Python Cheat Sheet

Dictionaries

d.items() returns an iterable with key-value pairs as tuples
d.keys() returns an iterable with all keys
d.values() returns an iterable with all values
dict(sorted(d.items())) returns a dict sorted by keys
dict(sorted(d.items(), key=lambda x: x[1])) returns a dict sorted by values
d.get(key, default=None) returns the value for a key
d.pop(key, default=None) removes the key and returns the value for key
d.popitem() removes the last item added and returns it as (key, value) (LIFO)

Flow Control

break: Terminate the current loop.
continue: Skip the current iteration of a loop and move to the next iteration.
pass: Do nothing.

Iterables

i.all() returns True if all items are True
i.any() returns True if any item is True
i.min() returns smallest item
i.max() returns largest item
i.sum() returns sum of all items
reversed(i) returns the reverse

sorted(list)
sorted(list, reverse=True)
sorted(string_list, key=str.lower)  # Sort by lower case
sorted(string_list, key=int)        # Sort by casted integer

for i, el in enumerate(list):
  print(i, el)  ## i = index, el = element

Merge 2+ lists into one list containing tuples of mapped items:

list(zip(i1, i2, ...))              #  length = shorted list

from itertools import zip_longest
zip_longest(i1, i2, fillvalue="?")  # length = longest list, filled with "?"

List Comprehension

[expression for item in iterable if condition]
[expression if condition else other_expression for item in iterable]

Filter a list with [x for x in iterable if condition].

List Misc

list.extend(iterable) extends the list by appending all items
list.insert(i, x) inserts x at position i
list.remove(x) removes the first occurrence of x
list.count(x) counts the number of x
list.pop(i) removes and returns item at position i

Queue and Stack

queue = []
queue.append(item)  # Add to end
queue.pop(0)        # Remove and return first

stack = []
stack.append(item)  # Add to end
stack.pop()         # Remove and return last

Sets

s = set()
s.add(item)
s.remove(item)
if item in s: ...

Substrings

substring = s[start:end:step]

Optional args:

start: Starting index (inclusive). Defaults to 0.
end: Stopping index (exclusive). Defaults to the end.
step: Defaults to 1.

s[::-1] reverses a string.

String Misc

All string functions return values, no in-place modifications.

s.strip() removes any leading and trailing whitespaces (also lstrip() and rstrip())
''.join(sorted(s)) sorts the characters in a string
s.zfill(n) adds 0 to the beginning until the string is n char long.
s.isdigit() returns True if all chars in the string are digits
s.isalpha() returns True if all chars in the string are in the alphabet
s.startswith(value) returns True if the string starts with value (can be a tuple for multiple checks)
s.upper() / s.lower() for case conversion
s.replace(old, new, count) returns string with the first count occurences of old replaced by new
list(s) converts a string into a list of chars
s.split(separator, maxsplit) splits a string (separator defaults to any whitespace, optionally stop splitting after maxsplit splits)
s.rsplit() splits from the right

Class

class ClassName:
    """Class description"""

    def __init__(self, arg1, arg2=None):
        """
        Initializes an object

        Args:
            [...]
        """
        self.arg1 = arg1
        self.arg2 = arg2

    def __str__(self):
        return (f"{self.arg1=}, {self.arg2=}")

    def function_a(self, new_arg2):
        self.new_arg2 = new_arg2

obj1 = ClassName("asdf")
print(obj1)

Tests

import pytest

def test_simple():
    assert function_a() == "Hello"

@pytest.mark.parametrize("input, expected", [
    (input1, "Hello"), (input2, "Hi")
])
def test_parameter(input, expected):
    assert function_b(input) == expected

stefanthoss/python-cheat-sheet.md