if __name__ == '__main__':
    main()

<list> = <list>[from_inclusive : to_exclusive : step_size]

<list>.append(<el>)
<list>.extend(<collection>)
<list> += [<el>]
<list> += <collection>

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

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>)

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.   

<view> = <dict>.keys()
<view> = <dict>.values()
<view> = <dict>.items()

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.

<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.

value = <dict>.pop(key)                         # Removes item from dictionary.
{k: v for k, v in <dict>.items() if k in keys}  # Filters dictionary by keys.

>>> 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> = set()

<set>.add(<el>)
<set>.update(<collection>)
<set> |= {<el>}
<set> |= <set>

<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>

<set>.remove(<el>)   # Throws error.
<set>.discard(<el>)  # Doesn't throw error.

<frozenset> = frozenset(<collection>)

<range> = range(to_exclusive)
<range> = range(from_inclusive, to_exclusive)
<range> = range(from_inclusive, to_exclusive, ±step_size)

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

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

>>> 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')

from itertools import count, repeat, cycle, chain, islice

<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.

<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.

<iter> = chain(<collection>, <collection>)  # Empties collections in order.
<iter> = chain.from_iterable(<collection>)  # Empties collections inside a collection in order.

<iter> = islice(<collection>, to_exclusive)
<iter> = islice(<collection>, from_inclusive, to_exclusive)
<iter> = islice(<collection>, from_inclusive, to_exclusive, step_size)

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

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

<type> = type(<el>)  # <class 'int'> / <class 'str'> / ...

from numbers import Integral, Rational, Real, Complex, Number
<bool> = isinstance(<el>, Number)

<bool> = callable(<el>)

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

<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.

<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.

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

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

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

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.

<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.

'\d' == '[0-9]'          # Digit
'\s' == '[ \t\n\r\f\v]'  # Whitespace
'\w' == '[a-zA-Z0-9_]'   # Alphanumeric

<str> = f'{<el_1>}, {<el_2>}'
<str> = '{}, {}'.format(<el_1>, <el_2>)

>>> Person = collections.namedtuple('Person', 'name height')
>>> person = Person('Jean-Luc', 187)
>>> f'{person.height}'
'187'
>>> '{p.height}'.format(p=person)
'187'

{<el>:<10}       # '<el>      '
{<el>:>10}       # '      <el>'
{<el>:^10}       # '   <el>   '
{<el>:->10}      # '------<el>'
{<el>:>0}        # '<el>'

{'abcde'!r:<10}  # "'abcde'   "

{'abcde':.3}     # 'abc'
{'abcde':10.3}   # 'abc       '

{1.23456:.3f}    # '1.235'
{1.23456:10.3f}  # '     1.235'

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

{65:c}           # 'A'
{3:08b}          # '00000011' -> Binary with leading zeros.
{3:0<8b}         # '11000000' -> Binary with trailing zeros.

<num>  = pow(<num>, <num>)  # Or: <num> ** <num>
<real> = abs(<num>)
<int>  = round(<real>)
<real> = round(<real>, ±ndigits)

from math import e, pi
from math import cos, acos, sin, asin, tan, atan, degrees, radians
from math import log, log10, log2
from math import inf, nan, isinf, isnan

from statistics import mean, median, variance, pvariance, pstdev

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

from itertools import product, combinations, combinations_with_replacement, permutations

>>> 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)]

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

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

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

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

from datetime import datetime
now = datetime.now()
now.month                      # 3
now.strftime('%Y%m%d')         # '20180315'
now.strftime('%Y%m%d%H%M%S')   # '20180315002834'
<datetime> = datetime.strptime('2015-05-12 00:39', '%Y-%m-%d %H:%M')

<function>(<positional_args>)                  # f(0, 0)
<function>(<keyword_args>)                     # f(x=0, y=0)
<function>(<positional_args>, <keyword_args>)  # f(0, y=0)

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)

args   = (1, 2)
kwargs = {'x': 3, 'y': 4, 'z': 5}
func(*args, **kwargs)  

func(1, 2, x=3, y=4, z=5)

def add(*a):
    return sum(a)

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

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)

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(*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)

<list>  = [*<collection> [, ...]]
<set>   = {*<collection> [, ...]}
<tuple> = (*<collection>, [...])
<dict>  = {**<dict> [, ...]}

head, *body, tail = <collection>

<function> = lambda: <return_value>
<function> = lambda <argument_1>, <argument_2>: <return_value>

<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}

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

out = []
for i in range(10):
    for j in range(10):
        out.append(i+j)

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

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

<expression_if_true> if <condition> else <expression_if_false>

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

from collections import namedtuple
Point     = namedtuple('Point', 'x y')
point     = Point(0, 0)

from enum import Enum
Direction = Enum('Direction', 'n e s w')
Cutlery   = Enum('Cutlery', {'fork': 1, 'knife': 2, 'spoon': 3})

# Warning: Objects will share the objects that are initialized in the dictionary!
Creature  = type('Creature', (), {'p': Point(0, 0), 'd': Direction.n})
creature  = Creature()

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

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

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

>>> multiply_by_3 = partial(operator.mul, 3)
>>> multiply_by_3(10)
30

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

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

@decorator_name
def function_that_gets_passed_to_decorator():
    ...

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

from functools import lru_cache

@lru_cache(maxsize=None)
def fib(n):
    return n if n < 2 else fib(n-2) + fib(n-1)

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 <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__

class <name>:
    def __init__(self, a=None):
        self.a = a

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

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

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 False 
    def __hash__(self):
        return hash(self.a)

class Counter:
    def __init__(self):
        self.i = 0
    def __call__(self):
        self.i += 1
        return self.i

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

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()

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

from copy import copy, deepcopy
<object> = copy(<object>)
<object> = deepcopy(<object>)

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()]

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

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>))

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

from functools import partial
LogicOp = Enum('LogicOp', {'AND': partial(lambda l, r: l and r),
                           'OR' : partial(lambda l, r: l or r)})

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

raise ValueError('A very specific message!')

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

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

>>> from pprint import pprint
>>> pprint(dir())
['__annotations__',
 '__builtins__',
 '__doc__', ...]

<str> = input(prompt=None)

while True:
    try:
        print(input())
    except EOFError:
        break

import sys
script_name = sys.argv[0]
arguments   = sys.argv[1:]

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>

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

<file>.seek(0)                 # Move to the start of the file.
<file>.seek(offset)            # Move 'offset' chars/bytes from the start.
<file>.seek(offset, <anchor>)  # Anchor: 0 start, 1 current pos., 2 end.

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

def write_to_file(filename, text):
    with open(filename, 'w', encoding='utf-8') as file:
        file.write(text)

from os import path, listdir
<bool> = path.exists('<path>')
<bool> = path.isfile('<path>')
<bool> = path.isdir('<path>')
<list> = listdir('<path>')

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

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

<bool> = <Path>.exists()
<bool> = <Path>.is_file()
<bool> = <Path>.is_dir()
<iter> = <Path>.iterdir()
<iter> = <Path>.glob('<pattern>')

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

<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.

import os
<str> = os.popen(<command>).read()

>>> import subprocess
>>> a = subprocess.run(['ls', '-a'], stdout=subprocess.PIPE)
>>> a.stdout
b'.\n..\nfile1.txt\nfile2.txt\n'
>>> a.returncode
0

import csv

def read_csv_file(filename):
    with open(filename, encoding='utf-8') as file:
        return csv.reader(file, delimiter=';')

def write_to_csv_file(filename, rows):
    with open(filename, 'w', encoding='utf-8') as file:
        writer = csv.writer(file, delimiter=';')
        writer.writerows(rows)

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

def read_json_file(filename):
    with open(filename, encoding='utf-8') as file:
        return json.load(file)

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)

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

def read_pickle_file(filename):
    with open(filename, 'rb') as file:
        return pickle.load(file)

def write_to_pickle_file(filename, an_object):
    with open(filename, 'wb') as file:
        pickle.dump(an_object, file)

import sqlite3
db = sqlite3.connect('<path>')
...
db.close()

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

db.execute('<query>')
db.commit()

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

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

<str>   = <bytes>.decode(encoding='utf-8') 
<int>   = int.from_bytes(<bytes>, byteorder='big|little', signed=False)
<hex>   = <bytes>.hex()

def read_bytes(filename):
    with open(filename, 'rb') as file:
        return file.read()

def write_bytes(filename, bytes_obj):
    with open(filename, 'wb') as file:
        file.write(bytes_obj)

from struct import pack, unpack, iter_unpack, calcsize
<bytes>  = pack('<format>', <value_1> [, <value_2>, ...])
<tuple>  = unpack('<format>', <bytes>)
<tuples> = iter_unpack('<format>', <bytes>)

>>> 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

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

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

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

<deque>.appendleft(<el>)
<el> = <deque>.popleft()

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

from threading import Thread, RLock

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

lock = RLock()
lock.acquire()
...
lock.release()

<list> = dir()      # Names of in-scope variables.
<dict> = locals()   # Dict of local variables. Also vars().
<dict> = globals()  # Dict of global variables.

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

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

<class> = type(<class_name>, <parents_tuple>, <attributes_dict>)

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

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

class MyMetaClass(type):
    def __new__(cls, name, parents, attrs):
        attrs['a'] = 'abcde'
        return type.__new__(cls, name, parents, attrs)

class MyClass(metaclass=MyMetaClass):
    b = 12345

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

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

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>)

>>> 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

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)

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

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

def reader(target):
    for i in range(10):
        target.send(i)
    target.close()

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

@coroutine
def printer():
    while True:
        item = (yield)
        print(item)

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

# $ 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)

# $ pip3 install matplotlib
from matplotlib import pyplot
pyplot.plot(<data_1> [, <data_2>, ...])
pyplot.savefig(<filename>, transparent=True)
pyplot.show()

# $ 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)

from curses import wrapper

def main():
    wrapper(draw)

def draw(screen):
    screen.clear()
    screen.addstr(0, 0, 'Press ESC to quit.')
    while screen.getch() != 27:
        pass

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

# $ 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')

# $ pip3 install bottle
from bottle import run, route, post, template, request, response
import json

run(host='localhost', port=8080)
run(host='0.0.0.0', port=80, server='cherrypy')

@route('/img/<image>')
def send_image(image):
    return static_file(image, 'images/', mimetype='image/png')

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

@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])

# $ 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]

from time import time
start_time = time()  # Seconds since Epoch.
...
duration = time() - start_time

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

from timeit import timeit
timeit('"-".join(str(a) for a in range(100))', 
       number=10000, globals=globals(), setup='pass')

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

$ 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)}

# $ 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>

# $ pip3 install numpy
import numpy as np

<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>)

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

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

<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.

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

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

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

left  = [[0.1], [0.6], [0.8]]  # Shape: (3, 1)
right = [[0.1 ,  0.6 ,  0.8]]  # Shape: (1, 3) <- !

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) <- !

>>> 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]

# $ pip3 install pillow
from PIL import Image

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')

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')

import wave
from struct import pack, iter_unpack

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)]

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))

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)

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)

# $ 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)

#!/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()