
```

### Shell Commands
```python
import os
<str> = os.popen('<shell_command>').read()
```

#### Using subprocess:
```python
>>> import subprocess, shlex
>>> a = subprocess.run(shlex.split('ls -a'), stdout=subprocess.PIPE)
>>> a.stdout
b'.\n..\nfile1.txt\nfile2.txt\n'
>>> a.returncode
0
```


JSON
----
**Text file format for storing collections of strings and numbers.**

```python
import json
<str>    = json.dumps(<object>, ensure_ascii=True, indent=None)
<object> = json.loads(<str>)
```

### Read Object from JSON File
```python
def read_json_file(filename):
    with open(filename, encoding='utf-8') as file:
        return json.load(file)
```

### Write Object to JSON File
```python
def write_to_json_file(filename, an_object):
    with open(filename, 'w', encoding='utf-8') as file:
        json.dump(an_object, file, ensure_ascii=False, indent=2)
```


Pickle
------
**Binary file format for storing objects.**

```python
import pickle
<bytes>  = pickle.dumps(<object>)
<object> = pickle.loads(<bytes>)
```

### Read Object from File
```python
def read_pickle_file(filename):
    with open(filename, 'rb') as file:
        return pickle.load(file)
```

### Write Object to File
```python
def write_to_pickle_file(filename, an_object):
    with open(filename, 'wb') as file:
        pickle.dump(an_object, file)
```


CSV
---
**Text file format for storing spreadsheets.**

```python
import csv
```

### Read
```python
<reader> = csv.reader(<file>, dialect='excel', delimiter=',')
<list>   = next(<reader>)           # Returns next row as a list of strings.
<list>   = list(<reader>)           # Returns list of remaining rows.
```
* **File must be opened with `'newline=""'` argument, or newlines embedded inside quoted fields will not be interpreted correctly!**

### Write
```python
<writer> = csv.writer(<file>, dialect='excel', delimiter=',')
<writer>.writerow(<collection>)     # Encodes objects using `str(<el>)`.
<writer>.writerows(<coll_of_coll>)  # Appends multiple rows.
```
* **File must be opened with `'newline=""'` argument, or an extra '\r' will be added on platforms that use '\r\n' linendings!**

### Parameters
* **`'dialect'` - Master parameter that sets the default values.**
* **`'delimiter'` - A one-character string used to separate fields.**
* **`'quotechar'` - Character for quoting fields that contain special characters.**
* **`'doublequote'` - Whether quotechars inside fields get doubled or escaped.**
* **`'skipinitialspace'` - Whether whitespace after delimiter gets stripped.**
* **`'lineterminator'` - How does writer terminate lines.**
* **`'quoting'` - Controls the amount of quoting: 0 - as necessary, 1 - all.**
* **`'escapechar'` - Character for escaping 'quotechar' if 'doublequote' is false.**

### Dialects
```text
+------------------+--------------+--------------+--------------+
|                  |     excel    |   excel-tab  |     unix     |
+------------------+--------------+--------------+--------------+
| delimiter        |       ','    |      '\t'    |       ','    |
| quotechar        |       '"'    |       '"'    |       '"'    |
| doublequote      |      True    |      True    |      True    |
| skipinitialspace |     False    |     False    |     False    |
| lineterminator   |    '\r\n'    |    '\r\n'    |      '\n'    |
| quoting          |         0    |         0    |         1    |
| escapechar       |      None    |      None    |      None    |
+------------------+--------------+--------------+--------------+
```

### Read Rows from CSV File
```python
def read_csv_file(filename):
    with open(filename, encoding='utf-8', newline='') as file:
        return list(csv.reader(file))
```

### Write Rows to CSV File
```python
def write_to_csv_file(filename, rows):
    with open(filename, 'w', encoding='utf-8', newline='') as file:
        writer = csv.writer(file)
        writer.writerows(rows)
```


SQLite
------
**Server-less database engine that stores each database into separate file.**

### Connect
**Opens a connection to the database file. Creates a new file if path doesn't exist.**
```python
import sqlite3
db = sqlite3.connect('<path>')                  # Also ':memory:'.
...
db.close()
```

### Read
**Returned values can be of type str, int, float, bytes or None.**
```python
<cursor> = db.execute('<query>')                # Can raise sqlite3.OperationalError.
<tuple>  = <cursor>.fetchone()                  # Returns next row. Also next(<cursor>).
<list>   = <cursor>.fetchall()                  # Returns remaining rows.
```

### Write
```python
db.execute('<query>')
db.commit()
```

#### Or:
```python
with db:
    db.execute('<query>')
```

### Placeholders
* **Passed values can be of type str, int, float, bytes, None, bool, datetime.date or datetime.datetme.**
* **Bools will be stored and returned as ints and dates as [ISO formatted strings](#encode).**
```python
db.execute('<query>', <list/tuple>)             # Replaces '?'s in query with values.
db.execute('<query>', <dict/namedtuple>)        # Replaces ':<key>'s with values.
db.executemany('<query>', <coll_of_above>)      # Runs execute() many times.
```

### Example
**In this example values are not actually saved because `'db.commit()'` is omitted!**
```python
>>> db = sqlite3.connect('test.db')
>>> db.execute('create table t (a, b, c)')
>>> db.execute('insert into t values (1, 2, 3)')
>>> db.execute('select * from t').fetchall()
[(1, 2, 3)]
``` 

### MySQL
**Has a very similar interface, with differences listed below.**
```python
# $ pip3 install mysql-connector
from mysql import connector
db = connector.connect(host=<str>, user=<str>, password=<str>, database=<str>)
<cursor> = db.cursor()
<cursor>.execute('<query>')                     # Only cursor has execute method.
<cursor>.execute('<query>', <list/tuple>)       # Replaces '%s's in query with values.
<cursor>.execute('<query>', <dict/namedtuple>)  # Replaces '%(<key>)s's with values.
```


Bytes
-----
**Bytes object is an immutable sequence of single bytes. Mutable version is called bytearray.**

```python
<bytes> = b'<str>'                       # Only accepts ASCII characters and \x00 - \xff.
<int>   = <bytes>[<index>]               # Returns int in range from 0 to 255.
<bytes> = <bytes>[<slice>]               # Returns bytes even if it has only one element.
<bytes> = <bytes>.join(<coll_of_bytes>)  # Joins elements using bytes object as separator.
```

### Encode
```python
<bytes> = bytes(<coll_of_ints>)          # Ints must be in range from 0 to 255.
<bytes> = bytes(<str>, 'utf-8')          # Or: <str>.encode('utf-8')
<bytes> = <int>.to_bytes(n_bytes, byteorder='big|little', signed=False)
<bytes> = bytes.fromhex('<hex>')
```

### Decode
```python
<list>  = list(<bytes>)                  # Returns ints in range from 0 to 255.
<str>   = str(<bytes>, 'utf-8')          # Or: <bytes>.decode('utf-8')
<int>   = int.from_bytes(<bytes>, byteorder='big|little', signed=False)
'<hex>' = <bytes>.hex()
```

### Read Bytes from File
```python
def read_bytes(filename):
    with open(filename, 'rb') as file:
        return file.read()
```

### Write Bytes to File
```python
def write_bytes(filename, bytes_obj):
    with open(filename, 'wb') as file:
        file.write(bytes_obj)
```


Struct
------
* **Module that performs conversions between a sequence of numbers and a bytes object.**
* **Machine’s native type sizes and byte order are used by default.**

```python
from struct import pack, unpack, iter_unpack
<bytes>  = pack('<format>', <num_1> [, <num_2>, ...])
<tuple>  = unpack('<format>', <bytes>)
<tuples> = iter_unpack('<format>', <bytes>)
```

### Example
```python
>>> pack('>hhl', 1, 2, 3)
b'\x00\x01\x00\x02\x00\x00\x00\x03'
>>> unpack('>hhl', b'\x00\x01\x00\x02\x00\x00\x00\x03')
(1, 2, 3)
```

### Format
#### For standard sizes start format string with:
* **`'='` - native byte order**
* **`'<'` - little-endian**
* **`'>'` - big-endian (also `'!'`)**

#### Integer types. Use capital letter for unsigned type. Standard sizes are in brackets:
* **`'x'` - pad byte**
* **`'b'` - char (1)**
* **`'h'` - short (2)**
* **`'i'` - int (4)**
* **`'l'` - long (4)**
* **`'q'` - long long (8)**

#### Floating point types:
* **`'f'` - float (4)**
* **`'d'` - double (8)**


Array
-----
**List that can only hold numbers of a predefined type. Available types and their sizes in bytes are listed above.**

```python
from array import array
<array> = array('<typecode>', <collection>)    # Array from coll. of numbers.
<array> = array('<typecode>', <bytes>)         # Array from bytes object.
<bytes> = bytes(<array>)                       # Or: <array>.tobytes()
```


Memory View
-----------
* **A sequence object that points to the memory of another object.**
* **Each element can reference a single or multiple consecutive bytes, depending on format.**
* **Order and number of elements can be changed with slicing.**

```python
<mview> = memoryview(<bytes/bytearray/array>)  # Immutable if bytes, else mutable.
<real>  = <mview>[<index>]                     # Returns an int or a float.
<mview> = <mview>[<slice>]                     # Mview with rearranged elements.
<mview> = <mview>.cast('<typecode>')           # Casts memoryview to the new format.
<mview>.release()                              # Releases the object's memory buffer.
```

### Decode
```python
<bin_file>.write(<mview>)                      # Appends mview to the binary file.
<bytes> = bytes(<mview>)                       # Creates a new bytes object.
<bytes> = <bytes>.join(<coll_of_mviews>)       # Joins mviews using bytes object as sep.
<list>  = list(<mview>)                        # Returns list of ints or floats.
<str>   = str(<mview>, 'utf-8')
<int>   = int.from_bytes(<mview>, byteorder='big|little', signed=False)
'<hex>' = <mview>.hex()
```


Deque
-----
**A thread-safe list with efficient appends and pops from either side. Pronounced "deck".**

```python
from collections import deque
<deque> = deque(<collection>, maxlen=None)
```

```python
<deque>.appendleft(<el>)                       # Opposite element is dropped if full.
<el> = <deque>.popleft()                       # Raises IndexError if empty.
<deque>.extendleft(<collection>)               # Collection gets reversed.
<deque>.rotate(n=1)                            # Rotates elements to the right.
```


Threading
---------
* **CPython interpreter can only run a single thread at a time.**
* **That is why using multiple threads won't result in a faster execution, unless there is an I/O operation in the thread.**
```python
from threading import Thread, RLock
```

### Thread
```python
thread = Thread(target=<function>, args=(<first_arg>, ))
thread.start()
...
<bool> = thread.is_alive()           # Checks if thread has finished executing.
thread.join()                        # Waits for thread to finish.
```
* **Use `'kwargs=<dict>'` to pass keyword arguments to the function.**
* **Use `'daemon=True'`, or the program will not be able to exit while the thread is alive.**

### Lock
```python
lock = RLock()
lock.acquire()                       # Waits for lock to be available.
...
lock.release()
```

#### Or:
```python
lock = RLock()
with lock:
    ...
```

### Thread Pool Executor
```python
from concurrent.futures import ThreadPoolExecutor
with ThreadPoolExecutor(max_workers=None) as executor:
    <iter>   = executor.map(lambda x: x + 1, range(3))         # (1, 2, 3)
    <iter>   = executor.map(lambda x, y: x + y, 'abc', '123')  # ('a1', 'b2', 'c3')
    <Future> = executor.submit(<function> [, <arg_1>, ...])
```

#### Future:
```python
<bool> = <Future>.done()             # Checks if thread has finished executing.
<obj>  = <Future>.result()           # Waits for thread to finish and returns result.
```

### Queue
**A thread-safe FIFO queue. For LIFO queue use LifoQueue.**
```python
from queue import Queue
<Queue> = Queue(maxsize=0)
```

```python
<Queue>.put(<el>)                    # Blocks until queue stops being full.
<Queue>.put_nowait(<el>)             # Raises queue.Full exception if full.
<el> = <Queue>.get()                 # Blocks until queue stops being empty.
<el> = <Queue>.get_nowait()          # Raises _queue.Empty exception if empty.
```


Operator
--------
**Module of functions that provide the functionality of operators.**
```python
from operator import add, sub, mul, truediv, floordiv, mod, pow, neg, abs
from operator import eq, ne, lt, le, gt, ge
from operator import and_, or_, not_
from operator import itemgetter, attrgetter, methodcaller
```

```python
import operator as op
sorted_by_second = sorted(<collection>, key=op.itemgetter(1))
sorted_by_both   = sorted(<collection>, key=op.itemgetter(1, 0))
product_of_elems = functools.reduce(op.mul, <collection>)
LogicOp          = enum.Enum('LogicOp', {'AND': op.and_, 'OR' : op.or_})
last_el          = op.methodcaller('pop')(<list>)
```


Introspection
-------------
**Inspecting code at runtime.**

### Variables
```python
<list> = dir()                       # Returns names of variables in current scope.
<dict> = locals()                    # Returns dict of local variables. Also vars().
<dict> = globals()                   # Returns dict of global variables.
```

### Attributes
```python
<dict> = vars(<object>)
<bool> = hasattr(<object>, '<attr_name>')
value  = getattr(<object>, '<attr_name>')
setattr(<object>, '<attr_name>', value)
```

### Parameters
```python
from inspect import signature
<sig>        = signature(<function>)
no_of_params = len(<sig>.parameters)
param_names  = list(<sig>.parameters.keys())
```


Metaprograming
--------------
**Code that generates code.**

### Type
**Type is the root class. If only passed an object it returns its type (class). Otherwise it creates a new class.**

```python
<class> = type('<class_name>', <parents_tuple>, <attributes_dict>)
```

```python
>>> Z = type('Z', (), {'a': 'abcde', 'b': 12345})
>>> z = Z()
```

### Meta Class
**Class that creates classes.**

```python
def my_meta_class(name, parents, attrs):
    attrs['a'] = 'abcde'
    return type(name, parents, attrs)
```

#### Or:
```python
class MyMetaClass(type):
    def __new__(cls, name, parents, attrs):
        attrs['a'] = 'abcde'
        return type.__new__(cls, name, parents, attrs)
```
* **New() is a class method that gets called before init(). If it returns an instance of its class, then that instance gets passed to init() as a 'self' argument.**
* **It receives the same arguments as init(), except for the first one that specifies the desired type of the returned instance (MyMetaClass in our case).**
* **Like in our case, new() can also be called directly, usually from a new() method of a child class (**`def __new__(cls): return super().__new__(cls)`**).**
* **The only difference between the examples above is that my\_meta\_class() returns a class of type type, while MyMetaClass() returns a class of type MyMetaClass.**

### Metaclass Attribute
**Right before a class is created it checks if it has a 'metaclass' attribute defined. If not, it recursively checks if any of his parents has it defined and eventually comes to type().**

```python
class MyClass(metaclass=MyMetaClass):
    b = 12345
```

```python
>>> MyClass.a, MyClass.b
('abcde', 12345)
```

### Type Diagram
```python
type(MyClass)     == MyMetaClass     # MyClass is an instance of MyMetaClass.
type(MyMetaClass) == type            # MyMetaClass is an instance of type.
```

```text
+-------------+-------------+
|   Classes   | Metaclasses |
+-------------+-------------|
|   MyClass --> MyMetaClass |
|             |     v       |
|    object -----> type <+  |
|             |    ^ +---+  |
|     str ---------+        |
+-------------+-------------+
```

### Inheritance Diagram
```python
MyClass.__base__     == object       # MyClass is a subclass of object.
MyMetaClass.__base__ == type         # MyMetaClass is a subclass of type.
```

```text
+-------------+-------------+
|   Classes   | Metaclasses |
+-------------+-------------|
|   MyClass   | MyMetaClass |
|      v      |     v       |
|    object <----- type     |
|      ^      |             |
|     str     |             |
+-------------+-------------+
```


Eval
----
```python
>>> from ast import literal_eval
>>> literal_eval('1 + 2')
3
>>> literal_eval('[1, 2, 3]')
[1, 2, 3]
>>> literal_eval('abs(1)')
ValueError: malformed node or string
```


Coroutine
---------
* **Any function that contains a `'(yield)'` expression returns a coroutine.**
* **Coroutines are similar to iterators, but data needs to be pulled out of an iterator by calling `'next(<iter>)'`, while we push data into the coroutine by calling `'<coroutine>.send(<el>)'`.**
* **Coroutines provide more powerful data routing possibilities than iterators.**

### Helper Decorator
* **All coroutines must first be "primed" by calling `'next(<coroutine>)'`.**
* **Remembering to call next() is easy to forget.**
* **Solved by wrapping coroutine functions with the following decorator:**

```python
def coroutine(func):
    def out(*args, **kwargs):
        cr = func(*args, **kwargs)
        next(cr)
        return cr
    return out
```

### Pipeline Example
```python
def reader(target):
    for i in range(10):
        target.send(i)
    target.close()

@coroutine
def adder(target):
    while True:
        value = (yield)
        target.send(value + 100)

@coroutine
def printer():
    while True:
        value = (yield)
        print(value, end=' ')
```

```python
>>> reader(adder(printer()))
100 101 102 103 104 105 106 107 108 109
```
<br>


Libraries
=========

Progress Bar
------------
```python
# $ pip3 install tqdm
from tqdm import tqdm
from time import sleep
for el in tqdm([1, 2, 3]):
    sleep(0.2)
```


Plot
----
```python
# $ pip3 install matplotlib
from matplotlib import pyplot
pyplot.plot(<y_data> [, label=<str>])
pyplot.plot(<x_data>, <y_data>)
pyplot.legend()                                # Adds a legend.
pyplot.savefig(<filename>)                     # Saves the figure.
pyplot.show()                                  # Displays the figure.
pyplot.clf()                                   # Clears the figure.
```


Table
-----
#### Prints a CSV file as an ASCII table:
```python
# $ pip3 install tabulate
import csv, tabulate
with open('test.csv', encoding='utf-8', newline='') as file:
    rows   = csv.reader(file)
    header = [a.title() for a in next(rows)]
    table  = tabulate.tabulate(rows, header)
    print(table)
```


Curses
------
#### Clears the terminal, prints a message and waits for an ESC key press:
```python
from curses import wrapper, curs_set, ascii
from curses import KEY_UP, KEY_RIGHT, KEY_DOWN, KEY_LEFT

def main():
    wrapper(draw)

def draw(screen):
    curs_set(0)                                # Makes cursor invisible.
    screen.nodelay(True)                       # Makes getch() non-blocking.
    screen.clear()
    screen.addstr(0, 0, 'Press ESC to quit.')
    while screen.getch() != ascii.ESC:
        pass

def get_border(screen):
    from collections import namedtuple
    P = namedtuple('P', 'x y')
    height, width = screen.getmaxyx()
    return P(width - 1, height - 1)

if __name__ == '__main__':
    main()
```


Logging
-------
```python
# $ pip3 install loguru
from loguru import logger
```

```python
logger.add('debug_{time}.log', colorize=True)  # Connects a log file.
logger.add('error_{time}.log', level='ERROR')  # Another file for errors or higher.
logger.<level>('A logging message.')
```
* **Levels: `'debug'`, `'info'`, `'success'`, `'warning'`, `'error'`, `'critical'`.**

### Exceptions
**Exception description, stack trace and values of variables are appended automatically.**

```python
try:
    ...
except <exception>:
    logger.exception('An error happened.')
```

### Rotation
**Argument that sets a condition when a new log file is created.**
```python
rotation=<int>|<datetime.timedelta>|<datetime.time>|<str>
```
* **`'<int>'` - Max file size in bytes.**
* **`'<timedelta>'` - Max age of a file.**
* **`'<time>'` - Time of day.**
* **`'<str>'` - Any of above as a string: `'100 MB'`, `'1 month'`, `'monday at 12:00'`, ...**

### Retention
**Sets a condition which old log files get deleted.**
```python
retention=<int>|<datetime.timedelta>|<str>
```
* **`'<int>'` - Max number of files.**
* **`'<timedelta>'` - Max age of a file.**
* **`'<str>'` - Max age as a string: `'1 week, 3 days'`, `'2 months'`, ...**


Scraping
--------
#### Scrapes Python's URL, version number and logo from Wikipedia page:
```python
# $ pip3 install requests beautifulsoup4
import requests
from bs4 import BeautifulSoup
URL = 'https://en.wikipedia.org/wiki/Python_(programming_language)'
try:
    html  = requests.get(URL).text
    doc   = BeautifulSoup(html, 'html.parser')
    table = doc.find('table', class_='infobox vevent')
    rows  = table.find_all('tr')
    link  = rows[11].find('a')['href']
    ver   = rows[6].find('div').text.split()[0]
    url_i = rows[0].find('img')['src']
    image = requests.get(f'https:{url_i}').content
    with open('test.png', 'wb') as file:
        file.write(image)
    print(link, ver)
except requests.exceptions.ConnectionError:
    print("You've got problems with connection.")
```


Web
---
```python
# $ pip3 install bottle
from bottle import run, route, static_file, template, post, request, response
import json
```

### Run
```python
run(host='localhost', port=8080)        # Runs locally.
run(host='0.0.0.0', port=80)            # Runs globally.
```

### Static Request
```python
@route('/img/<image>')
def send_image(image):
    return static_file(image, 'img_dir/', mimetype='image/png')
```

### Dynamic Request
```python
@route('/<sport>')
def send_page(sport):
    return template('<h1>{{title}}</h1>', title=sport)
```

### REST Request
```python
@post('/odds/<sport>')
def odds_handler(sport):
    team = request.forms.get('team')
    home_odds, away_odds = 2.44, 3.29
    response.headers['Content-Type'] = 'application/json'
    response.headers['Cache-Control'] = 'no-cache'
    return json.dumps([team, home_odds, away_odds])
```

#### Test:
```python
# $ pip3 install requests
>>> import requests
>>> url  = 'http://localhost:8080/odds/football'
>>> data = {'team': 'arsenal f.c.'}
>>> response = requests.post(url, data=data)
>>> response.json()
['arsenal f.c.', 2.44, 3.29]
```


Profiling
---------
### Stopwatch
```python
from time import time
start_time = time()                     # Seconds since the Epoch.
...
duration = time() - start_time
```

#### High performance:
```python
from time import perf_counter
start_time = perf_counter()             # Seconds since restart.
...
duration = perf_counter() - start_time
```

### Timing a Snippet
```python
>>> from timeit import timeit
>>> timeit('"-".join(str(a) for a in range(100))',
...        number=10000, globals=globals(), setup='pass')
0.34986
```

### Profiling by Line
```python
# $ pip3 install line_profiler memory_profiler
@profile
def main():
    a = [*range(10000)]
    b = {*range(10000)}
main()
```

```text
$ kernprof -lv test.py
Line #   Hits     Time  Per Hit   % Time  Line Contents
=======================================================
     1                                    @profile
     2                                    def main():
     3      1   1128.0   1128.0     27.4      a = [*range(10000)]
     4      1   2994.0   2994.0     72.6      b = {*range(10000)}
```

```text
$ python3 -m memory_profiler test.py
Line #         Mem usage      Increment   Line Contents
=======================================================
     1        35.387 MiB     35.387 MiB   @profile
     2                                    def main():
     3        35.734 MiB      0.348 MiB       a = [*range(10000)]
     4        36.160 MiB      0.426 MiB       b = {*range(10000)}
```

### Call Graph
#### Generates a PNG image of a call graph with highlighted bottlenecks:
```python
# $ pip3 install pycallgraph
from pycallgraph import output, PyCallGraph
from datetime import datetime
time_str = datetime.now().strftime('%Y%m%d%H%M%S')
filename = f'profile-{time_str}.png'
drawer = output.GraphvizOutput(output_file=filename)
with PyCallGraph(drawer):
    <code_to_be_profiled>
```


NumPy
-----
**Array manipulation mini language. Can run up to one hundred times faster than equivalent Python code.**

```python
# $ pip3 install numpy
import numpy as np
```

```python
<array> = np.array(<list>)
<array> = np.arange(from_inclusive, to_exclusive, ±step_size)
<array> = np.ones(<shape>)
<array> = np.random.randint(from_inclusive, to_exclusive, <shape>)
```

```python
<array>.shape = <shape>
<view>  = <array>.reshape(<shape>)
<view>  = np.broadcast_to(<array>, <shape>)
```

```python
<array> = <array>.sum(axis)
indexes = <array>.argmin(axis)
```

* **Shape is a tuple of dimension sizes.**
* **Axis is an index of dimension that gets collapsed. Leftmost dimension has index 0.**

### Indexing
```bash
<el>       = <2d_array>[0, 0]        # First element.
<1d_view>  = <2d_array>[0]           # First row.
<1d_view>  = <2d_array>[:, 0]        # First column. Also [..., 0].
<3d_view>  = <2d_array>[None, :, :]  # Expanded by dimension of size 1.
```

```bash
<1d_array> = <2d_array>[<1d_row_indexes>, <1d_column_indexes>]
<2d_array> = <2d_array>[<2d_row_indexes>, <2d_column_indexes>]
```

```bash
<2d_bools> = <2d_array> > 0
<1d_array> = <2d_array>[<2d_bools>]
```

* **If row and column indexes differ in shape, they are combined with broadcasting.**

### Broadcasting
**Broadcasting is a set of rules by which NumPy functions operate on arrays of different sizes and/or dimensions.**

```python
left  = [[0.1], [0.6], [0.8]]        # Shape: (3, 1)
right = [ 0.1 ,  0.6 ,  0.8 ]        # Shape: (3)
```

#### 1. If array shapes differ in length, left-pad the shorter shape with ones:
```python
left  = [[0.1], [0.6], [0.8]]        # Shape: (3, 1)
right = [[0.1 ,  0.6 ,  0.8]]        # Shape: (1, 3) <- !
```

#### 2. If any dimensions differ in size, expand the ones that have size 1 by duplicating their elements:
```python
left  = [[0.1, 0.1, 0.1], [0.6, 0.6, 0.6], [0.8, 0.8, 0.8]]  # Shape: (3, 3) <- !
right = [[0.1, 0.6, 0.8], [0.1, 0.6, 0.8], [0.1, 0.6, 0.8]]  # Shape: (3, 3) <- !
```

#### 3. If neither non-matching dimension has size 1, rise an error.


### Example
#### For each point returns index of its nearest point (`[0.1, 0.6, 0.8] => [1, 2, 1]`):

```python
>>> points = np.array([0.1, 0.6, 0.8])
[ 0.1,  0.6,  0.8]
>>> wrapped_points = points.reshape(3, 1)
[[ 0.1],
 [ 0.6],
 [ 0.8]]
>>> distances = wrapped_points - points
[[ 0. , -0.5, -0.7],
 [ 0.5,  0. , -0.2],
 [ 0.7,  0.2,  0. ]]
>>> distances = np.abs(distances)
[[ 0. ,  0.5,  0.7],
 [ 0.5,  0. ,  0.2],
 [ 0.7,  0.2,  0. ]]
>>> i = np.arange(3)
[0, 1, 2]
>>> distances[i, i] = np.inf
[[ inf,  0.5,  0.7],
 [ 0.5,  inf,  0.2],
 [ 0.7,  0.2,  inf]]
>>> distances.argmin(1)
[1, 2, 1]
```


Image
-----
```python
# $ pip3 install pillow
from PIL import Image
```

```python
<Image> = Image.new('<mode>', (width, height))
<Image> = Image.open('<path>')
<Image> = <Image>.convert('<mode>')
<Image>.save('<path>')
<Image>.show()
```

```python
<tuple/int> = <Image>.getpixel((x, y))          # Returns a pixel.
<Image>.putpixel((x, y), <tuple/int>)           # Writes a pixel to image.
<ImagingCore> = <Image>.getdata()               # Returns a sequence of pixels.
<Image>.putdata(<list/ImagingCore>)             # Writes a sequence of pixels.
<Image>.paste(<Image>, (x, y))                  # Writes an image to image.
```

```bash
<2d_array> = np.array(<Image>)                  # NumPy array from greyscale image.
<3d_array> = np.array(<Image>)                  # NumPy array from color image.
<Image>    = Image.fromarray(<array>)           # Image from NumPy array.
```

### Modes
* **`'1'` - 1-bit pixels, black and white, stored with one pixel per byte.**
* **`'L'` - 8-bit pixels, greyscale.**
* **`'RGB'` - 3x8-bit pixels, true color.**
* **`'RGBA'` - 4x8-bit pixels, true color with transparency mask.**
* **`'HSV'` - 3x8-bit pixels, Hue, Saturation, Value color space.**

### Examples
#### Creates a PNG image of a rainbow gradient:
```python
WIDTH, HEIGHT = 100, 100
size = WIDTH * HEIGHT
hues = [255 * i/size for i in range(size)]
img = Image.new('HSV', (WIDTH, HEIGHT))
img.putdata([(int(h), 255, 255) for h in hues])
img.convert('RGB').save('test.png')
```

#### Adds noise to a PNG image:
```python
from random import randint
add_noise = lambda value: max(0, min(255, value + randint(-20, 20)))
img = Image.open('test.png').convert('HSV')
img.putdata([(add_noise(h), s, v) for h, s, v in img.getdata()])
img.convert('RGB').save('test.png')
```

### Drawing
```python
from PIL import ImageDraw
```

```python
<ImageDraw> = ImageDraw.Draw(<Image>)
<ImageDraw>.point((x, y), fill=None)
<ImageDraw>.line((x1, y1, x2, y2 [, ...]), fill=None, width=0, joint=None) 
<ImageDraw>.arc((x1, y1, x2, y2), from_deg, to_deg, fill=None, width=0)
<ImageDraw>.rectangle((x1, y1, x2, y2), fill=None, outline=None, width=0)
<ImageDraw>.polygon((x1, y1, x2, y2 [, ...]), fill=None, outline=None)
<ImageDraw>.ellipse((x1, y1, x2, y2), fill=None, outline=None, width=0)
```
* **Use `'fill=<color>'` to set the primary color.**
* **Use `'outline=<color>'` to set the secondary color.**
* **Color can be specified as a tuple, int, `'#rrggbb'` string or a color name.**


Animation
---------
#### Creates a GIF of a bouncing ball:
```python
# $ pip3 install pillow imageio
from PIL import Image, ImageDraw
import imageio
WIDTH, R = 126, 10
frames = []
for velocity in range(15):
    y = sum(range(velocity+1))
    frame = Image.new('L', (WIDTH, WIDTH))
    draw  = ImageDraw.Draw(frame)
    draw.ellipse((WIDTH/2-R, y, WIDTH/2+R, y+2*R), fill='white')
    frames.append(frame)
frames += reversed(frames[1:-1])
imageio.mimsave('test.gif', frames, duration=0.03)
```


Audio
-----
```python
import wave
```

```python
<Wave_read>  = wave.open('<path>', 'rb')
framerate    = <Wave_read>.getframerate()       # Number of frames per second.
nchannels    = <Wave_read>.getnchannels()       # Number of samples per frame.
sampwidth    = <Wave_read>.getsampwidth()       # Sample size in bytes.
nframes      = <Wave_read>.getnframes()         # Number of frames.
<params>     = <Wave_read>.getparams()          # Immutable collection of above.
<bytes>      = <Wave_read>.readframes(nframes)  # Returns next 'nframes' frames.
```

```python
<Wave_write> = wave.open('<path>', 'wb')
<Wave_write>.setframerate(<int>)                # 44100 for CD, 48000 for video.
<Wave_write>.setnchannels(<int>)                # 1 for mono, 2 for stereo.
<Wave_write>.setsampwidth(<int>)                # 2 for CD quality sound.
<Wave_write>.setparams(<params>)                # Sets all parameters.
<Wave_write>.writeframes(<bytes>)               # Appends frames to file.
```
* **Bytes object contains a sequence of frames, each consisting of one or more samples.**
* **In stereo signal first sample of a frame belongs to the left channel.**
* **Each sample consists of one or more bytes that, when converted to an integer, indicate the displacement of a speaker membrane at a given moment.**
* **If sample width is one, then the integer should be encoded unsigned.**
* **For all other sizes the integer should be encoded signed with little-endian byte order.**

### Sample Values
```text
+-----------+-------------+------+-------------+
| sampwidth |     min     | zero |     max     |
+-----------+-------------+------+-------------+
|     1     |           0 |  128 |         255 |
|     2     |      -32768 |    0 |       32767 |
|     3     |    -8388608 |    0 |     8388607 |
|     4     | -2147483648 |    0 |  2147483647 |
+-----------+-------------+------+-------------+
```

### Read Float Samples from WAV File
```python
def read_wav_file(filename):
    def get_int(a_bytes):
        an_int = int.from_bytes(a_bytes, 'little', signed=width!=1)
        return an_int - 128 * (width == 1)
    with wave.open(filename, 'rb') as file:
        width  = file.getsampwidth()
        frames = file.readframes(file.getnframes())
    byte_samples = (frames[i: i + width] for i in range(0, len(frames), width))
    return [get_int(b) / pow(2, width * 8 - 1) for b in byte_samples]
```

### Write Float Samples to WAV File
```python
def write_to_wav_file(filename, float_samples, nchannels=1, sampwidth=2, framerate=44100):
    def get_bytes(a_float):
        a_float = max(-1, min(1 - 2e-16, a_float))
        a_float += sampwidth == 1
        a_float *= pow(2, sampwidth * 8 - 1)
        return int(a_float).to_bytes(sampwidth, 'little', signed=sampwidth!=1) 
    with wave.open(filename, 'wb') as file:
        file.setnchannels(nchannels)
        file.setsampwidth(sampwidth)
        file.setframerate(framerate)
        file.writeframes(b''.join(get_bytes(f) for f in float_samples))
```

### Examples
#### Saves a sine wave to a mono WAV file:
```python
from math import pi, sin
samples_f = (sin(i * 2 * pi * 440 / 44100) for i in range(100000))
write_to_wav_file('test.wav', samples_f)
```

#### Adds noise to a mono WAV file:
```python
from random import random
add_noise = lambda value: value + (random() - 0.5) * 0.03
samples_f = (add_noise(f) for f in read_wav_file('test.wav'))
write_to_wav_file('test.wav', samples_f)
```

#### Plays a WAV file:
```python
# $ pip3 install simpleaudio
import simpleaudio
with wave.open('test.wav', 'rb') as file:
    p = file.getparams()
    frames = file.readframes(p.nframes)
    simpleaudio.play_buffer(frames, p.nchannels, p.sampwidth, p.framerate)
```

### Text to Speech
```python
# $ pip3 install pyttsx3
import pyttsx3
engine = pyttsx3.init()
engine.say('Sally sells seashells by the seashore.')
engine.runAndWait()
```


Synthesizer
-----------
#### Plays Popcorn by Gershon Kingsley:
```python
# $ pip3 install simpleaudio
import simpleaudio, math, struct
from itertools import chain, repeat
F  = 44100
P1 = '71♪,69,,71♪,66,,62♪,66,,59♪,,,'
P2 = '71♪,73,,74♪,73,,74,,71,,73♪,71,,73,,69,,71♪,69,,71,,67,,71♪,,,'
get_pause   = lambda seconds: repeat(0, int(seconds * F))
sin_f       = lambda i, hz: math.sin(i * 2 * math.pi * hz / F)
get_wave    = lambda hz, seconds: (sin_f(i, hz) for i in range(int(seconds * F)))
get_hz      = lambda key: 8.176 * 2 ** (int(key) / 12)
parse_note  = lambda note: (get_hz(note[:2]), 0.25 if '♪' in note else 0.125)
get_samples = lambda note: get_wave(*parse_note(note)) if note else get_pause(0.125)
samples_f   = chain.from_iterable(get_samples(n) for n in f'{P1}{P1}{P2}'.split(','))
samples_b   = b''.join(struct.pack('<h', int(f * 30000)) for f in samples_f)
simpleaudio.play_buffer(samples_b, 1, 2, F)
```


Basic Script Template
---------------------
```python
#!/usr/bin/env python3
#
# Usage: .py
#

from collections import namedtuple
from dataclasses import make_dataclass
from enum import Enum
from sys import argv
import re


def main():
    pass


###
##  UTIL
#

def read_file(filename):
    with open(filename, encoding='utf-8') as file:
        return file.readlines()


if __name__ == '__main__':
    main()

```