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)**__,__ **[`Dict`](#dictionary)**__,__ **[`Set`](#set)**__,__ **[`Range`](#range)**__,__ **[`Enumerate`](#enumerate)**__,__ **[`Namedtuple`](#named-tuple)**__,__ **[`Iterator`](#iterator)**__,__ **[`Generator`](#generator)**__.__  
**&nbsp;&nbsp;&nbsp;** **2. Types:** **&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;**  **[`Type`](#type)**__,__ **[`String`](#string)**__,__ **[`Regex`](#regex)**__,__ **[`Format`](#format)**__,__ **[`Numbers`](#numbers)**__,__ **[`Combinatorics`](#combinatorics)**__,__ **[`Datetime`](#datetime)**__.__  
**&nbsp;&nbsp;&nbsp;** **3. Syntax:** **&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;**  **[`Arguments`](#arguments)**__,__ **[`Splat`](#splat-operator)**__,__ **[`Inline`](#inline)**__,__ **[`Closure`](#closure)**__,__ **[`Decorator`](#decorator)**__,__ **[`Class`](#class)**__,__ **[`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`](#command-execution)**__.__  
**&nbsp;&nbsp;&nbsp;** **5. Data:** **&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;**  **[`CSV`](#csv)**__,__ **[`JSON`](#json)**__,__ **[`Pickle`](#pickle)**__,__ **[`SQLite`](#sqlite)**__,__ **[`Bytes`](#bytes)**__,__ **[`Struct`](#struct)**__,__ **[`Array`](#array)**__,__ **[`MemoryView`](#memory-view)**__,__ **[`Deque`](#deque)**__.__  
**&nbsp;&nbsp;&nbsp;** **6. Advanced:** **&nbsp;&nbsp;&nbsp;**  **[`Threading`](#threading)**__,__ **[`Introspection`](#introspection)**__,__ **[`Metaprograming`](#metaprograming)**__,__ **[`Operator`](#operator)**__,__ **[`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`](#profile)**__,__  
**&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)**__,__ **[`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
index = <list>.index(<el>)     # Returns first index of item.
<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.
```


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.
```

```python
value  = <dict>.get(key, default=None)          # Returns default if key does not exist.
value  = <dict>.setdefault(key, default=None)   # Same, but also adds default to dict.
<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>.update(<dict>)                           # Or: dict_a = {**dict_a, **dict_b}.
<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
value = <dict>.pop(key)                         # Removes item from dictionary.
{k: v for k, v in <dict>.items() if k in keys}  # Filters dictionary by keys.
```

### Counter
```python
>>> from collections import Counter
>>> colors = ['red', 'blue', 'yellow', 'blue', 'red', 'blue']
>>> counter = Counter(colors)
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
<set>.remove(<el>)                            # Raises KeyError.
<set>.discard(<el>)                           # Doesn't raise an error.
```

### Frozenset
#### Is hashable, meaning it can be used as a key in a dictionary or as an element in a set.
```python
<frozenset> = frozenset(<collection>)
```


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]):
    ...
```


Named Tuple
-----------
* **Tuple is an immutable and hashable list.**
* **Named tuple is its 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')
```


Iterator
--------
```python
from itertools import count, repeat, cycle, chain, islice
```

```python
<iter> = iter(<collection>)
<iter> = iter(<function>, to_exclusive)     # Sequence of return values until 'to_exclusive'.
<el>   = next(<iter> [, default])           # Raises StopIteration or returns 'default' on end.
```

```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 indefinitely.
```

```python
<iter> = chain(<coll.>, <coll.>, ...)       # 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)
<iter> = islice(<collection>, from_inclusive, to_exclusive, step_size)
```


Generator
---------
**Convenient way to implement the iterator protocol.**

```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 builtin 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 numbers import Integral, Rational, Real, Complex, Number
from collections.abc import Sequence, Collection, Iterable
```

```python
>>> isinstance(123, Number)
True
>>> isinstance([1, 2, 3], Iterable)
True
```


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 any whitespace character.
<list> = <str>.split(sep=None, maxsplit=-1)  # Splits on 'sep' str at most 'maxsplit' times.
<str>  = <str>.join(<collection>)            # Joins elements using string as separator.
```

```python
<str>  = <str>.replace(old, new [, count])   # Replaces 'old' with 'new' at most 'count' times.
<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>.index(<sub_str>)              # Returns start index of first match.
```

```python
<bool> = <str>.isnumeric()                   # True if str contains only numeric characters.
<list> = textwrap.wrap(<str>, width)         # Nicely breaks string into lines.
```

### Char
```python
<str> = chr(<int>)  # Converts int to unicode char.
<int> = ord(<str>)  # Converts unicode char to int.
```

```python
>>> ord('0'), ord('9')
(48, 57)
>>> ord('A'), ord('Z')
(65, 90)
>>> ord('a'), ord('z')
(97, 122)
```


Regex
-----
```python
import re
<str>   = re.sub(<regex>, new, text, count=0)  # Substitutes all occurrences.
<list>  = re.findall(<regex>, text)            # Returns all occurrences.
<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 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.
```

* **Parameter `'flags=re.IGNORECASE'` can be used with all functions.**
* **Parameter `'flags=re.DOTALL'` makes dot also accept newline.**
* **Use `r'\1'` or `'\\1'` for backreference.**
* **Use `'?'` to make an operator non-greedy.**

### Match Object
```python
<str>   = <Match>.group()   # Whole match.
<str>   = <Match>.group(1)  # Part in first bracket.
<tuple> = <Match>.groups()  # All bracketed parts.
<int>   = <Match>.start()   # Start index of a match.
<int>   = <Match>.end()     # Exclusive end index of a match.
```

### Special Sequences
**Expressions below hold true for strings that contain only ASCII characters. Use capital letters for negation.**
```python
'\d' == '[0-9]'             # Digit
'\s' == '[ \t\n\r\f\v]'     # Whitespace
'\w' == '[a-zA-Z0-9_]'      # Alphanumeric
```


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

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

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

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

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

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


Numbers
-------
### Basic Functions
```python
<num>  = pow(<num>, <num>)  # Or: <num> ** <num>
<real> = abs(<num>)
<int>  = round(<real>)
<real> = round(<real>, ±ndigits)
```

### 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>)
```


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 system's timezone.**

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

### 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.**

### 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(<tz>)                     # Aware datetime from current tz time.
```

### Timezone
```python
<tz>     = UTC                              # UTC timezone. London without DST.
<tz>     = tzlocal()                        # Local timezone.
<tz>     = gettz('<Cont.>/<City>')          # Timezone from 'Continent/City_Name' str.
```

```python
<DTa>    = <DT>.astimezone(<tz>)            # Datetime, converted to passed timezone.
<Ta/DTa> = <T/DT>.replace(tzinfo=<tz>)      # Unconverted object with new timezone.
```

### Encode
```python
<D/T/DT> = D/T/DT.fromisoformat('<iso>')    # Object from ISO string.
<DT>     = DT.strptime(<str>, '<format>')   # Datetime from str, according to format.
<D/DTn>  = D/DT.fromordinal(<int>)          # D/DTn from days since Christ.
<DTa>    = DT.fromtimestamp(<real>, <tz>)   # DTa from seconds since Epoch in tz time.
```
* **ISO strings come in following forms: `'YYYY-MM-DD'`, `'HH:MM:SS.ffffff[±<offset>]'`, or both separated by `'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()             # ISO string representation.
<str>    = <D/T/DT>.strftime('<format>')    # Custom string representation.
<int>    = <D/DT>.toordinal()               # Days since Christ, ignoring time and tz.
<float>  = <DT>.timestamp()                 # Seconds since Epoch in local time or tz.
```

### 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"
```

#### Rest of the codes:
* **`'a'` - Weekday, abbreviated name.**
* **`'b'` - Month, abbreviated name.**


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(*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)
```

```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)
<int>  = 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
<expression_if_true> if <condition> else <expression_if_false>
```

```python
>>> [a if a else 'zero' for a in (0, 1, 0, 3)]
['zero', 1, 'zero', 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')
Cutlery   = Enum('Cutlery', {'fork': 1, 'knife': 2, 'spoon': 3})
```

```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
```

### 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 metadata of function add() to function 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)
```

* **Recursion depth is limited 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__
```

### 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'>]
```

### 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 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)
```

### Collection
* **Methods do not depend on each other, so they can be skipped if not needed.**
* **Any object with defined getitem() is considered iterable, even if it lacks iter().**
```python
class MyCollection:
    def __init__(self, a):
        self.a = a
    def __len__(self):
        return len(self.a)
    def __getitem__(self, i):
        return self.a[i]
    def __setitem__(self, i, el):
        self.a[i] = el
    def __contains__(self, el):
        return el in self.a
    def __iter__(self):
        for el in self.a:
            yield el
```

### 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
```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!
```


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()

    @classmethod
    def get_member_names(cls):
        return [a.name for a in cls.__members__.values()]
```

```python
<member> = <enum>.<member_name>
<member> = <enum>['<member_name>']
<member> = <enum>(<value>)
name     = <member>.name
value    = <member>.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>))
```

### 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)})
```


Exceptions
----------
```python
while True:
    try:
        x = int(input('Please enter a number: '))
    except ValueError:
        print('Oops!  That was no valid number.  Try again...')
    else:
        print('Thank you.')
        break
```

### Raising Exception
```python
raise ValueError('A very specific message!')
```

### Finally
```python
>>> try:
...     raise KeyboardInterrupt
... finally:
...     print('Goodbye, world!')
Goodbye, world!
Traceback (most recent call last):
  File "<stdin>", line 2, in <module>
KeyboardInterrupt
```


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

* **Use `'file=sys.stderr'` for errors.**

### Pretty Print
```python
>>> from pprint import pprint
>>> pprint(dir())
['__annotations__',
 '__builtins__',
 '__doc__', ...]
```


Input
-----
* **Reads a line from user input or pipe if present.**
* **Trailing newline gets stripped.**
* **Prompt string is printed to the standard output before reading input.**

```python
<str> = input(prompt=None)
```

#### Prints lines until EOF:
```python
while True:
    try:
        print(input())
    except EOFError:
        break
```


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)                    # Argument
p.add_argument('<name>', type=<type>, nargs='+')                  # Arguments
args  = p.parse_args()
value = args.<name>
```

* **Use `'help=<str>'` for argument description.**
* **Use `'type=FileType(<mode>)'` for files.**


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

```python
<file> = open('<path>', mode='r', encoding=None)
```

### 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.**

### File
```python
<file>.seek(0)                      # Moves to the start of the file.
<file>.seek(offset)                 # Moves 'offset' chars/bytes from the start.
<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.
<list>      = <file>.readlines()    # Returns a list of 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(<list>)           # Writes a list of strings or bytes objects.
<file>.flush()                      # Flushes write buffer.
```
* **Methods do not add or strip trailing newlines.**

### 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 path, listdir
<bool> = path.exists('<path>')
<bool> = path.isfile('<path>')
<bool> = path.isdir('<path>')
<list> = listdir('<path>')
```

```python
>>> from glob import glob
>>> glob('../*.gif')
['1.gif', 'card.gif']
```

### Pathlib
```python
from pathlib import Path
cwd    = Path()
<Path> = Path('<path>' [, '<path>', <Path>, ...])
<Path> = <Path> / '<dir>' / '<file>'
```

```python
<bool> = <Path>.exists()
<bool> = <Path>.is_file()
<bool> = <Path>.is_dir()
<iter> = <Path>.iterdir()
```

```python
<iter> = <Path>.glob('<pattern>')
```

```python
<str>  = str(<Path>)               # Returns path as a string.
<tup.> = <Path>.parts              # Returns all components as strings.
<Path> = <Path>.resolve()          # Returns absolute Path without symlinks.
```

```python
<str>  = <Path>.name               # Final component.
<str>  = <Path>.stem               # Final component without extension.
<str>  = <Path>.suffix             # Final component's extension.
<Path> = <Path>.parent             # Path without final component.
```


Command Execution
-----------------
```python
import os
<str> = os.popen(<command>).read()
```

### 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
```


CSV
---
```python
import csv
```

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

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


JSON
----
```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
------
```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)
```


SQLite
------
```python
import sqlite3
db = sqlite3.connect('<path>')
...
db.close()
```

### Read
```python
cursor = db.execute('<query>')
if cursor:
    <tuple> = cursor.fetchone()  # First row.
    <list>  = cursor.fetchall()  # Remaining rows.
```

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


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

```python
<bytes> = b'<str>'
<int>   = <bytes>[<index>]
<bytes> = <bytes>[<slice>]
<ints>  = list(<bytes>)
<bytes> = b''.join(<coll_of_bytes>)
```

### Encode
```python
<bytes> = <str>.encode(encoding='utf-8')
<bytes> = <int>.to_bytes(<length>, byteorder='big|little', signed=False)
<bytes> = bytes.fromhex('<hex>')
```

### Decode
```python
<str>   = <bytes>.decode(encoding='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 Python values and a C struct, represented as a Python bytes object.**
* **Machine’s native type sizes and byte order are used by default.**

```python
from struct import pack, unpack, iter_unpack, calcsize
<bytes>  = pack('<format>', <value_1> [, <value_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)
>>> calcsize('>hhl')
8
```

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

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


Array
-----
**List that can hold only elements of predefined type. Available types are listed above.**

```python
from array import array
<array> = array('<typecode>' [, <collection>])
```


Memory View
-----------
**Used for accessing the internal data of an object that supports the buffer protocol.**

```python
<memoryview> = memoryview(<bytes> / <bytearray> / <array>)
<memoryview>.release()
```


Deque
-----
**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>)
<el> = <deque>.popleft()
```

```python
<deque>.extendleft(<collection>)  # Collection gets reversed.
<deque>.rotate(n=1)               # Rotates elements to the right.
```


Threading
---------
```python
from threading import Thread, RLock
```

### Thread
```python
thread = Thread(target=<function>, args=(<first_arg>, ))
thread.start()
...
thread.join()
```

### Lock
```python
lock = RLock()
lock.acquire()
...
lock.release()
```

#### Or:
```python
with lock:
    ...
```


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

### Variables
```python
<list> = dir()      # Names of variables in current scope.
<dict> = locals()   # Dict of local variables. Also vars().
<dict> = globals()  # 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 the 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 class.**

```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 class of returned instance (**`'MyMetaClass'` **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)`**), in which case init() is not called.**

### Metaclass Attribute
**When class is created it checks if it has metaclass 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 (str is an instance of type, ...):
```text
+---------+-------------+
| classes | metaclasses |
+---------+-------------|
| MyClass > MyMetaClass |
|         |     v       |
|  object ---> type <+  |
|         |    ^ +---+  |
|   str -------+        |
+---------+-------------+
```

#### Inheritance diagram (str is a subclass of object, ...):
```text
+---------+-------------+
| classes | metaclasses |
+---------+-------------|
| MyClass | MyMetaClass |
|    v    |     v       |
|  object <--- type     |
|    ^    |             |
|   str   |             |
+---------+-------------+
```


Operator
--------
```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 not_, and_, or_
from operator import itemgetter, attrgetter, methodcaller
```

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


Eval
----
### Basic
```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
```

### Using Abstract Syntax Trees
```python
import ast
from ast import Num, BinOp, UnaryOp
import operator as op

LEGAL_OPERATORS = {ast.Add:    op.add,      # <el> + <el>
                   ast.Sub:    op.sub,      # <el> - <el>
                   ast.Mult:   op.mul,      # <el> * <el>
                   ast.Div:    op.truediv,  # <el> / <el>
                   ast.Pow:    op.pow,      # <el> ** <el>
                   ast.BitXor: op.xor,      # <el> ^ <el>
                   ast.USub:   op.neg}      # - <el>

def evaluate(expression):
    root = ast.parse(expression, mode='eval')
    return eval_node(root.body)

def eval_node(node):
    node_type = type(node)
    if node_type == Num:
        return node.n
    if node_type not in [BinOp, UnaryOp]:
        raise TypeError(node)
    operator_type = type(node.op)
    if operator_type not in LEGAL_OPERATORS:
        raise TypeError(f'Illegal operator {node.op}')
    operator = LEGAL_OPERATORS[operator_type]
    if node_type == BinOp:
        left, right = eval_node(node.left), eval_node(node.right)
        return operator(left, right)
    elif node_type == UnaryOp:
        operand = eval_node(node.operand)
        return operator(operand)
```

```python
>>> evaluate('2 ^ 6')
4
>>> evaluate('2 ** 6')
64
>>> evaluate('1 + 2 * 3 ** (4 ^ 5) / (6 + -7)')
-5.0
```


Coroutine
---------
* **Similar to generator, but generator pulls data through the pipe with iteration, while coroutine pushes data into the pipeline with send().**
* **Coroutines provide more powerful data routing possibilities than iterators.**
* **If you build a collection of simple data processing components, you can glue them together into complex arrangements of pipes, branches, merging, etc.**

### Helper Decorator
* **All coroutines must be "primed" by first calling next().**
* **Remembering to call next() is easy to forget.**
* **Solved by wrapping coroutines with a 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)

reader(adder(printer()))  # 100, 101, ..., 109
```

<br><br>

Libraries
=========

Progress Bar
------------
```python
# $ pip3 install tqdm
from tqdm import tqdm
from time import sleep
for i in tqdm([1, 2, 3]):
    sleep(0.2)
for i in tqdm(range(100)):
    sleep(0.02)
```


Plot
----
```python
# $ pip3 install matplotlib
from matplotlib import pyplot
pyplot.plot(<data_1> [, <data_2>, ...])
pyplot.savefig(<filename>)
pyplot.show()
```


Table
-----
#### Prints a CSV file as an ASCII table:
```python
# $ pip3 install tabulate
from tabulate import tabulate
import csv
with open(<filename>, encoding='utf-8') as file:
    lines   = csv.reader(file, delimiter=';')
    headers = [header.title() for header in next(lines)]
    table   = tabulate(lines, headers)
    print(table)
```


Curses
------
```python
from curses import wrapper, ascii

def main():
    wrapper(draw)

def draw(screen):
    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', 'y x')
    height, width = screen.getmaxyx()
    return P(height-1, width-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
**Error description, stack trace and values of variables are appended automatically.**

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

### Rotation
**Parameter 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 are 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
--------
```python
# $ pip3 install requests beautifulsoup4
>>> import requests
>>> from bs4 import BeautifulSoup
>>> url   = 'https://en.wikipedia.org/wiki/Python_(programming_language)'
>>> page  = requests.get(url)
>>> doc   = BeautifulSoup(page.text, '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]
>>> link, ver
('https://www.python.org/', '3.7.2')
```

### Selenium
**Library for scraping dynamically generated web content.**

```python
# $ brew cask install chromedriver
# $ pip3 install selenium
>>> from selenium import webdriver
>>> driver = webdriver.Chrome()
>>> driver.get(url)
>>> xpath  = '//*[@id="mw-content-text"]/div/table[1]/tbody/tr[7]/td/div'
>>> driver.find_element_by_xpath(xpath).text.split()[0]
'3.7.2'
```


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

### Run
```python
run(host='localhost', port=8080)
run(host='0.0.0.0', port=80, server='cherrypy')
```

### Static Request
```python
@route('/img/<image>')
def send_image(image):
    return static_file(image, 'images/', 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]
```


Profile
-------
### Basic
```python
from time import time
start_time = time()  # Seconds since Epoch.
...
duration = time() - start_time
```

### High Performance
```python
from time import perf_counter as pc
start_time = pc()    # Seconds since restart.
...
duration = pc() - 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
```

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

#### Usage:
```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)}
```

### 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(output=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
```

#### Creates a PNG image of a rainbow gradient:
```python
width  = 100
height = 100
size   = width * height
pixels = [255 * i/size for i in range(size)]

img = Image.new('HSV', (width, height))
img.putdata([(int(a), 255, 255) for a in pixels])
img.convert(mode='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(mode='HSV')
img.putdata([(add_noise(h), s, v) for h, s, v in img.getdata()])
img.convert(mode='RGB').save('test.png')
```

### 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.**


Audio
-----
```python
import wave
from struct import pack, iter_unpack
```

### Read Frames from WAV File
```python
def read_wav_file(filename):
    with wave.open(filename, 'rb') as wf:
        frames = wf.readframes(wf.getnframes())
        return [a[0] for a in iter_unpack('<h', frames)]
```

### Write Frames to WAV File
```python
def write_to_wav_file(filename, frames_int, mono=True):
    frames_short = (pack('<h', a) for a in frames_int)
    with wave.open(filename, 'wb') as wf:
        wf.setnchannels(1 if mono else 2)
        wf.setsampwidth(2)
        wf.setframerate(44100)
        wf.writeframes(b''.join(frames_short))
```

### Examples
#### Saves a sine wave to a mono WAV file:
```python
from math import pi, sin
frames_f = (sin(i * 2 * pi * 440 / 44100) for i in range(100000))
frames_i = (int(a * 30000) for a in frames_f)
write_to_wav_file('test.wav', frames_i)
```

#### Adds noise to a mono WAV file:
```python
from random import randint
add_noise = lambda value: max(-32768, min(32767, value + randint(-500, 500)))
frames_i  = (add_noise(a) for a in read_wav_file('test.wav'))
write_to_wav_file('test.wav', frames_i)
```

#### Plays Popcorn:
```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_n   = lambda note: (get_hz(note[:2]), 0.25 if '♪' in note else 0.125)
get_note  = lambda note: get_wave(*parse_n(note)) if note else get_pause(0.125)
frames_i  = chain.from_iterable(get_note(n) for n in f'{P1}{P1}{P2}'.split(','))
frames_b  = b''.join(struct.pack('<h', int(a * 30000)) for a in frames_i)
simpleaudio.play_buffer(frames_b, 1, 2, F)
```


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

from collections import namedtuple
from enum import Enum
import re
import sys


def main():
    pass


###
##  UTIL
#

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


if __name__ == '__main__':
    main()

```