Python · Fundamentals · Quick Reference

Python
Cheatsheet

A quick reference guide for Python fundamentals, syntax, and common operations — from basics and data types through to file handling, exceptions, and useful one-liners.

Language: Python 3

Level: Beginner → Intermediate

Topics: 10 sections

📖 View Official Docs 📦 PyPI

🐍 1. Basics

Variables, printing, input, and comments.

# Variables — no type declaration needed
name   = "Kenneth"
age    = 25
score  = 98.5
active = True

# Print
print("Hello, World!")
print(f"Name: {name}, Age: {age}")   # f-string

# Input
user = input("Enter your name: ")

# Multiple assignment
x, y, z = 1, 2, 3

# Check type
type(name)    # <class 'str'>

📦 2. Data Types

Python's core built-in types: strings, lists, tuples, sets, and dicts.

Strings

s = "hello world"
s.upper()        # HELLO WORLD
s.title()        # Hello World
s.split(" ")     # ['hello','world']
s.replace("hello","hi")
s[0:5]           # hello (slice)
len(s)           # 11
",".join(["a","b"]) # "a,b"

Lists

lst = [1, 2, 3, 4]
lst.append(5)    # add to end
lst.insert(0, 0) # insert at index
lst.remove(3)    # remove value
lst.pop()        # remove last
lst.sort()       # sort in-place
lst[1:3]         # [2, 3]
lst[::-1]        # reversed

Dictionaries

d = {"name": "Ken", "age": 25}
d["age"]          # 25
d.get("city", "N/A") # safe get
d["city"] = "London"
d.keys()
d.values()
d.items()        # key-value pairs
"name" in d       # True

Tuples & Sets

# Tuple — immutable
t = (1, 2, 3)
t[0]             # 1
a, b, c = t      # unpack

# Set — unique values
s = {1, 2, 2, 3}  # {1, 2, 3}
s.add(4)
s.discard(1)
s1 & s2          # intersection
s1 | s2          # union

🔀 3. Control Flow

if / elif / else

if age >= 18:
    print("Adult")
elif age >= 13:
    print("Teen")
else:
    print("Child")

# Ternary
label = "Adult" if age >= 18 else "Minor"

Loops

# for loop
for i in range(5):
    print(i)

for item in ["a", "b", "c"]:
    print(item)

# while loop
n = 0
while n < 5:
    n += 1

# enumerate
for i, v in enumerate(lst):
    print(i, v)

⚙️ 4. Functions

# Basic function
def greet(name, greeting="Hello"):
    return f"{greeting}, {name}!"

greet("Ken")            # Hello, Ken!
greet("Ken", "Hi")      # Hi, Ken!

# *args and **kwargs
def add_all(*args):
    return sum(args)

def show_info(**kwargs):
    for k, v in kwargs.items():
        print(f"{k}: {v}")

# Lambda (anonymous function)
square = lambda x: x ** 2
square(4)               # 16

# Type hints (Python 3.5+)
def add(a: int, b: int) -> int:
    return a + b

⚡ 5. List Comprehensions

Compact syntax for building lists, dicts, and sets in a single line.

# List comprehension
squares = [x**2 for x in range(10)]

# With condition
evens = [x for x in range(20) if x % 2 == 0]

# Nested comprehension
matrix = [[i*j for j in range(3)] for i in range(3)]

# Dict comprehension
word_len = {w: len(w) for w in ["apple", "cat", "dog"]}

# Set comprehension
unique_lens = {len(w) for w in ["hi", "hey", "hello"]}

# Generator expression (memory-efficient)
total = sum(x**2 for x in range(1000))

📁 6. Working with Files

# Read entire file
with open("file.txt", "r") as f:
    content = f.read()

# Read line by line
with open("file.txt") as f:
    for line in f:
        print(line.strip())

# Read all lines into a list
lines = f.readlines()

# Write to file
with open("output.txt", "w") as f:
    f.write("Hello\n")

# Append to file
with open("output.txt", "a") as f:
    f.write("New line\n")

# File modes: "r" read  "w" write  "a" append  "rb" read binary

🛡️ 7. Exception Handling

# try / except / else / finally
try:
    result = 10 / 0
except ZeroDivisionError as e:
    print(f"Error: {e}")
except (TypeError, ValueError):
    print("Type or value error")
else:
    print("No error occurred")
finally:
    print("Always runs")

# Raise custom exception
raise ValueError("Invalid input")

# Custom exception class
class AppError(Exception):
    pass

raise AppError("Something went wrong")

📚 8. Modules & Imports

# Standard imports
import os
import sys
import math
import datetime
import json
import re

# Aliased import
import numpy as np
import pandas as pd

# Selective import
from math import sqrt, pi
from os.path import join, exists

# Useful os / sys snippets
os.getcwd()              # current directory
os.listdir(".")          # list files
os.path.join("a", "b")   # path join
sys.argv                  # command-line args

🔧 9. Common Built-in Functions

Function	Description
len(x)	Length of a sequence or collection
range(start, stop, step)	Generate a range of integers
enumerate(iterable)	Returns (index, value) pairs
zip(a, b)	Pair elements from two iterables
map(func, iterable)	Apply function to each element
filter(func, iterable)	Keep elements where function returns True
sorted(iterable, key, reverse)	Return a new sorted list
min(x) / max(x)	Minimum or maximum value
sum(iterable)	Sum all numeric elements
any(iterable) / all(iterable)	True if any/all elements are truthy
isinstance(obj, type)	Check object type
int() / str() / float() / list()	Type conversion

✨ 10. Useful One-Liners

# Swap variables
a, b = b, a

# Flatten a nested list
flat = [x for sub in nested for x in sub]

# Count items in a list
from collections import Counter
counts = Counter(["a", "b", "a", "c", "a"])

# Reverse a string
rev = s[::-1]

# Remove duplicates preserving order
unique = list(dict.fromkeys(lst))

# Check palindrome
is_pal = s == s[::-1]

# Merge two dicts (Python 3.9+)
merged = dict1 | dict2

# Unpack and ignore
first, *rest = [1, 2, 3, 4]

# Sort dict by value
sorted(d.items(), key=lambda x: x[1])

# Read CSV in one line
rows = [line.split(",") for line in open("f.csv")]