8000 GitHub - dillionaire/python-cheatsheet: Comprehensive Python Cheatsheet
[go: up one dir, main page]
Skip to content

dillionaire/python-cheatsheet

BranchesTags
 
 

Folders and files

NameName
Last commit message
Last commit date

Latest commit

 

History

2,733 Commits
pdf
pdf
 
 
web
web
 
 
README.md
README.md
 
 
index.html
index.html
 
 
parse.js
parse.js
 
 

Repository files navigation

Comprehensive Python Cheatsheet

Download text file, Fork me on GitHub or Check out FAQ.

Monty Python

Contents

    1. Collections:   List, Dictionary, Set, Tuple, Range, Enumerate, Iterator, Generator.
    2. Types:            Type, String, Regular_Exp, Format, Numbers, Combinatorics, Datetime.
    3. Syntax:           Function, Inline, Import, Decorator, Class, Duck_Type, Enum, Except.
    4. System:          Exit, Print, Input, Command_Line_Arguments, Open, Path, OS_Commands.
    5. Data:               JSON, Pickle, CSV, SQLite, Bytes, Struct, Array, Memory_View, Deque.
    6. Advanced:     Operator, Match_Stmt, Logging, Introspection, Threading, Coroutines.
    7. Libraries:        Progress_Bar, Plot, Table, Console_App, GUI, Scraping, Web, Profile.
    8. Multimedia:    NumPy, Image, Animation, Audio, Synthesizer, Pygame, Pandas, Plotly.

Main

if __name__ == '__main__':      # Skips next line if file was imported.
    main()                      # Runs `def main(): ...` function.

List

<list> = [<el_1>, <el_2>, ...]  # Creates new list. Also list(<collection>).
<el>   = <list>[index]          # First index is 0. Last -1. Allows assignments.
<list> = <list>[<slice>]        # Also <list>[from_inclusive : to_exclusive : ±step].
<list>.append(<el>)             # Appends element to the end. Also <list> += [<el>].
<list>.extend(<collection>)     # Appends elements to the end. Also <list> += <coll>.
<list>.sort()                   # Sorts elements in ascending order.
<list>.reverse()                # Reverses the list in-place.
<list> = sorted(<collection>)   # Returns new list with sorted elements.
<iter> = reversed(<list>)       # Returns reversed iterator of elements.
<el>  = max(<collection>)       # Returns largest element. Also min(<el_1>, ...).
<num> = sum(<collection>)       # Returns sum of elements. Also math.prod(<coll>).
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>))
  • For details about sort(), sorted(), min() and max() see Sortable.
  • Module operator has function itemgetter() that can replace listed lambdas.
  • This text uses the term collection instead of iterable. For rationale see Collection.
<int> = len(<list>)             # Returns number of items. Also works on dict, set and string.
<int> = <list>.count(<el>)      # Returns number of occurrences. Also `if <el> in <coll>: ...`.
<int> = <list>.index(<el>)      # Returns index of the first occurrence or raises ValueError.
<el>  = <list>.pop()            # Removes and returns item from the end or at index if passed.
<list>.insert(<int>, <el>)      # Inserts item at index and moves the rest to the right.
<list>.remove(<el>)             # Removes first occurrence of the item or raises ValueError.
<list>.clear()                  # Removes all items. Also works on dictionary and set.

Dictionary

<dict> = {key_1: val_1, key_2: val_2, ...}      # Use `<dict>[key]` to get or set the value.
<view> = <dict>.keys()                          # Collection of keys that reflects changes.
<view> = <dict>.values()                        # Collection of values that reflects changes.
<view> = <dict>.items()                         # Coll. of key-value tuples that reflects chgs.
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>)        # Returns a dict with default value `<type>()`.
<dict> = collections.defaultdict(lambda: 1)     # Returns a dict with default value 1.
<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.
<dict>.update(<dict>)                           # Adds items. Replaces ones with matching keys.
value = <dict>.pop(key)                         # Removes item or raises KeyError if missing.
{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}  # Filters the dictionary by keys.

Counter

>>> from collections import Counter
>>> counter = Counter(['blue', 'blue', 'blue', 'red', 'red'])
>>> counter['yellow'] += 1
>>> print(counter.most_common())
[('blue', 3), ('red', 2), ('yellow', 1)]

Set

<set> = {<el_1>, <el_2>, ...}                   # Use `set()` for empty 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>
<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.
<frozenset> = frozenset(<collection>)

Tuple

Tuple is an immutable and hashable list.

<tuple> = ()                               # Empty tuple.
<tuple> = (<el>,)                          # Or: <el>,
<tuple> = (<el_1>, <el_2> [, ...])         # Or: <el_1>, <el_2> [, ...]

Named Tuple

Tuple's subclass with named elements.

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

Range

Immutable and hashable sequence of integers.

<range> = range(stop)                      # range(to_exclusive)
<range> = range(start, stop)               # range(from_inclusive, to_exclusive)
<range> = range(start, stop, ±step)        # range(from_inclusive, to_exclusive, ±step_size)
>>> [i for i in range(3)]
[0, 1, 2]

Enumerate

for i, el in enumerate(<coll>, start=0):   # Returns next element and its index on each pass.
    ...

Iterator

<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

import itertools as it
<iter> = it.count(start=0, step=1)         # Returns updated value endlessly. Accepts floats.
<iter> = it.repeat(<el> [, times])         # Returns element endlessly or 'times' times.
<iter> = it.cycle(<collection>)            # Repeats the sequence endlessly.
<iter> = it.chain(<coll>, <coll> [, ...])  # Empties collections in order (figuratively).
<iter> = it.chain.from_iterable(<coll>)    # Empties collections inside a collection in order.
<iter> = it.islice(<coll>, to_exclusive)   # Only returns first 'to_exclusive' elements.
<iter> = it.islice(<coll>, from_inc, …)    # `to_exclusive, +step_size`. Indices can be None.

Generator

  • Any function that contains a yield statement returns a generator.
  • Generators and iterators are interchangeable.
def count(start, step):
    while True:
        yield start
        start += step
>>> 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.
<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 built-in names, so they must be imported:

from types import FunctionType, MethodType, LambdaType, GeneratorType, ModuleType

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. ABC can also manually decide whether or not a specific class is its virtual subclass, usually based on which methods the class has implemented. For instance, Iterable ABC looks for method iter(), while Collection ABC looks for iter(), contains() and len().

>>> from collections.abc import Iterable, Collection, Sequence
>>> isinstance([1, 2, 3], Iterable)
True
+------------------+------------+------------+------------+
|                  |  Iterable  | Collection |  Sequence  |
+------------------+------------+------------+------------+
| list, range, str |    yes     |    yes     |    yes     |
| dict, set        |    yes     |    yes     |            |
| iter             |    yes     |            |            |
+------------------+------------+------------+------------+

>>> from numbers import Number, Complex, Real, Rational, Integral
>>> isinstance(123, Number)
True
+--------------------+----------+----------+----------+----------+----------+
|                    |  Number  |  Complex |   Real   | Rational | Integral |
+--------------------+----------+----------+----------+----------+----------+
| int                |   yes    |   yes    |   yes    |   yes    |   yes    |
| fractions.Fraction |   yes    |   yes    |   yes    |   yes    |          |
| float              |   yes    |   yes    |   yes    |          |          |
| complex            |   yes    |   yes    |          |          |          |
| decimal.Decimal    |   yes    |          |          |          |          |
+--------------------+----------+----------+----------+----------+----------+

String

Immutable sequence of characters.

<str>  = <str>.strip()                       # Strips all whitespace characters from both ends.
<str>  = <str>.strip('<chars>')              # Strips passed characters. Also lstrip/rstrip().
<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)    # On [\n\r\f\v\x1c-\x1e\x85\u2028\u2029] and \r\n.
<str>  = <str>.join(<coll_of_strings>)       # Joins elements using string as a separator.
<bool> = <sub_str> in <str>                  # Checks if string contains the substring.
<bool> = <str>.startswith(<sub_str>)         # Pass tuple of strings for multiple options.
<int>  = <str>.find(<sub_str>)               # Returns start index of the first match or -1.
<int>  = <str>.index(<sub_str>)              # Same, but raises ValueError if there's no match.
<str>  = <str>.lower()                       # Changes the case. Also upper/capitalize/title().
<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.
<str>  = chr(<int>)                          # Converts int to Unicode character.
<int>  = ord(<str>)                          # Converts Unicode character to int.
  • Use 'unicodedata.normalize("NFC", <str>)' on strings like 'Motörhead' before comparing them to other strings, because 'ö' can be stored as one or two characters.
  • 'NFC' converts such characters to a single character, while 'NFD' converts them to two.

Property Methods

<bool> = <str>.isdecimal()                   # Checks for [0-9]. Also [०-९] and [٠-٩].
<bool> = <str>.isdigit()                     # Checks for [²³¹…] and isdecimal().
<bool> = <str>.isnumeric()                   # Checks for [¼½¾…], [零〇一…] and isdigit().
<bool> = <str>.isalnum()                     # Checks for [a-zA-Z…] and isnumeric().
<bool> = <str>.isprintable()                 # Checks for [ !#$%…] and isalnum().
<bool> = <str>.isspace()                     # Checks for [ \t\n\r\f\v\x1c-\x1f\x85\xa0…].

Regex

Functions for regular expression matching.

import re
<str>   = re.sub(r'<regex>', new, text, count=0)  # Substitutes all occurrences with 'new'.
<list>  = re.findall(r'<regex>', text)            # Returns all occurrences as strings.
<list>  = re.split(r'<regex>', text, maxsplit=0)  # Add brackets around regex to keep matches.
<Match> = re.search(r'<regex>', text)             # First occurrence of the pattern or None.
<Match> = re.match(r'<regex>', text)              # Searches only at the beginning of the text.
<iter>  = re.finditer(r'<regex>', text)           # Returns all occurrences as Match objects.
  • Raw string literals do not interpret escape sequences, thus enabling us to use regex-specific escape sequences that cause SyntaxWarning in normal string literals (since 3.12).
  • Argument 'new' of re.sub() can be a function that accepts Match object and returns a str.
  • 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 '.' also accept the '\n'.
  • 're.compile(<regex>)' returns a Pattern object with methods sub(), findall(), etc.

Match Object

<str>   = <Match>.group()                         # Returns the whole match. Also group(0).
<str>   = <Match>.group(1)                        # Returns part inside the first brackets.
<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

'\d' == '[0-9]'                                   # Also [०-९…]. Matches a decimal character.
'\w' == '[a-zA-Z0-9_]'                            # Also [ª²³…]. Matches an alphanumeric or _.
'\s' == '[ \t\n\r\f\v]'                           # Also [\x1c-\x1f…]. Matches a whitespace.
  • By default, decimal characters and alphanumerics from all alphabets are matched unless 'flags=re.ASCII' is used. It restricts special sequence matches to the first 128 Unicode characters and also prevents '\s' from accepting '\x1c', '\x1d', '\x1e' and '\x1f' (non-printable characters that divide text into files, tables, rows and fields, respectively).
  • Use a capital letter for negation (all non-ASCII characters will be matched when used in combination with ASCII flag).

Format

<str> = f'{<el_1>}, {<el_2>}'            # Curly brackets can also contain expressions.
<str> = '{}, {}'.format(<el_1>, <el_2>)  # Or: '{0}, {a}'.format(<el_1>, a=<el_2>)
<str> = '%s, %s' % (<el_1>, <el_2>)      # Redundant and inferior C-style formatting.

Example

>>> Person = collections.namedtuple('Person', 'name height')
>>> person = Person('Jean-Luc', 187)
>>> f'{person.name} is {person.height / 100} meters tall.'
'Jean-Luc is 1.87 meters tall.'

General Options

{<el>:<10}                               # '<el>      '
{<el>:^10}                               # '   <el>   '
{<el>:>10}                               # '      <el>'
{<el>:.<10}                              # '<el>......'
{<el>:0}                                 # '<el>'
  • Objects are rendered using 'format(<el>, "<options>")'.
  • Options can be generated dynamically: f'{<el>:{<str/int>}[…]}'.
  • Adding '=' to the expression prepends it to the output: f'{1+1=}' returns '1+1=2'.
  • Adding '!r' to the expression converts object to string by calling its repr() method.

Strings

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

Numbers

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

Floats

{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:

+--------------+----------------+----------------+----------------+----------------+
|              |    {<float>}   |   {<float>:f}  |   {<float>:e}  |   {<float>:%}  |
+--------------+----------------+-------------
19AA
---+----------------+----------------+
|  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%' |
+--------------+----------------+----------------+----------------+----------------+

+--------------+----------------+----------------+----------------+----------------+
|              |  {<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%'   |
+--------------+----------------+----------------+----------------+----------------+
  • '{<float>:g}' is '{<float>:.6}' with stripped zeros, exponent starting at '1e+06'.
  • When both rounding up and rounding down are possible, the one that returns result with even last digit is chosen. That makes '{6.5:.0f}' a '6' and '{7.5:.0f}' an '8'.
  • This rule only effects numbers that can be represented exactly by a float (.5, .25, …).

Ints

{90:c}                                   # 'Z'. Unicode character with value 90.
{90:b}                                   # '1011010'. Binary representation of the int.
{90:X}                                   # '5A'. Hexadecimal with upper-case letters.

Numbers

<int>      = int(<float/str/bool>)           # Or: math.trunc(<float>)
<float>    = float(<int/str/bool>)           # Or: <int/float>e±<int>
<complex>  = complex(real=0, imag=0)         # Or: <int/float> ± <int/float>j
<Fraction> = fractions.Fraction(0, 1)        # Or: Fraction(numerator=0, denominator=1)
<Decimal>  = decimal.Decimal(<str/int>)      # Or: Decimal((sign, digits, exponent))
  • 'int(<str>)' and 'float(<str>)' raise ValueError on malformed strings.
  • Decimal numbers are stored exactly, unlike most floats where '1.1 + 2.2 != 3.3'.
  • Floats can be compared with: 'math.isclose(<float>, <float>)'.
  • Precision of decimal operations is set with: 'decimal.getcontext().prec = <int>'.
  • Bools can be used anywhere ints can, because bool is a subclass of int: 'True + 1 == 2'.

Built-in Functions

<num> = pow(<num>, <num>)                    # Or: <number> ** <number>
<num> = abs(<num>)                           # <float> = abs(<complex>)
<num> = round(<num> [, ±ndigits])            # Also math.floor/ceil(<number>).
<num> = min(<collection>)                    # Also max(<num>, <num> [, ...]).
<num> = sum(<collection>)                    # Also math.prod(<collection>).

Math

from math import pi, inf, nan, isnan         # `inf*0` and `nan+1` return nan.
from math import sqrt, factorial             # `sqrt(-1)` raises ValueError.
from math import sin, cos, tan               # Also: asin, degrees, radians.
from math import log, log10, log2            # Log accepts base as second arg.

Statistics

from statistics import mean, median, mode    # Mode returns the most common value.
from statistics import variance, stdev       # Also: pvariance, pstdev, quantiles.

Random

from random import random, randint, uniform  # Also: gauss, choice, shuffle, seed.
<float> = random()                           # Returns a float inside [0, 1).
<num>   = randint/uniform(a, b)              # Returns an int/float inside [a, b].
<float> = gauss(mean, stdev)                 # Also triangular(low, high, mode).
<el>    = choice(<sequence>)                 # Keeps it intact. Also sample(pop, k).
shuffle(<list>)                              # Shuffles the list in place.

Hexadecimal Numbers

<int> = ±0x<hex>                             # Or: ±0b<bin>
<int> = int('±<hex>', 16)                    # Or: int('±<bin>', 2)
<int> = int('±0x<hex>', 0)                   # Or: int('±0b<bin>', 0)
<str> = hex(<int>)                           # Returns '[-]0x<hex>'. Also bin().

Bitwise Operators

<int> = <int> & <int>                        # And (0b1100 & 0b1010 == 0b1000).
<int> = <int> | <int>                        # Or  (0b1100 | 0b1010 == 0b1110).
<int> = <int> ^ <int>                        # Xor (0b1100 ^ 0b1010 == 0b0110).
<int> = <int> << n_bits                      # Left shift. Use >> for right.
<int> = ~<int>                               # Not. Also -<int> - 1.

Combinatorics

import itertools as it
>>> list(it.product('abc', repeat=2))        #   a  b  c
[('a', 'a'), ('a', 'b'), ('a', 'c'),         # a x  x  x
 ('b', 'a'), ('b', 'b'), ('b', 'c'),         # b x  x  x
 ('c', 'a'), ('c', 'b'), ('c', 'c')]         # c x  x  x
>>> list(it.permutations('abc', 2))          #   a  b  c
[('a', 'b'), ('a', 'c'),                     # a .  x  x
 ('b', 'a'), ('b', 'c'),                     # b x  .  x
 ('c', 'a'), ('c', 'b')]                     # c x  x  .
>>> list(it.combinations('abc', 2))          #   a  b  c
[('a', 'b'), ('a', 'c'),                     # a .  x  x
 ('b', 'c')                                  # b .  .  x
]                                            # c .  .  .

Datetime

Provides 'date', 'time', 'datetime' and 'timedelta' classes. All are immutable and hashable.

# $ pip3 install python-dateutil
from datetime import date, time, datetime, timedelta, timezone
import zoneinfo, dateutil.tz
<D>  = date(year, month, day)               # Only accepts valid dates from 1 to 9999 AD.
<T>  = time(hour=0, minute=0, second=0)     # Also: `microsecond=0, tzinfo=None, fold=0`.
<DT> = datetime(year, month, day, hour=0)   # Also: `minute=0, second=0, microsecond=0, …`.
<TD> = timedelta(weeks=0, days=0, hours=0)  # Also: `minutes=0, seconds=0, microseconds=0`.
  • Aware times and datetimes have defined timezone, while naive don't. If object is naive, it is presumed to be in the system's timezone!
  • 'fold=1' means the second pass in case of time jumping back for one hour.
  • Timedelta normalizes arguments to ±days, seconds (< 86 400) and microseconds (< 1M). Its str() method returns '[±D, ]H:MM:SS[.…]' and total_seconds() a float of all seconds.
  • Use '<D/DT>.weekday()' to get the day of the week as an int, with Monday being 0.

Now

<D/DTn> = D/DT.today()                      # Current local date or naive DT. Also DT.now().
<DTa>   = DT.now(<tzinfo>)                  # Aware DT from current time in passed timezone.
  • To extract time use '<DTn>.time()', '<DTa>.time()' or '<DTa>.timetz()'.

Timezone

<tzinfo> = timezone.utc                     # London without daylight saving time (DST).
<tzinfo> = timezone(<timedelta>)            # Timezone with fixed offset from UTC.
<tzinfo> = dateutil.tz.tzlocal()            # Local timezone with dynamic offset from UTC.
<tzinfo> = zoneinfo.ZoneInfo('<iana_key>')  # 'Continent/City_Name' zone with dynamic offset.
<DTa>    = <DT>.astimezone([<tzinfo>])      # Converts DT to the passed or local fixed zone.
<Ta/DTa> = <T/DT>.replace(tzinfo=<tzinfo>)  # Changes object's timezone without conversion.
  • Timezones returned by tzlocal(), ZoneInfo() and implicit local timezone of naive objects have offsets that vary through time due to DST and historical changes of the base offset.
  • To get ZoneInfo() to work on Windows run '> pip3 install tzdata'.

Encode

<D/T/DT> = D/T/DT.fromisoformat(<str>)      # Object from ISO string. Raises ValueError.
<DT>     = DT.strptime(<str>, '<format>')   # Datetime from custom string. See Format.
<D/DTn>  = D/DT.fromordinal(<int>)          # D/DT from days since the Gregorian NYE 1.
<DTn>    = DT.fromtimestamp(<float>)        # Local naive DT from seconds since the Epoch.
<DTa>    = DT.fromtimestamp(<float>, <tz>)  # Aware datetime from seconds since the Epoch.
  • ISO strings come in following forms: 'YYYY-MM-DD', 'HH:MM:SS.mmmuuu[±HH:MM]', or both separated by an arbitrary character. All parts following the hours are optional.
  • Python uses the Unix Epoch: '1970-01-01 00:00 UTC', '1970-01-01 01:00 CET', ...

Decode

<str>    = <D/T/DT>.isoformat(sep='T')      # Also `timespec='auto/hours/minutes/seconds/…'`.
<str>    = <D/T/DT>.strftime('<format>')    # Custom string representation of the object.
<int>    = <D/DT>.toordinal()               # Days since Gregorian NYE 1, ignoring time and tz.
<float>  = <DTn>.timestamp()                # Seconds since the Epoch, from local naive DT.
<float>  = <DTa>.timestamp()                # Seconds since the Epoch, from aware datetime.

Format

>>> dt = datetime.strptime('2025-08-14 23:39:00.00 +0200', '%Y-%m-%d %H:%M:%S.%f %z')
>>> dt.strftime("%dth of %B '%y (%a), %I:%M %p %Z")
"14th of August '25 (Thu), 11:39 PM UTC+02:00"
  • '%z' accepts '±HH[:]MM' and returns '±HHMM' or empty string if object is naive.
  • '%Z' accepts 'UTC/GMT' and local timezone's code and returns timezone's name, 'UTC[±HH:MM]' if timezone is nameless, or an empty string if object is naive.

Arithmetics

<bool>   = <D/T/DTn> > <D/T/DTn>            # Ignores time jumps (fold attribute). Also ==.
<bool>   = <DTa>     > <DTa>                # Ignores time jumps if they share tzinfo object.
<TD>     = <D/DTn>   - <D/DTn>              # Ignores jumps. Convert to UTC for actual delta.
<TD>     = <DTa>     - <DTa>                # Ignores jumps if they share tzinfo object.
<D/DT>   = <D/DT>    ± <TD>                 # Returned datetime can fall into missing hour.
<TD>     = <TD>      * <float>              # Also `<TD> = abs(<TD>)`, `<TD> = <TD> ± <TD>`.
<float>  = <TD>      / <TD>                 # Also `(<int>, <TD>) = divmod(<TD>, <TD>)`.

Function

Independent block of code that returns a value when called.

def <func_name>(<nondefault_args>): ...                  # E.g. `def func(x, y): ...`.
def <func_name>(<default_args>): ...                     # E.g. `def func(x=0, y=0): ...`.
def <func_name>(<nondefault_args>, <default_args>): ...  # E.g. `def func(x, y=0): ...`.
  • Function returns None if it doesn't encounter 'return <obj/exp>' statement.
  • Run 'global <var_name>' inside the function before assigning to global variable.
  • Default values are evaluated when function is first encountered in the scope. Any mutation of a mutable default value will persist between invocations!

Function Call

<obj> = <function>(<positional_args>)                    # E.g. `func(0, 0)`.
<obj> = <function>(<keyword_args>)                       # E.g. `func(x=0, y=0)`.
<obj> = <function>(<positional_args>, <keyword_args>)    # E.g. `func(0, y=0)`.

Splat Operator

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

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

Is the same as:

func(1, 2, z=3)

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

Allowed compositions of arguments inside function definition and the ways they can be called:

+--------------------+------------+--------------+----------------+------------------+
|                    | f(1, 2, 3) | f(1, 2, z=3) | f(1, y=2, z=3) | f(x=1, y=2, z=3) |
+--------------------+------------+--------------+----------------+------------------+
| f(x, *args, **kw): |     yes    |      yes     |       yes      |       yes        |
| f(*args, z, **kw): |            |      yes     |       yes      |       yes        |
| f(x, **kw):        |            |              |       yes      |       yes        |
| f(*, x, **kw):     |            |              |                |       yes        |
+--------------------+------------+--------------+----------------+------------------+

Other Uses

<list>  = [*<collection> [, ...]]  # Or: list(<coll>) [+ ...]
<tuple> = (*<collection>, [...])   # Or: tuple(<coll>) [+ ...]
<set>   = {*<collection> [, ...]}  # Or: set(<coll>) [| ...]
<dict>  = {**<dict> [, ...]}       # Or: <dict> | ...
head, *body, tail = <collection>   # Head or tail can be omitted.

Inline

Lambda

<func> = lambda: <return_value>                     # A single statement function.
<func> = lambda <arg_1>, <arg_2>: <return_value>    # Also allows default arguments.

Comprehensions

<list> = [i+1 for i in range(10)]                   # Or: [1, 2, ..., 10]
<iter> = (i for i in range(10) if i > 5)            # Or: iter([6, 7, 8, 9])
<set>  = {i+5 for i in range(10)}                   # Or: {5, 6, ..., 14}
<dict> = {i: i*2 for i in range(10)}                # Or: {0: 0, 1: 2, ..., 9: 18}
>>> [l+r for l in 'abc' for r in 'abc']             # Inner loop is on the right side.
['aa', 'ab', 'ac', ..., 'cc']

Map, Filter, Reduce

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

Any, All

<bool> = any(<collection>)                          # Is `bool(<el>)` True for any el?
<bool> = all(<collection>)                          # True for all? Also True if empty.

Conditional Expression

<obj> = <exp> if <condition> else <exp>             # Only one expression is evaluated.
>>> [i if i else 'zero' for i in (0, 1, 2, 3)]      # `any([0, '', [], None]) == False`
['zero', 1, 2, 3]

And, Or

<obj> = <exp> and <exp> [and ...]                   # Returns first false or last operand.
<obj> = <exp> or <exp> [or ...]                     # Returns first true or last operand.

Walrus Operator

>>> [i for a in '0123' if (i := int(a)) > 0]        # Assigns to variable mid-sentence.
[1, 2, 3]

Named Tuple, Enum, Dataclass

from collections import namedtuple
Point = namedtuple('Point', 'x y')                  # Creates tuple's subclass.
point = Point(0, 0)                                 # Returns its instance.

from enum import Enum
Direction = Enum('Direction', 'N E S W')            # Creates Enum's subclass.
direction = Direction.N                             # Returns its member.

from dataclasses import make_dataclass
Player = make_dataclass('Player', ['loc', 'dir'])   # Creates a class.
player = Player(point, direction)                   # Returns its instance.

Imports

Mechanism that makes code in one file available to another file.

import <module>            # Imports a built-in or '<module>.py'.
import <package>           # Imports a built-in or '<package>/__init__.py'.
import <package>.<module>  # Imports a built-in or '<package>/<module>.py'.
  • Package is a collection of modules, but it can also define its own objects.
  • On a filesystem this corresponds to a directory of Python files with an optional init script.
  • Running 'import <package>' does not automatically provide access to the package's modules unless they are explicitly imported in its init script.
  • Directory of the file that is passed to python command serves as a root of local imports.
  • For relative imports use 'from .[…][<pkg/module>[.…]] import <obj>'.

Closure

We have/get a closure in Python when a nested function references a value of its enclosing function and then the enclosing function returns its nested function.

def get_multiplier(a):
    def out(b):
        return a * b
    return out
>>> multiply_by_3 = get_multiplier(3)
>>> multiply_by_3(10)
30
  • Any value that is referenced from within multiple nested functions gets shared.

Partial

from functools import partial
<function> = partial(<function> [, <arg_1> [, ...]])
>>> def multiply(a, b):
...     return a * b
>>> multiply_by_3 = partial(multiply, 3)
>>> multiply_by_3(10)
30
  • Partial is also useful in cases when a function needs to be passed as an argument because it enables us to set its arguments beforehand.
  • A few examples being: 'defaultdict(<func>)', 'iter(<func>, to_exc)' and dataclass's 'field(default_factory=<func>)'.

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'.

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

  • A decorator takes a function, adds some functionality and returns it.
  • It can be any callable, but is usually implemented as a function that returns a closure.
@decorator_name
def function_that_gets_passed_to_decorator():
    ...

Debugger Example

Decorator that prints function's name every time the function is 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
  • 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'.

Cache

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

from functools import cache

@cache
def fib(n):
    return n if n < 2 else fib(n-2) + fib(n-1)
  • Potential problem with cache is that it can grow indefinitely. To clear stored values run '<func>.cache_clear()', or use '@lru_cache(maxsize=<int>)' decorator instead.
  • CPython interpreter limits recursion depth to 3000 by default. To increase it run 'sys.setrecursionlimit(<int>)'.

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
  • Using only '@debug' to decorate the add() function would not work here, because debug would then receive the add() function as a 'print_result' argument. Decorators can however manually check if the argument they received is a function and act accordingly.

Class

A template for creating user-defined objects.

class MyClass:
    def __init__(self, a):
        self.a = a
    def __str__(self):
        return str(self.a)
    def __repr__(self):
        class_name = self.__class__.__name__
        return f'{class_name}({self.a!r})'

    @classmethod
    def get_class_name(cls):
        return cls.__name__
>>> obj = MyClass(1)
>>> obj.a, str(obj), repr(obj)
(1, '1', 'MyClass(1)')
  • Return value of str() should be readable and of repr() unambiguous.
  • If only repr() is defined, it will also be used for str().
  • Methods decorated with '@staticmethod' do not receive 'self' nor 'cls' as their first argument.

Expressions that call the str() method:

print(<obj>)
f'{<obj>}'
logging.warning(<obj>)
csv.writer(<file>).writerow([<obj>])
raise Exception(<obj>)

Expressions that call the repr() method:

print/str/repr([<obj>])
print/str/repr({<obj>: <obj>})
f'{<obj>!r}'
Z = make_dataclass('Z', ['a']); print/str/repr(Z(<obj>))
>>> <obj>

Inheritance

class Person:
    def __init__(self, name):
        self.name = name

class Employee(Person):
    def __init__(self, name, staff_num):
        super().__init__(name)
        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 or an attribute:

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

Type Annotations

  • They add type hints to variables, arguments and functions ('def f() -> <type>:').
  • Hints are used by type checkers like mypy, data validation libraries such as Pydantic and lately also by Cython compiler. However, they are not enforced by CPython interpreter.
from collections import abc

<name>: <type> [| ...] [= <obj>]
<name>: list/set/abc.Iterable/abc.Sequence[<type>] [= <obj>]
<name>: dict/tuple[<type>, ...] [= <obj>]

Dataclass

Decorator that uses class variables to generate init(), repr() and eq() special methods.

from dataclasses import dataclass, field, make_dataclass

@dataclass(order=False, frozen=False)
class <class_name>:
    <attr_name>: <type>
    <attr_name>: <ty
19AA
pe> = <default_value>
    <attr_name>: list/dict/set = field(default_factory=list/dict/set)
  • Objects can be made sortable with 'order=True' and immutable with 'frozen=True'.
  • For object to be hashable, all attributes must be hashable and 'frozen' must be True.
  • Function field() is needed because '<attr_name>: list = []' would make a list that is shared among all instances. Its 'default_factory' argument can be any callable.
  • For attributes of arbitrary type use 'typing.Any'.
P = make_dataclass('P', ['x', 'y'])
P = make_dataclass('P', [('x', float), ('y', float)])
P = make_dataclass('P', [('x', float, 0), ('y', float, 0)])

Property

Pythonic way of implementing getters and setters.

class Person:
    @property
    def name(self):
        return ' '.join(self._name)

    @name.setter
    def name(self, value):
        self._name = value.split()
>>> person = Person()
>>> person.name = '\t Guido  van Rossum \n'
>>> person.name
'Guido van Rossum'

Slots

Mechanism that restricts objects to attributes listed in 'slots', reduces their memory footprint.

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

Copy

from copy import copy, deepcopy
<object> = copy/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 user-defined 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. False is returned if both return NotImplemented.
  • Ne() automatically works on any object that has eq() defined.
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().
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 and the rest will be automatically generated.
  • Functions sorted() and min() only require lt() method, while max() only requires gt(). However, it is best to define them all so that confusion doesn't arise in other contexts.
  • When two lists, strings or dataclasses are compared, their values get compared in order until a pair of unequal values is found. The comparison of this two values is then returned. The shorter sequence is considered smaller in case of all values being equal.
  • To sort collection of strings in proper alphabetical order pass 'key=locale.strxfrm' to sorted() after running 'locale.setlocale(locale.LC_COLLATE, "en_US.UTF-8")'.
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 exception.
  • Iter() should return 'self', i.e. unmodified object on which it was called.
class Counter:
    def __init__(self):
        self.i = 0
    def __next__(self):
        self.i += 1
        return self.i
    def __iter__(self):
        return self
>>> counter = Counter()
>>> next(counter), next(counter), next(counter)
(1, 2, 3)

Python has many different iterator objects:

Callable

  • All functions and classes have a call() method, hence are callable.
  • Use 'callable(<obj>)' or 'isinstance(<obj>, collections.abc.Callable)' to check if object is callable. Calling an uncallable object raises 'TypeError'.
  • When this cheatsheet uses '<function>' as an argument, it means '<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

  • With statements only work on objects that have enter() and exit() special methods.
  • 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.
  • The exit() method can suppress the exception by returning a true value.
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()
>>> 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.
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
>>> 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(). Len() should return the number of items.
  • 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'. The main drawback of this decision is that the reader could think a certain function doesn't accept iterators when it does, since iterators are the only built-in objects that are iterable but are not collections.
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 getitem() and len().
  • Getitem() should return an item at the passed 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. It returns reversed iterator of object's items.
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)

Discrepancies between glossary definitions and abstract base classes:

  • Python's glossary defines iterable as any object with special methods iter() and/or getitem() and sequence as any object with getitem() and len(). It doesn't define collection.
  • Passing ABC Iterable to isinstance() or issubclass() only checks whether object/class has special method iter(), while ABC Collection checks for iter(), contains() and len().

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. It however recognizes list, tuple, range, str, bytes, bytearray, array, memoryview and deque, since they are registered as Sequence's virtual subclasses.
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:

+------------+------------+------------+------------+--------------+
|            |  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      |
+------------+------------+------------+------------+--------------+
  • Method iter() is required for 'isinstance(<obj>, abc.Iterable)' to return True, however any object with getitem() will work with any code expecting an iterable.
  • MutableSequence, Set, MutableSet, Mapping and MutableMapping ABCs are also extendable. Use '<abc>.__abstractmethods__' to get names of required methods.

Enum

Class of named constants called members.

from enum import Enum, auto
class <enum_name>(Enum):
    <member_name> = auto()              # Increment of the last numeric value or 1.
    <member_name> = <value>             # Values don't have to be hashable.
    <member_name> = <el_1>, <el_2>      # Values can be collections (this is a tuple).
  • Methods receive the member they were called on as the 'self' argument.
  • Accessing a member named after a reserved keyword causes SyntaxError.
<member> = <enum>.<member_name>         # Returns a member. Raises AttributeError.
<member> = <enum>['<member_name>']      # Returns a member. Raises KeyError.
<member> = <enum>(<value>)              # Returns a member. Raises ValueError.
<str>    = <member>.name                # Returns member's name.
<obj>    = <member>.value               # Returns member's value.
<list>   = list(<enum>)                 # Returns enum's members.
<list>   = [a.name for a in <enum>]     # Returns enum's member names.
<list>   = [a.value for a in <enum>]    # Returns enum's member values.
<enum>   = type(<member>)               # Returns member's enum.
<iter>   = itertools.cycle(<enum>)      # Returns endless iterator of members.
<member> = random.choice(list(<enum>))  # Returns a random member.

Inline

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:

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

Exceptions

try:
    <code>
except <exception>:
    <code>

Complex Example

try:
    <code_1>
except <exception_a>:
    <code_2_a>
except <exception_b>:
    <code_2_b>
else:
    <code_2_c>
finally:
    <code_3>
  • Code inside the 'else' block will only be executed if 'try' block had no exceptions.
  • Code inside the 'finally' block will always be executed (unless a signal is received).
  • All variables that are initialized in executed blocks are also visible in all subsequent blocks, as well as outside the try statement (only function block delimits scope).
  • To catch signals use 'signal.signal(signal_number, <func>)'.

Catching Exceptions

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 full error message to stderr.
  • Use 'print(<name>)' to print just the cause of the exception (its arguments).
  • Use 'logging.exception(<str>)' to log the passed message, followed by the full error message of the caught exception. For details see Logging.
  • Use 'sys.exc_info()' to get exception type, object, and traceback of caught exception.

Raising Exceptions

raise <exception>
raise <exception>()
raise <exception>(<obj> [, ...])

Re-raising caught exception:

except <exception> [as <name>]:
    ...
    raise

Exception Object

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)
trace_str = ''.join(traceback.format_tb(<name>.__traceback__))
error_msg = ''.join(traceback.format_exception(type(<name>), <name>, <name>.__traceback__))

Built-in Exceptions

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 such as ZeroDivisionError.
      +-- AssertionError          # Raised by `assert <exp>` if expression returns false value.
      +-- AttributeError          # Raised when object doesn't have requested attribute/method.
      +-- EOFError                # Raised by input() when it hits an end-of-file condition.
      +-- LookupError             # Base class for errors when a collection can't find an item.
      |    +-- IndexError         # Raised when a sequence index is out of range.
      |    +-- KeyError           # Raised when a dictionary key or set element is missing.
      +-- MemoryError             # Out of memory. May be too late to start deleting variables.
      +-- NameError               # Raised when nonexistent name (variable/func/class) is used.
      |    +-- UnboundLocalError  # Raised when local name is used before it's being defined.
      +-- OSError                 # Errors such as FileExistsError/TimeoutError (see #Open).
      |    +-- ConnectionError    # Errors such as BrokenPipeError/ConnectionAbortedError.
      +-- RuntimeError            # Raised by errors that don't fall into other categories.
      |    +-- NotImplementedEr…  # Can be raised by abstract methods or by unfinished code.
      |    +-- RecursionError     # Raised when the maximum recursion depth is exceeded.
      +-- StopIteration           # Raised when an empty iterator is passed to next().
      +-- TypeError               # When an argument of the wrong type is passed to function.
      +-- ValueError              # When argument has the right type but inappropriate value.

Collections and their exceptions:

+-----------+------------+------------+------------+
|           |    List    |    Set     |    Dict    |
+-----------+------------+------------+------------+
| getitem() | IndexError |            |  KeyError  |
| pop()     | IndexError |  KeyError  |  KeyError  |
| remove()  | ValueError |  KeyError  |            |
| index()   | ValueError |            |            |
+-----------+------------+------------+------------+

Useful built-in exceptions:

raise TypeError('Argument is of the wrong type!')
raise ValueError('Argument has the right type but an inappropriate value!')
raise RuntimeError('I am too lazy to define my own exception!')

User-defined Exceptions

class MyError(Exception): pass
class MyInputError(MyError): pass

Exit

Exits the interpreter by raising SystemExit exception.

import sys
sys.exit()                        # Exits with exit code 0 (success).
sys.exit(<int>)                   # Exits with the passed exit code.
sys.exit(<obj>)                   # Prints to stderr and exits with 1.

Print

print(<el_1>, ..., sep=' ', end='\n', file=sys.stdout, flush=False)
  • Use 'file=sys.stderr' for messages about errors.
  • Stdout and stderr streams hold output in a buffer until they receive a string containing '\n' or '\r', buffer reaches 4096 characters, 'flush=True' is used, or program exits.

Pretty Print

from pprint import pprint
pprint(<collection>, width=80, depth=None, compact=False, sort_dicts=True)
  • Each item is printed on its own line if collection exceeds 'width' characters.
  • Nested collections that are 'depth' levels deep get printed as '...'.

Input

<str> = input()
  • Reads a line from the user input or pipe if present (trailing newline gets stripped).
  • If argument is passed, it gets printed to the standard output before input is read.
  • EOFError is raised if user hits EOF (ctrl-d/ctrl-z⏎) or input stream gets exhausted.

Command Line Arguments

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

Argument Parser

from argparse import ArgumentParser, FileType
p = ArgumentParser(description=<str>)                             # Returns a parser.
p.add_argument('-<short_name>', '--<name>', action='store_true')  # Flag (defaults to False).
p.add_argument('-<short_name>', '--<name>', type=<type>)          # Option (defaults to None).
p.add_argument('<name>', type=<type>, nargs=1)                    # Mandatory first argument.
p.add_argument('<name>', type=<type>, nargs='+')                  # Mandatory remaining args.
p.add_argument('<name>', type=<type>, nargs='?/*')                # Optional argument/s.
args  = p.parse_args()                                            # Exits on parsing error.
<obj> = args.<name>                                               # Returns `<type>(<arg>)`.
  • Use 'help=<str>' to set argument description that will be displayed in help message.
  • Use 'default=<obj>' to set option's or optional argument's default value.
  • Use 'type=FileType(<mode>)' for files. Accepts 'encoding', but 'newline' is None.

Open

Opens a file and returns the corresponding file object.

<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 every '\n', '\r' and '\r\n'.

Modes

  • 'r' - Read. Used by default.
  • 'w' - Write. Deletes existing contents.
  • 'x' - Write or fail if the file already exists.
  • 'a' - Append. Creates new file if it doesn't exist.
  • 'w+' - Read and write. Deletes existing contents.
  • 'r+' - Read and write from the start.
  • 'a+' - Read and write from the end.
  • 'b' - Binary mode ('rb', 'wb', 'xb', …).

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

<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, origin)    # Origin: 0 start, 1 current position, 2 end.
<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.
<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. Runs every 4096/8192 B.
<file>.close()                      # Closes the file after flushing write buffer.
  • Methods do not add or strip trailing newlines, not even writelines().

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)

Paths

import os, glob
from pathlib import Path
<str>  = os.getcwd()                # Returns working dir. Starts as shell's $PWD.
<str>  = os.path.join(<path>, ...)  # Joins two or more pathname components.
<str>  = os.path.realpath(<path>)   # Resolves symlinks and calls path.abspath().
<str>  = os.path.basename(<path>)   # Returns final component of the path.
<str>  = os.path.dirname(<path>)    # Returns path without the final component.
<tup.> = os.path.splitext(<path>)   # Splits on last period of the final component.
<list> = os.listdir(path='.')       # Returns filenames located at the path.
<list> = glob.glob('<pattern>')     # Returns paths matching the wildcard pattern.
<bool> = os.path.exists(<path>)     # Or: <Path>.exists()
<bool> = os.path.isfile(<path>)     # Or: <DirEntry/Path>.is_file()
<bool> = os.path.isdir(<path>)      # Or: <DirEntry/Path>.is_dir()
<stat> = os.stat(<path>)            # Or: <DirEntry/Path>.stat()
<num>  = <stat>.st_mtime/st_size/…  # Modification time, size in bytes, etc.

DirEntry

Unlike listdir(), scandir() returns DirEntry objects that cache isfile, isdir, and on Windows also stat information, thus significantly increasing the performance of code that requires it.

<iter> = os.scandir(path='.')       # Returns DirEntry objects located at the path.
<str>  = <DirEntry>.path            # Returns the whole path as a string.
<str>  = <DirEntry>.name            # Returns final component as a string.
<file> = open(<DirEntry>)           # Opens the file and returns a file object.

Path Object

<Path> = Path(<path> [, ...])       # Accepts strings, Paths, and DirEntry objects.
<Path> = <path> / <path> [/ ...]    # First or second path must be a Path object.
<Path> = <Path>.resolve()           # Returns absolute path with resolved symlinks.
<Path> = Path()                     # Returns relative CWD. Also Path('.').
<Path> = Path.cwd()                 # Returns absolute CWD. Also Path().resolve().
<Path> = Path.home()                # Returns user's home directory (absolute).
<Path> = Path(__file__).resolve()   # Returns module's path if CWD wasn't changed.
<Path> = <Path>.parent              # Returns Path without the final component.
<str>  = <Path>.name                # Returns final component as a string.
<str>  = <Path>.stem                # Returns final component w/o last extension.
<str>  = <Path>.suffix              # Returns last extension prepended with a dot.
<tup.> = <Path>.parts               # Returns all path's components as strings.
<iter> = <Path>.iterdir()           # Returns directory contents as Path objects.
<iter> = <Path>.glob('<pattern>')   # Returns Paths matching the wildcard pattern.
<str>  = str(<Path>)                # Returns path as a string.
<file> = open(<Path>)               # Also <Path>.read/write_text/bytes(<args>).

OS Commands

import os, shutil, subprocess
os.chdir(<path>)          
19AA
          # Changes the current working directory.
os.mkdir(<path>, mode=0o777)        # Creates a directory. Permissions are in octal.
os.makedirs(<path>, mode=0o777)     # Creates all path's dirs. Also `exist_ok=False`.
shutil.copy(from, to)               # Copies the file. 'to' can exist or be a dir.
shutil.copy2(from, to)              # Also copies creation and modification time.
shutil.copytree(from, to)           # Copies the directory. 'to' must not exist.
os.rename(from, to)                 # Renames/moves the file or directory.
os.replace(from, to)                # Same, but overwrites file 'to' even on Windows.
shutil.move(from, to)               # Rename() that moves into 'to' if it's a dir.
os.remove(<path>)                   # Deletes the file.
os.rmdir(<path>)                    # Deletes the empty directory.
shutil.rmtree(<path>)               # Deletes the directory.
  • Paths can be either strings, Paths, or DirEntry objects.
  • Functions report OS related errors by raising either OSError or one of its subclasses.

Shell Commands

<pipe> = os.popen('<commands>')     # Executes commands in sh/cmd. Returns combined stdout.
<str>  = <pipe>.read(size=-1)       # Reads 'size' chars or until EOF. Also readline/s().
<int>  = <pipe>.close()             # Returns None if last command exited with returncode 0.

Sends '1 + 1' to the basic calculator and captures its output:

>>> subprocess.run('bc', input='1 + 1\n', capture_output=True, text=True)
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:

>>> from shlex import split
>>> os.popen('echo 1 + 1 > test.in')
>>> subprocess.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.

import json
<str>  = json.dumps(<list/dict>)    # Converts collection to JSON string.
<coll> = json.loads(<str>)          # Converts JSON string to collection.

Read Collection from JSON File

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

Write Collection to JSON File

def write_to_json_file(filename, collection):
    with open(filename, 'w', encoding='utf-8') as file:
        json.dump(collection, file, ensure_ascii=False, indent=2)

Pickle

Binary file format for storing Python objects.

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

Read Object from Pickle File

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

Write Object to Pickle File

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.

import csv

Read

<reader> = csv.reader(<file>)       # Also: `dialect='excel', delimiter=','`.
<list>   = next(<reader>)           # Returns next row as a list of strings.
<list>   = list(<reader>)           # Returns a list of remaining rows.
  • File must be opened with a 'newline=""' argument, or newlines embedded inside quoted fields will not be interpreted correctly!
  • To print the spreadsheet to the console use Tabulate library.
  • For XML and binary Excel files (xlsx, xlsm and xlsb) use Pandas library.
  • Reader accepts any iterator of strings, not just files.

Write

<writer> = csv.writer(<file>)       # Also: `dialect='excel', delimiter=','`.
<writer>.writerow(<collection>)     # Encodes objects using `str(<el>)`.
<writer>.writerows(<coll_of_coll>)  # Appends multiple rows.
  • File must be opened with a 'newline=""' argument, or '\r' will be added in front of every '\n' on platforms that use '\r\n' line endings!
  • Open existing file with 'mode="a"' to append to it or 'mode="w"' to overwrite it.

Parameters

  • 'dialect' - Master parameter that sets the default values. String or a 'csv.Dialect' object.
  • 'delimiter' - A one-character string used to separate fields.
  • 'lineterminator' - How writer terminates rows. Reader looks for '\n', '\r' and '\r\n'.
  • 'quotechar' - Character for quoting fields containing delimiters, quotechars, '\n' or '\r'.
  • 'escapechar' - Character for escaping quotechars.
  • 'doublequote' - Whether quotechars inside fields are/get doubled or escaped.
  • 'quoting' - 0: As necessary, 1: All, 2: All but numbers which are read as floats, 3: None.
  • 'skipinitialspace' - Is space character at the start of the field stripped by the reader.

Dialects

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

Read Rows from CSV File

def read_csv_file(filename, **csv_params):
    with open(filename, encoding='utf-8', newline='') as file:
        return list(csv.reader(file, **csv_params))

Write Rows to CSV File

def write_to_csv_file(filename, rows, mode='w', **csv_params):
    with open(filename, mode, encoding='utf-8', newline='') as file:
        writer = csv.writer(file, **csv_params)
        writer.writerows(rows)

SQLite

A server-less database engine that stores each database into its own file.

import sqlite3
<conn> = sqlite3.connect(<path>)               # Opens existing or new file. Also ':memory:'.
<conn>.close()                                 # Closes connection. Discards uncommitted data.

Read

<cursor> = <conn>.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

<conn>.execute('<query>')                      # Can raise a subclass of sqlite3.Error.
<conn>.commit()                                # Saves all changes since the last commit.
<conn>.rollback()                              # Discards all changes since the last commit.

Or:

with <conn>:                                   # Exits the block with commit() or rollback(),
    <conn>.execute('<query>')                  # depending on whether any exception occurred.

Placeholders

<conn>.execute('<query>', <list/tuple>)        # Replaces '?'s in query with values.
<conn>.execute('<query>', <dict/namedtuple>)   # Replaces ':<key>'s with values.
<conn>.executemany('<query>', <coll_of_coll>)  # Runs execute() multiple times.
  • Passed values can be of type str, int, float, bytes, None, or bool (stored as 1 or 0).

Example

Values are not actually saved in this example because 'conn.commit()' is omitted!

>>> conn = sqlite3.connect('test.db')
>>> conn.execute('CREATE TABLE person (person_id INTEGER PRIMARY KEY, name, height)')
>>> conn.execute('INSERT INTO person VALUES (NULL, ?, ?)', ('Jean-Luc', 187)).lastrowid
1
>>> conn.execute('SELECT * FROM person').fetchall()
[(1, 'Jean-Luc', 187)]

SQLAlchemy

Library for interacting with various DB systems via SQL, method chaining, or ORM.

# $ pip3 install sqlalchemy
from sqlalchemy import create_engine, text
<engine> = create_engine('<url>')              # Url: 'dialect://user:password@host/dbname'.
<conn>   = <engine>.connect()                  # Creates a connection. Also <conn>.close().
<cursor> = <conn>.execute(text('<query>'), …)  # `<dict>`. Replaces ':<key>'s with values.
with <conn>.begin(): ...                       # Exits the block with commit or rollback.
+-----------------+--------------+----------------------------------+
| Dialect         | pip3 install |           Dependencies           |
+-----------------+--------------+----------------------------------+
| mysql           | mysqlclient  | www.pypi.org/project/mysqlclient |
| postgresql      | psycopg2     | www.pypi.org/project/psycopg2    |
| mssql           | pyodbc       | www.pypi.org/project/pyodbc      |
| oracle+oracledb | oracledb     | www.pypi.org/project/oracledb    |
+-----------------+--------------+----------------------------------+

Bytes

A bytes object is an immutable sequence of single bytes. Mutable version is called bytearray.

<bytes> = b'<str>'                       # Only accepts ASCII characters and \x00-\xff.
<int>   = <bytes>[index]                 # Returns an 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 as a separator.

Encode

<bytes> = bytes(<coll_of_ints>)          # Ints must be in range from 0 to 255.
<bytes> = bytes(<str>, 'utf-8')          # Encodes the string. Also <str>.encode().
<bytes> = bytes.fromhex('<hex>')         # Hex pairs can be separated by whitespaces.
<bytes> = <int>.to_bytes(n_bytes, …)     # `byteorder='big/little', signed=False`.

Decode

<list>  = list(<bytes>)                  # Returns ints in range from 0 to 255.
<str>   = str(<bytes>, 'utf-8')          # Returns a string. Also <bytes>.decode().
<str>   = <bytes>.hex()                  # Returns hex pairs. Accepts `sep=<str>`.
<int>   = int.from_bytes(<bytes>, …)     # `byteorder='big/little', signed=False`.

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

  • Module that performs conversions between a sequence of numbers and a bytes object.
  • System’s type sizes, byte order, and alignment rules are used by default.
from struct import pack, unpack

<bytes> = pack('<format>', <el_1> [, ...])  # Packs objects according to format string.
<tuple> = unpack('<format>', <bytes>)       # Use iter_unpack() to get iterator of tuples.
>>> 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 and manual alignment (padding) start format string with:

  • '=' - System's byte order (usually little-endian).
  • '<' - Little-endian (i.e. least significant byte first).
  • '>' - Big-endian (also '!').

Besides numbers, pack() and unpack() also support bytes objects as a part of the sequence: