Comprehensive Python Cheatsheet
===============================
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Contents
--------
** ** **1. Collections:** ** ** **[`List`](#list)**__,__ **[`Dictionary`](#dictionary)**__,__ **[`Set`](#set)**__,__ **[`Tuple`](#tuple)**__,__ **[`Range`](#range)**__,__ **[`Enumerate`](#enumerate)**__,__ **[`Iterator`](#iterator)**__,__ **[`Generator`](#generator)**__.__
** ** **2. Types:** ** ** **[`Type`](#type)**__,__ **[`String`](#string)**__,__ **[`Regular_Exp`](#regex)**__,__ **[`Format`](#format)**__,__ **[`Numbers`](#numbers)**__,__ **[`Combinatorics`](#combinatorics)**__,__ **[`Datetime`](#datetime)**__.__
** ** **3. Syntax:** ** ** **[`Args`](#arguments)**__,__ **[`Inline`](#inline)**__,__ **[`Closure`](#closure)**__,__ **[`Decorator`](#decorator)**__,__ **[`Class`](#class)**__,__ **[`Duck_Type`](#duck-types)**__,__ **[`Enum`](#enum)**__,__ **[`Exception`](#exceptions)**__.__
** ** **4. System:** ** ** **[`Exit`](#exit)**__,__ **[`Print`](#print)**__,__ **[`Input`](#input)**__,__ **[`Command_Line_Arguments`](#command-line-arguments)**__,__ **[`Open`](#open)**__,__ **[`Path`](#path)**__,__ **[`OS_Commands`](#oscommands)**__.__
** ** **5. Data:** ** ** **[`JSON`](#json)**__,__ **[`Pickle`](#pickle)**__,__ **[`CSV`](#csv)**__,__ **[`SQLite`](#sqlite)**__,__ **[`Bytes`](#bytes)**__,__ **[`Struct`](#struct)**__,__ **[`Array`](#array)**__,__ **[`Memory_View`](#memory-view)**__,__ **[`Deque`](#deque)**__.__
** ** **6. Advanced:** ** ** **[`Threading`](#threading)**__,__ **[`Operator`](#operator)**__,__ **[`Introspection`](#introspection)**__,__ **[`Metaprograming`](#metaprograming)**__,__ **[`Eval`](#eval)**__,__ **[`Coroutines`](#coroutines)**__.__
** ** **7. Libraries:** ** ** **[`Progress_Bar`](#progress-bar)**__,__ **[`Plot`](#plot)**__,__ **[`Table`](#table)**__,__ **[`Curses`](#curses)**__,__ **[`Logging`](#logging)**__,__ **[`Scraping`](#scraping)**__,__ **[`Web`](#web)**__,__ **[`Profile`](#profiling)**__,__
** ** **[`NumPy`](#numpy)**__,__ **[`Image`](#image)**__,__ **[`Audio`](#audio)**__,__ **[`Pygame`](#pygame)**__.__
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, el: out * el, <collection>)
list_of_chars = list(<str>)
```
* **Module [operator](#operator) provides functions itemgetter() and mul() that offer the same functionality as [lambda](#lambda) expressions above.**
```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 a dictionary, filtered by keys.
```
### Counter
```python
>>> from collections import Counter
>>> colors = ['blue', 'blue', 'blue', 'red', 'red']
>>> 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 updated value endlessly. Accepts floats.
<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
```
### Abstract Base Classes
**Each abstract base class specifies a set of virtual subclasses. These classes are then recognized by isinstance() and issubclass() as subclasses of the ABC, although they are really not.**
```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 \n,\r,\r\n. 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
+---------------+----------+----------+----------+----------+----------+
| | [ !#$%…] | [a-zA-Z] | [¼½¾] | [²³¹] | [0-9] |
+---------------+----------+----------+----------+----------+----------+
| isprintable() | yes | yes | yes | yes | yes |
| isalnum() | | yes | yes | yes | yes |
| isnumeric() | | | yes | yes | yes |
| isdigit() | | | | yes | yes |
| isdecimal() | | | | | yes |
+---------------+----------+----------+----------+----------+----------+
```
* **Also: `'isspace()'` checks for `'[ \t\n\r\f\v…]'`.**
Regex
-----
```python
import re
<str> = re.sub(<regex>, new, text, count=0) # Substitutes all occurrences with 'new'.
<list> = re.findall(<regex>, text) # Returns all occurrences as strings.
<list> = re.split(<regex>, text, maxsplit=0) # Use brackets in regex to include the matches.
<Match> = re.search(<regex>, text) # Searches for first occurrence of the pattern.
<Match> = re.match(<regex>, text) # Searches only at the beginning of the text.
<iter> = re.finditer(<regex>, text) # Returns all occurrences as match objects.
```
* **Search() and match() return None if they can't find a match.**
* **Argument `'flags=re.IGNORECASE'` can be used with all functions.**
* **Argument `'flags=re.MULTILINE'` makes `'^'` and `'$'` match the start/end of each line.**
* **Argument `'flags=re.DOTALL'` makes dot also accept the `'\n'`.**
* **Use `r'\1'` or `'\\1'` for backreference.**
* **Add `'?'` after an operator to make it non-greedy.**
### Match Object
```python
<str> = <Match>.group() # Returns the whole match. Also group(0).
<str> = <Match>.group(1) # Returns part in the first bracket.
<tuple> = <Match>.groups() # Returns all bracketed parts.
<int> = <Match>.start() # Returns start index of the match.
<int> = <Match>.end() # Returns exclusive end index of the match.
```
### Special Sequences
* **By default digits, alphanumerics and whitespaces from all alphabets are matched, unless `'flags=re.ASCII'` argument is used.**
* **Use a capital letter for negation.**
```python
'\d' == '[0-9]' # Matches any digit.
'\w' == '[a-zA-Z0-9_]' # Matches any alphanumeric.
'\s' == '[ \t\n\r\f\v]' # Matches any whitespace.
```
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>'
{<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':10.3} # 'abc '
{'abcde':.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 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%' |
+---------------+-----------------+-----------------+-----------------+-----------------+
```
### Integers
```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, isinf, isnan
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: 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 first!**
```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 objects 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 the second pass in case of time jumping back for one hour.**
* **`'<DTa> = resolve_imaginary(<DTa>)'` fixes DTs that fall into the 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('<Continent>/<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 an arbitrary character. Offset is formatted as: `'HH:MM'`.**
* **Epoch on Unix systems 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 tz.
<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> # Returned datetime can fall into missing hour.
<TD> = <D/DTn> - <D/DTn> # Returns the difference, ignoring time jumps.
<TD> = <DTa> - <DTa> # Ignores time jumps if they share tzinfo object.
<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>)'`.**
### Non-Local
**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 = 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 has 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>'` as 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 optionally return an object.**
* **Exit() should release the resources.**
* **Any exception that happens inside the with block is passed to the exit() method.**
* **If it wishes to suppress the exception it must return a true value.**
```python
class MyOpen:
def __init__(self, filename):
self.filename = filename
def __enter__(self):
self.file = open(self.filename)
return self.file
def __exit__(self, exc_type, exception, traceback):
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):
return iter(self.a)
def __contains__(self, el):
return el in self.a
```
```python
>>> obj = MyIterable([1, 2, 3])
>>> [el for el in obj]
[1, 2, 3]
>>> 1 in obj
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)
```
### 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, abc.Sequence)'` would return False even if MySequence had all the methods defined.**
```python
from collections import abc
class MyAbcSequence(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})
```
#### User-defined functions cannot 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 built-in functions and\_() and or\_() from the 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.**
* **Use `'traceback.print_exc()'` to print the error message.**
### Raising Exceptions
```python
raise <exception>
raise <exception>()
raise <exception>(<el> [, ...])
```
#### Re-raising caught exception:
```python
except <exception> as <name>:
...
raise
```
### Exception Object
```python
arguments = <name>.args
exc_type = <name>.__class__
filename = <name>.__traceback__.tb_frame.f_code.co_filename
func_name = <name>.__traceback__.tb_frame.f_code.co_name
line = linecache.getline(filename, <name>.__traceback__.tb_lineno)
error_msg = traceback.format_exception(exc_type, <name>, <name>.__traceback__)
```
### 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.
+-- 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 collection fails.
| +-- IndexError # Raised when a sequence index is out of range.
| +-- KeyError # Raised when a dictionary key or set element 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 into other categories.
| +-- RecursionError # Raised when the maximum recursion depth is exceeded.
+-- StopIteration # Raised by next() when run on an empty iterator.
+-- 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 to/from bytes fails.
```
#### Collections and their exceptions:
```text
+-----------+------------+------------+------------+
| | list | dict | set |
+-----------+------------+------------+------------+
| getitem() | IndexError | KeyError | |
| pop() | IndexError | KeyError | KeyError |
| remove() | ValueError | | KeyError |
| index() | ValueError | | |
+-----------+------------+------------+------------+
```
#### Useful built-in exceptions:
```python
raise TypeError('Argument is of wrong type!')
raise ValueError('Argument is of right type but inappropriate value!')
raise RuntimeError('None of above!')
```
### User-defined Exceptions
```python
class MyError(Exception):
pass
class MyInputError(MyError):
pass
```
Exit
----
**Exits the interpreter by raising SystemExit exception.**
```python
import sys
sys.exit() # Exits with exit code 0 (success).
sys.exit(<el>) # Prints to stderr and exits with 1.
sys.exit(<int>) # Exits with passed exit code.
```
Print
-----
```python
print(<el_1>, ..., sep=' ', end='\n', file=sys.stdout, flush=False)
```
* **Use `'file=sys.stderr'` for messages about errors.**
* **Use `'flush=True'` to forcibly flush the stream.**
### Pretty Print
```python
from pprint import pprint
pprint(<collection>, width=80, depth=None, compact=False, sort_dicts=True)
```
* **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 (ctrl-d/z) or input stream gets exhausted.**
Command Line Arguments
----------------------
```python
import sys
script_name = sys.argv[0]
arguments = sys.argv[1:]
```
### Argument Parser
```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 raised when reading with `'r'` or `'r+'`.**
* **`'FileExistsError'` can be raised when writing with `'x'`.**
* **`'IsADirectoryError'` and `'PermissionError'` can be raised 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 position, 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/bytes 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
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>) # Returns absolute path.
```
```python
<str> = path.basename(<path>) # Returns final component of the path.
<str> = path.dirname(<path>) # Returns path without the final component.
<tup.> = path.splitext(<path>) # Splits on last period of the 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
from os import scandir
```
```python
<iter> = scandir(path='.') # Returns DirEntry objects located at path.
<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 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 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 the current working directory.
os.mkdir(<path>, mode=0o777) # Creates a directory. Mode is in octal.
```
```python
shutil.copy(from, to) # Copies the file. 'to' can exist or be a dir.
shutil.copytree(from, to) # Copies the directory. 'to' must not exist.
```
```python
os.rename(from, to) # Renames/moves 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 the empty directory.
shutil.rmtree(<path>) # Deletes the directory.
```
### Shell Commands
```python
import os
<str> = os.popen('<shell_command>').read()
```
#### Sends '1 + 1' to the basic calculator and captures its output:
```python
>>> from subprocess import run
>>> run('bc', input='1 + 1\n', capture_output=True, encoding='utf-8')
CompletedProcess(args='bc', returncode=0, stdout='2\n', stderr='')
```
#### Sends test.in to the basic calculator running in standard mode and saves its output to test.out:
```python
>>> from shlex import split
>>> os.popen('echo 1 + 1 > test.in')
>>> run(split('bc -s'), stdin=open('test.in'), stdout=open('test.out', 'w'))
CompletedProcess(args=['bc', '-s'], returncode=0)
>>> open('test.out').read()
'2\n'
```
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 '\r' will be added in front of every '\n' on platforms that use '\r\n' line endings!**
### 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'` - Specifies how writer terminates rows.**
* **`'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 a 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 a subclass of sqlite3.Error.
<tuple> = <cursor>.fetchone() # Returns next row. Also next(<cursor>).
<list> = <cursor>.fetchall() # Returns remaining rows. Also list(<cursor>).
```
### 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>') # Can raise a subclass of connector.Error.
<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 type sizes start format string with:
* **`'='` - native byte order**
* **`'<'` - little-endian**
* **`'>'` - big-endian (also `'!'`)**
#### Integer types. Use a 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 collection of numbers.
<array> = array('<typecode>', <bytes>) # Array from bytes object.
<array> = array('<typecode>', <array>) # Treats array as a sequence of numbers.
<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>) # Writes 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.
<array> = array('<typecode>', <mview>) # Treats mview as a sequence of numbers.
<list> = list(<mview>) # Returns list of ints or floats.
<str> = str(<mview>, 'utf-8') # Treats mview as a bytes object.
<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.
<deque>.extendleft(<collection>) # Collection gets reversed.
<el> = <deque>.popleft() # Raises IndexError if empty.
<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 at least one of the threads contains an I/O operation.**
```python
from threading import Thread, RLock, Semaphore, Event, Barrier
```
### 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:
...
```
### Semaphore, Event, Barrier
```python
<Semaphore> = Semaphore(value=1) # Lock that can be acquired 'value' times.
<Event> = Event() # Method wait() blocks until set() is called.
<Barrier> = Barrier(n_times) # Method wait() blocks until it's called 'n_times'.
```
### Thread Pool Executor
```python
from concurrent.futures import ThreadPoolExecutor
with ThreadPoolExecutor(max_workers=None) as executor: # Does not exit until done.
<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>, ...]) # Also visible outside block.
```
#### 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
elementwise_sum = map(op.add, list_a, list_b)
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() # Names of local variables (incl. functions).
<dict> = vars() # Dict of local variables. Also locals().
<dict> = globals() # Dict of global variables.
```
### Attributes
```python
<list> = dir(<object>) # Names of object's attributes (incl. methods).
<dict> = vars(<object>) # Dict of object's fields. Also <obj>.__dict__.
```
```python
<bool> = hasattr(<object>, '<attr_name>')
value = getattr(<object>, '<attr_name>')
setattr(<object>, '<attr_name>', value)
delattr(<object>, '<attr_name>')
```
### Parameters
```python
from inspect import signature
<sig> = signature(<function>)
no_of_params = len(<sig>.parameters)
param_names = list(<sig>.parameters.keys())
param_kinds = [a.kind for a in <sig>.parameters.values()]
```
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
**A 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 the '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
```
Coroutines
----------
* **Coroutines have a lot in common with threads, but unlike threads, they only give up control when they call another coroutine and they don’t use as much memory.**
* **Coroutine definition starts with `'async'` and its call with `'await'`.**
* **`'asyncio.run(<coroutine>)'` is the main entry point for asynchronous programs.**
* **Functions wait(), gather() and as_completed() can be used when multiple coroutines need to be started at the same time.**
* **Asyncio module also provides its own [Queue](#queue), [Event](#semaphore-event-barrier), [Lock](#lock) and [Semaphore](#semaphore-event-barrier) classes.**
#### Runs a terminal game where you control an asterisk that must avoid numbers:
```python
import asyncio, collections, curses, enum, random
P = collections.namedtuple('P', 'x y') # Position
D = enum.Enum('D', 'n e s w') # Direction
def main(screen):
curses.curs_set(0) # Makes cursor invisible.
screen.nodelay(True) # Makes getch() non-blocking.
asyncio.run(main_coroutine(screen)) # Starts running asyncio code.
async def main_coroutine(screen):
state = {'*': P(0, 0), **{id_: P(30, 10) for id_ in range(10)}}
moves = asyncio.Queue()
coros = [*[random_controller(id_, moves) for id_ in range(10)],
human_controller(screen, moves),
model(moves, state, *screen.getmaxyx()),
view(state, screen)]
await asyncio.wait(coros, return_when=asyncio.FIRST_COMPLETED)
async def random_controller(id_, moves):
while True:
moves.put_nowait((id_, random.choice(list(D))))
await asyncio.sleep(random.random() / 2)
async def human_controller(screen, moves):
while True:
ch = screen.getch()
key_mappings = {259: D.n, 261: D.e, 258: D.s, 260: D.w}
if ch in key_mappings:
moves.put_nowait(('*', key_mappings[ch]))
await asyncio.sleep(0.01)
async def model(moves, state, height, width):
while state['*'] not in {p for id_, p in state.items() if id_ != '*'}:
id_, d = await moves.get()
p = state[id_]
deltas = {D.n: P(0, -1), D.e: P(1, 0), D.s: P(0, 1), D.w: P(-1, 0)}
new_p = P(*[sum(a) for a in zip(p, deltas[d])])
if 0 <= new_p.x < width-1 and 0 <= new_p.y < height:
state[id_] = new_p
async def view(state, screen):
while True:
screen.clear()
for id_, p in state.items():
screen.addstr(p.y, p.x, str(id_))
await asyncio.sleep(0.01)
curses.wrapper(main)
```
<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('<path>') # 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 the 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. It can run up to one hundred times faster than the 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 the index of a dimension that gets collapsed. The 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, raise 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 the 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 the image.
```
```bash
<2d_array> = np.array(<Image>) # Creates NumPy array from greyscale image.
<3d_array> = np.array(<Image>) # Creates NumPy array from color image.
<Image> = Image.fromarray(<array>) # Creates image from NumPy array of floats.
```
### 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+R*2), 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') # Opens the WAV file.
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') # Truncates existing file.
<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 the file.
```
* **Bytes object contains a sequence of frames, each consisting of one or more samples.**
* **In a stereo signal, the 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
from simpleaudio import play_buffer
with wave.open('test.wav', 'rb') as file:
p = file.getparams()
frames = file.readframes(p.nframes)
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)
```
Pygame
------
### Basic Example
```python
# $ pip3 install pygame
import pygame as pg
pg.init()
screen = pg.display.set_mode((500, 500))
rect = pg.Rect(240, 240, 20, 20)
while all(event.type != pg.QUIT for event in pg.event.get()):
deltas = {pg.K_UP: (0, -3), pg.K_RIGHT: (3, 0), pg.K_DOWN: (0, 3), pg.K_LEFT: (-3, 0)}
for delta in (deltas.get(i) for i, on in enumerate(pg.key.get_pressed()) if on):
rect = rect.move(delta) if delta else rect
screen.fill((0, 0, 0))
pg.draw.rect(screen, (255, 255, 255), rect)
pg.display.flip()
```
### Rectangle
**Object for storing rectangular coordinates.**
```python
<Rect> = pg.Rect(x, y, width, height)
<int> = <Rect>.x/y/centerx/centery/…
<tup.> = <Rect>.topleft/center/…
<Rect> = <Rect>.move((x, y))
```
```python
<bool> = <Rect>.collidepoint((x, y)) # Tests if a point is inside a rectangle.
<bool> = <Rect>.colliderect(<Rect>) # Tests if two rectangles overlap.
<int> = <Rect>.collidelist(<list_of_Rect>) # Returns index of first colliding Rect or -1.
<list> = <Rect>.collidelistall(<list_of_Rect>) # Returns indices of all colliding Rects.
```
### Surface
**Object for representing images.**
```python
<Surf> = pg.display.set_mode((width, height)) # Returns the display surface.
<Surf> = pg.Surface((width, height)) # Creates a new surface.
<Surf> = pg.image.load('<path>') # Loads the image.
<Surf> = <Surf>.subsurface(<Rect>) # Returns a subsurface.
```
```python
<Surf>.fill(color) # Fills the whole surface.
<Surf>.set_at((x, y), color) # Updates pixel.
<Surf>.blit(<Surface>, (x, y)) # Draws passed surface to the surface.
```
```python
<Surf> = pg.transform.flip(<Surf>, xbool, ybool)
<Surf> = pg.transform.rotate(<Surf>, degrees)
<Surf> = pg.transform.scale(<Surf>, (width, height))
```
```python
pg.draw.line(<Surf>, color, (x1, y1), (x2, y2), width)
pg.draw.arc(<Surf>, color, <Rect>, from_radians, to_radians)
pg.draw.rect(<Surf>, color, <Rect>)
pg.draw.polygon(<Surf>, color, points)
pg.draw.ellipse(<Surf>, color, <Rect>)
```
### Font
```python
<Font> = pg.font.SysFont('<name>', size, bold=False, italic=False)
<Font> = pg.font.Font('<path>', size)
<Surf> = <Font>.render(text, antialias, color, background=None)
```
### Sound
```python
<Sound> = pg.mixer.Sound('<path>') # Loads the WAV file.
<Sound>.play() # Starts playing the sound.
```
### Basic Mario Brothers Example
```python
import collections, dataclasses, enum, io, pygame, urllib.request, itertools as it
from random import randint
P = collections.namedtuple('P', 'x y') # Position
D = enum.Enum('D', 'n e s w') # Direction
SIZE, MAX_SPEED = 50, P(5, 10) # Screen size, Speed limit
def main():
def get_screen():
pygame.init()
return pygame.display.set_mode(2 * [SIZE*16])
def get_images():
url = 'https://gto76.github.io/python-cheatsheet/web/mario_bros.png'
img = pygame.image.load(io.BytesIO(urllib.request.urlopen(url).read()))
return [img.subsurface(get_rect(x, 0)) for x in range(img.get_width() // 16)]
def get_mario():
Mario = dataclasses.make_dataclass('Mario', 'rect spd facing_left frame_cycle'.split())
return Mario(get_rect(1, 1), P(0, 0), False, it.cycle(range(3)))
def get_tiles():
positions = [p for p in it.product(range(SIZE), repeat=2) if {*p} & {0, SIZE-1}] + \
[(randint(1, SIZE-2), randint(2, SIZE-2)) for _ in range(SIZE**2 // 10)]
return [get_rect(*p) for p in positions]
def get_rect(x, y):
return pygame.Rect(x*16, y*16, 16, 16)
run(get_screen(), get_images(), get_mario(), get_tiles())
def run(screen, images, mario, tiles):
clock = pygame.time.Clock()
while all(event.type != pygame.QUIT for event in pygame.event.get()):
keys = {pygame.K_UP: D.n, pygame.K_RIGHT: D.e, pygame.K_DOWN: D.s, pygame.K_LEFT: D.w}
pressed = {keys.get(i) for i, on in enumerate(pygame.key.get_pressed()) if on}
update_speed(mario, tiles, pressed)
update_position(mario, tiles)
draw(screen, images, mario, tiles, pressed)
clock.tick(28)
def update_speed(mario, tiles, pressed):
x, y = mario.spd
x += 2 * ((D.e in pressed) - (D.w in pressed))
x -= x / abs(x) if x else 0
y += 1 if D.s not in get_boundaries(mario.rect, tiles) else (-10 if D.n in pressed else 0)
mario.spd = P(*[max(-limit, min(limit, s)) for limit, s in zip(MAX_SPEED, P(x, y))])
def update_position(mario, tiles):
old_p, delta = mario.rect.topleft, P(0, 0)
larger_speed = max(abs(s) for s in mario.spd)
for _ in range(int(larger_speed)):
mario.spd = stop_on_collision(mario.spd, get_boundaries(mario.rect, tiles))
delta = P(*[a + s/larger_speed for a, s in zip(delta, mario.spd)])
mario.rect.topleft = [sum(pair) for pair in zip(old_p, delta)]
def get_boundaries(rect, tiles):
deltas = {D.n: P(0, -1), D.e: P(1, 0), D.s: P(0, 1), D.w: P(-1, 0)}
return {d for d, delta in deltas.items() if rect.move(delta).collidelist(tiles) != -1}
def stop_on_collision(spd, bounds):
return P(x=0 if (D.w in bounds and spd.x < 0) or (D.e in bounds and spd.x > 0) else spd.x,
y=0 if (D.n in bounds and spd.y < 0) or (D.s in bounds and spd.y > 0) else spd.y)
def draw(screen, images, mario, tiles, pressed):
def get_frame_index():
if D.s not in get_boundaries(mario.rect, tiles):
return 4
return next(mario.frame_cycle) if {D.w, D.e} & pressed else 6
screen.fill((85, 168, 255))
mario.facing_left = (D.w in pressed) if {D.e, D.w} & pressed else mario.facing_left
screen.blit(images[get_frame_index() + mario.facing_left * 9], mario.rect)
for rect in tiles:
screen.blit(images[19 if {*rect.topleft} & {0, (SIZE-1)*16} else 18], rect)
pygame.display.flip()
if __name__ == '__main__':
main()
```
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()
```