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Collections
List
Dictionary
Counter
Set
Frozenset
Range
Enumerate
Named Tuple
Iterator
Generator
Types
ABC
String
Char
Regex
Format
Numbers
Combinatorics
Datetime
Syntax
Arguments
Splat Operator
Lambda comprehension
Dataclass
Closure
Decorator
Class
Duck Types
Enum
Exceptions
System
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Intro

#!/usr/bin/env python3
from collections import namedtuple
from enum import Enum
import re
import sys


def main():
    pass


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


if __name__ == '__main__':
    main()

Collections

ListDictionaryCounterSetFrozensetRangeEnumerateNamed TupleIteratorGenerator

List

<list> = <list>[from_inclusive : to_exclusive : ±step_size]
<list>.append(<el>)            # Or: <list> += [<el>]
<list>.extend(<collection>)    # Or: <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.

Dictionary

<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.
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.

Counter

>>> 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()
<set>.add(<el>)                               # Or: <set> |= {<el>}
<set>.update(<collection>)                    # Or: <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>)                            # Raises KeyError.
<set>.discard(<el>)                           # Doesn't raise an error.

Frozenset

Is hashable, meaning it can be used as a key in a dictionary or as an element in a set.

<frozenset> = frozenset(<collection>)

Range

<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

Enumerate

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

Named Tuple

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

Iterator

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(<coll.>, <coll.>, ...)       # 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)

Generator

Convenient way to implement the iterator protocol.

def count(start, step):
    while True:
        yield start
        start += step
>>> counter = count(10, 2)
>>> next(counter), next(counter), next(counter)
(10, 12, 14)

Types

<type> = type(<el>)                # Or: <el>.__class__
<bool> = isinstance(<el>, <type>)  # Or: issubclass(type(<el>), <type>)
>>> type('a'), 'a'.__class__, str
(<class 'str'>, <class 'str'>, <class 'str'>)

Some types do not have builtin names, so they must be imported:

from types import FunctionType, MethodType, LambdaType, GeneratorType

ABC

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

from numbers import Integral, Rational, Real, Complex, Number
from collections.abc import Sequence, Collection, Iterable
>>> isinstance(123, Number)
True
>>> isinstance([1, 2, 3], Iterable)
True

String

<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.

Char

<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)

Regex

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.

Match Object

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

Special Sequences

Expressions below hold true for strings that contain only ASCII characters. Use capital letters for negation.

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

Format

General OptionsString OptionsNumber Options
<str> = f'{<el_1>}, {<el_2>}'
<str> = '{}, {}'.format(<el_1>, <el_2>)
>>> 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

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

String Options

'!r' calls object’s repr() method, instead of format(), to get a string.

{'abcde'!r:<10}  # "'abcde'   "
{'abcde':.3}     # 'abc'
{'abcde':10.3}   # 'abc       '

Number Options

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

Float types:

{1.23456:10.3f}  # '     1.235'
{1.23456:10.3e}  # ' 1.235e+00'
{1.23456:10.3%}  # '  123.456%'

Int types:

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

Numbers

Basic FunctionsMathStatisticsRandom

Basic Functions

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

Math

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

Statistics

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

Random

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

Combinatorics

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')]

Datetime

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

Constructors

<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)

Now

<D/DTn>  = D/DT.today()                     # Current local date or naive datetime.
<DTn>    = DT.utcnow()                      # Naive datetime from current UTC time.
<DTa>    = DT.now(<tz>)                     # Aware datetime from current tz time.

Timezone

<tz>     = UTC                              # UTC timezone. London without DST.
<tz>     = tzlocal()                        # Local timezone.
<tz>     = gettz('<Cont.>/<City>')          # Timezone from 'Continent/City_Name' str.
<DTa>    = <DT>.astimezone(<tz>)            # Datetime, converted to passed timezone.
<Ta/DTa> = <T/DT>.replace(tzinfo=<tz>)      # Unconverted object with new timezone.

Encode

<D/T/DT> = D/T/DT.fromisoformat('<iso>')    # Object from ISO string.
<DT>     = DT.strptime(<str>, '<format>')   # Datetime from str, according to format.
<D/DTn>  = D/DT.fromordinal(<int>)          # D/DTn from days since Christ.
<DTa>    = DT.fromtimestamp(<real>, <tz>)   # DTa from seconds since Epoch in tz time.

Decode

<str>    = <D/T/DT>.isoformat()             # ISO string representation.
<str>    = <D/T/DT>.strftime('<format>')    # Custom string representation.
<int>    = <D/DT>.toordinal()               # Days since Christ, ignoring time and tz.
<float>  = <DT>.timestamp()                 # Seconds since Epoch in local time or tz.

Format

>>> from datetime import datetime
>>> dt = datetime.strptime('2015-05-14 23:39:00.00 +0200', '%Y-%m-%d %H:%M:%S.%f %z')
>>> dt.strftime("%A, %dth of %B '%y, %I:%M%p %Z")
"Thursday, 14th of May '15, 11:39PM UTC+02:00"

Rest of the codes:

Syntax

ArgumentsSplat OperatorLambda comprehensionDataclassClosureDecoratorClassDuck TypesEnumExceptions

Arguments

Inside Function Call

<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

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 CallInside Function DefinitionLegal argument combinations:Other Uses

Inside Function Call

Splat expands a collection into positional arguments, while splatty-splat expands a dictionary into keyword arguments.

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

is the same as:

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.

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)

Other Uses

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

Lambda comprehension

LambdaComprehensionMap, Filter, ReduceAny, AllIf - Else

Lambda

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

Comprehension

<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)]

Is the same as:

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

Map, Filter, Reduce

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

Any, All

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

If - Else

<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]

Dataclass

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

Closure

PartialNonlocal

We have a closure in Python when:

def get_multiplier(a):
    def out(b):
        return a * b
    return out
>>> multiply_by_3 = get_multiplier(3)
>>> multiply_by_3(10)
30

Partial

from functools import partial
<function> = partial(<function> [, <arg_1>, <arg_2>, ...])
>>> import operator as op
>>> multiply_by_3 = partial(op.mul, 3)
>>> multiply_by_3(10)
30

Nonlocal

If variable is being assigned to anywhere in the scope, it is regarded as a local variable, unless it is declared as a ‘global’ or a ‘nonlocal’.

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

Debugger ExampleLRU CacheParametrized Decorator

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

@decorator_name
def function_that_gets_passed_to_decorator():
    ...

Debugger Example

Decorator that prints function’s name every time it gets called.

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

LRU Cache

Decorator that caches function’s return values. All function’s arguments must be hashable.

from functools import lru_cache

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

Parametrized Decorator

A decorator that accepts arguments and returns a normal decorator that accepts a function.

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

Constructor OverloadingInheritanceMultiple InheritanceCopy
class <name>:
    def __init__(self, a):
        self.a = a
    def __repr__(self):
        class_name = self.__class__.__name__
        return f'{class_name}({self.a!r})'
    def __str__(self):
        return str(self.a)

    @classmethod
    def get_class_name(cls):
        return cls.__name__

Constructor Overloading

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

Inheritance

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

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:

>>> C.mro()
[<class 'C'>, <class 'A'>, <class 'B'>, <class 'object'>]

Copy

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

Duck Types

ComparableHashableCollectionCallableContext Manager

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

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

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

Collection

class MyCollection:
    def __init__(self, a):
        self.a = a
    def __len__(self):
        return len(self.a)
    def __getitem__(self, i):
        return self.a[i]
    def __setitem__(self, i, el):
        self.a[i] = el
    def __contains__(self, el):
        return el in self.a
    def __iter__(self):
        for el in self.a:
            yield el

Callable

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)

Context Manager

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!

Enum

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>))

Inline

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

*Functions can not be values, so they must be wrapped**:

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

Exceptions

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

Raising Exception

raise ValueError('A very specific message!')

Finally

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

System

PrintInputCommand Line ArgumentsOpenPathCommand Execution

Print

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

Pretty Print

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

Input

<str> = input(prompt=None)

Prints lines until EOF:

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

Command Line Arguments

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

Argparse

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>

Open

ModesFileRead Text from FileWrite Text to File

Opens a file and returns a corresponding file object.

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

Modes

File

<file>.seek(0)                      # Moves to the start of the file.
<file>.seek(offset)                 # Moves 'offset' chars/bytes from the start.
<file>.seek(offset, <anchor>)       # Anchor: 0 start, 1 current pos., 2 end.
<str/bytes> = <file>.read(size=-1)  # Reads 'size' chars/bytes or until EOF.
<str/bytes> = <file>.readline()     # Returns a line.
<list>      = <file>.readlines()    # Returns a list of lines.
<str/bytes> = next(<file>)          # Returns a line using buffer. Do not mix.
<file>.write(<str/bytes>)           # Writes a string or bytes object.
<file>.writelines(<list>)           # Writes a list of strings or bytes objects.
<file>.flush()                      # Flushes write buffer.

Read Text from File

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

Write Text to File

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

Path

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']

Pathlib

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 a 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.

Command Execution

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

Subprocess

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

Data

CSVJSONPickleSQLiteBytesStructArrayMemory ViewDeque

CSV

import csv

** Read Rows from CSV File**

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

Write Rows to CSV File

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

JSON

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

Read Object from JSON File

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

Write Object to JSON 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)

Pickle

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

Read Object from File

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

Write Object to File

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

SQLite

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

Read

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

Write

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

Bytes

EncodeDecodeRead Bytes from FileWrite Bytes to File

Bytes object is an immutable sequence of single bytes. Mutable version is called ‘bytearray’.

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

Encode

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

Decode

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

Read Bytes from File

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

Write Bytes to File

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

Struct

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

Example

>>> pack('>hhl', 1, 2, 3)
b'\x00\x01\x00\x02\x00\x00\x00\x03'
>>> unpack('>hhl', b'\x00\x01\x00\x02\x00\x00\x00\x03')
(1, 2, 3)
>>> calcsize('>hhl')
8

Format

For standard sizes start format string with:

Use capital letter for unsigned type. Standard sizes are in brackets:

Array

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

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

Memory View

Used for accessing the internal data of an object that supports the buffer protocol.

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

Deque

Thread-safe list with efficient appends and pops from either side. Pronounced “deck”.

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.

Advanced

ThreadingIntrospectionMetaprogramingOperatorEvalCoroutine

Threading

from threading import Thread, RLock

Thread

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

Lock

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

Or:

with lock:
    ...

Introspection

Inspecting code at runtime.

Variables

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

Attributes

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

Parameters

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

Metaprograming

TypeMeta ClassMetaclass Attribute

Code that generates code.

Type

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

<class> = type(<class_name>, <parents_tuple>, <attributes_dict>)
>>> Z = type('Z', (), {'a': 'abcde', 'b': 12345})
>>> z = Z()

Meta Class

Class that creates class.

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

Or:

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

Metaclass Attribute

When class is created it checks if it has metaclass defined. If not, it recursively checks if any of his parents has it defined and eventually comes to type().

class MyClass(metaclass=MyMetaClass):
    b = 12345
>>> MyClass.a, MyClass.b
('abcde', 12345)

Type diagram (str is an instance of type, …):

+---------+-------------+
| classes | metaclasses |
+---------+-------------|
| MyClass > MyMetaClass |
|         |     v       |
|  object ---> type <+  |
|         |    ^ +---+  |
|   str -------+        |
+---------+-------------+

Inheritance diagram (str is a subclass of object, …):

+---------+-------------+
| classes | metaclasses |
+---------+-------------|
| MyClass | MyMetaClass |
|    v    |     v       |
|  object <--- type     |
|    ^    |             |
|   str   |             |
+---------+-------------+

Operator

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>)

Eval

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

Using Abstract Syntax Trees

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

Coroutine

Helper DecoratorPipeline Example

Helper Decorator

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

Pipeline Example

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

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

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

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

Libraries

Progress BarPlotTableCursesLoggingWeb ScrapingWebProfileNumPyImageAudio

Progress Bar

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

Plot

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

Table

Prints a CSV file as an ASCII table:

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

Curses

from curses import wrapper, ascii

def main():
    wrapper(draw)

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

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

if __name__ == '__main__':
    main()

Logging

# $ pip3 install loguru
from loguru import logger
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.')

Exceptions

Error description, stack trace and values of variables are appended automatically.

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

Rotation

Parameter that sets a condition when a new log file is created.

rotation=<int>|<datetime.timedelta>|<datetime.time>|<str>

Retention

Sets a condition which old log files are deleted.

retention=<int>|<datetime.timedelta>|<str>

Web Scraping

# $ pip3 install requests beautifulsoup4
>>> import requests
>>> from bs4 import BeautifulSoup
>>> url   = 'https://en.wikipedia.org/wiki/Python_(programming_language)'
>>> page  = requests.get(url)
>>> doc   = BeautifulSoup(page.text, 'html.parser')
>>> table = doc.find('table', class_='infobox vevent')
>>> rows  = table.find_all('tr')
>>> link  = rows[11].find('a')['href']
>>> ver   = rows[6].find('div').text.split()[0]
>>> link, ver
('https://www.python.org/', '3.7.2')

Selenium

Library for scraping dynamically generated web content.

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

Web

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

Run

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

Static Request

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

Dynamic Request

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

REST Request

@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:

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

Profile

High PerformanceTiming a SnippetLine ProfilerCall Graph
from time import time
start_time = time()  # Seconds since Epoch.
...
duration = time() - start_time

High Performance

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

Timing a Snippet

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

Line Profiler

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

Usage:

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

Call Graph

Generates a PNG image of a call graph with highlighted bottlenecks:

# $ pip3 install pycallgraph
from pycallgraph import output, PyCallGraph
from datetime import datetime
time_str = datetime.now().strftime('%Y%m%d%H%M%S')
filename = f'profile-{time_str}.png'
drawer = output.GraphvizOutput(output_file=filename)
with PyCallGraph(output=drawer):
    <code_to_be_profiled>

NumPy

IndexingBroadcastingExample

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

# $ 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)

Indexing

<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>]

Broadcasting

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

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:

    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:

    left  = [[0.1, 0.1, 0.1], [0.6, 0.6, 0.6], [0.8, 0.8, 0.8]]  # Shape: (3, 3) <- !
    right = [[0.1, 0.6, 0.8], [0.1, 0.6, 0.8], [0.1, 0.6, 0.8]]  # Shape: (3, 3) <- !
    
  3. If neither non-matching dimension has size 1, rise an error.

Example

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

>>> 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

# $ pip3 install pillow
from PIL import Image

Create a PNG image of a rainbow gradient:

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')

Add noise to a PNG image:

from random import randint
add_noise = lambda value: max(0, min(255, value + randint(-20, 20)))
img = Image.open('test.png').convert(mode='HSV')
img.putdata([(add_noise(h), s, v) for h, s, v in img.getdata()])
img.convert(mode='RGB').save('test.png')

Modes

Audio

import wave
from struct import pack, iter_unpack

Read Frames from WAV File

def read_wav_file(filename):
    with wave.open(filename, 'rb') as wf:
        frames = wf.readframes(wf.getnframes())
        return [a[0] for a in iter_unpack('<h', frames)]

Write Frames to WAV File

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))

Save a sine wave to a mono WAV file:

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)

Add noise to a mono WAV file:

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)

Play Popcorn:

# $ 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)