Python Cheat Sheet — Quick Reference for Python Developers

Basics & Variables

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x = 10

Variable assignment

x, y, z = 1, 2, 3

Multiple assignment

type(x)

Check type of variable

isinstance(x, int)

Check if instance of type

int("42")

Convert string to integer

str(42)

Convert to string

float("3.14")

Convert to float

bool(0) # False

Convert to boolean

input("Enter: ")

Read user input

print(f"Hello {name}")

F-string formatting

Strings

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s.upper()

Convert to uppercase

s.lower()

Convert to lowercase

s.strip()

Remove leading/trailing whitespace

s.split(",")

Split string into list

",".join(lst)

Join list into string

s.replace("old", "new")

Replace substring

s.find("sub")

Find index of substring (-1 if not found)

s.startswith("pre")

Check if starts with prefix

s.endswith(".py")

Check if ends with suffix

s[1:5]

Slice string (index 1 to 4)

s[::-1]

Reverse string

len(s)

Get string length

s.isdigit()

Check if all characters are digits

s.title()

Title Case Each Word

Lists

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lst = [1, 2, 3]

Create a list

lst.append(4)

Add item to end

lst.insert(0, "first")

Insert at index

lst.remove(2)

Remove first occurrence of value

lst.pop()

Remove and return last item

lst.pop(0)

Remove and return item at index

lst.sort()

Sort list in place

sorted(lst)

Return new sorted list

lst.reverse()

Reverse list in place

lst.index(2)

Find index of value

lst.count(2)

Count occurrences

lst.extend([4, 5])

Add multiple items

[x**2 for x in range(10)]

List comprehension

[x for x in lst if x > 2]

Filtered list comprehension

lst[::2]

Every other element

Dictionaries

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d = {"key": "value"}

Create a dictionary

d["key"]

Access value by key

d.get("key", "default")

Get with default value

d["new_key"] = "val"

Add or update key

del d["key"]

Delete a key

d.keys()

Get all keys

d.values()

Get all values

d.items()

Get key-value pairs

d.update({"a": 1})

Merge dictionaries

d.pop("key")

Remove key and return value

"key" in d

Check if key exists

{k: v for k, v in d.items()}

Dict comprehension

d | {"new": 1}

Merge operator (Python 3.9+)

Sets & Tuples

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s = {1, 2, 3}

Create a set

s.add(4)

Add element to set

s.discard(2)

Remove element (no error if missing)

s1 | s2

Union of two sets

s1 & s2

Intersection of two sets

s1 - s2

Difference (in s1 but not s2)

t = (1, 2, 3)

Create a tuple (immutable)

a, b, c = t

Unpack tuple

a, *rest = (1,2,3,4)

Extended unpacking

Control Flow

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if x > 0: ... elif x == 0: ... else: ...

If/elif/else

for item in iterable:

For loop

for i, val in enumerate(lst):

Loop with index

for k, v in d.items():

Loop over dict

while condition:

While loop

break

Exit loop

continue

Skip to next iteration

x if condition else y

Ternary operator

match value: case 1: ... case _: ...

Match statement (Python 3.10+)

Functions

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def func(a, b=10):

Function with default argument

def func(*args, **kwargs):

Variable arguments

lambda x: x * 2

Lambda (anonymous) function

def func() -> int:

Type hint for return value

def func(x: str, y: int = 0):

Type hints for parameters

map(func, iterable)

Apply function to each element

filter(func, iterable)

Filter elements by function

from functools import reduce

Import reduce for accumulation

@decorator

Apply decorator to function

Classes & OOP

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

Basic class with constructor

class Puppy(Dog):

Inheritance

super().__init__()

Call parent constructor

@property

Getter property

@staticmethod

Static method (no self)

@classmethod

Class method (cls instead of self)

def __str__(self):

String representation

def __len__(self):

Support len()

from dataclasses import dataclass
@dataclass
class Point:
    x: float
    y: float

Dataclass (auto __init__, __repr__)

File I/O

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with open("f.txt") as f:
    text = f.read()

Read entire file

with open("f.txt") as f:
    lines = f.readlines()

Read file as list of lines

with open("f.txt", "w") as f:
    f.write("text")

Write to file

with open("f.txt", "a") as f:
    f.write("more")

Append to file

import json
with open("d.json") as f:
    data = json.load(f)

Read JSON file

json.dumps(data, indent=2)

Convert to JSON string

import csv
with open("d.csv") as f:
    reader = csv.reader(f)

Read CSV file

from pathlib import Path
p = Path("dir/file.txt")

Modern path handling

Path("dir").mkdir(exist_ok=True)

Create directory

list(Path(".").glob("*.py"))

Find files by pattern

Error Handling

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try:
    risky()
except ValueError as e:
    print(e)

Try/except with specific error

except (TypeError, KeyError):

Catch multiple exceptions

finally:

Always runs after try/except

raise ValueError("msg")

Raise an exception

assert x > 0, "x must be positive"

Assert condition

class MyError(Exception): pass

Custom exception class

Modules & Imports

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import os

Import module

from os import path

Import specific item

import numpy as np

Import with alias

from . import module

Relative import

if __name__ == "__main__":

Run only when executed directly

pip install package

Install package

pip freeze > requirements.txt

Export dependencies

python -m venv env

Create virtual environment

Common Built-ins

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len(obj)

Length of object

range(start, stop, step)

Generate sequence of numbers

zip(lst1, lst2)

Pair elements from two iterables

any(iterable)

True if any element is truthy

all(iterable)

True if all elements are truthy

min(iterable) / max(iterable)

Minimum / maximum value

sum(iterable)

Sum of all elements

abs(-5)

Absolute value

round(3.14159, 2)

Round to decimal places

dir(obj)

List attributes of object

help(func)

Get documentation

Useful Standard Library

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import datetime
datetime.datetime.now()

Current date and time

import re
re.findall(r"\d+", text)

Find all pattern matches

import os
os.environ.get("KEY")

Get environment variable

import sys
sys.argv

Command-line arguments

from collections import Counter
Counter(lst)

Count element frequencies

from collections import defaultdict
dd = defaultdict(list)

Dict with default factory

import itertools
itertools.chain(a, b)

Chain iterables together

import random
random.choice(lst)

Random element from list

import hashlib
hashlib.sha256(b"text").hexdigest()

SHA-256 hash

import subprocess
subprocess.run(["ls", "-la"])

Run shell command

🐍 Python Cheat Sheet