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Python Cheat Sheet: Syntax, Data Types, Functions, and More

7 min read •
Beginner Python Programming Cheat Sheet Reference

Quick Answer: Variables: x = 10. Strings: f"Hello {name}". Lists: [1, 2, 3]. Dicts: {"key": "value"}. Loop: for item in list:. Function: def greet(name): return f"Hi {name}". File: with open("file.txt") as f: data = f.read().

Basics

# Variables (no type declaration needed)
name = "Sam"
age = 25
price = 19.99
is_active = True

# Print
print("Hello World")
print(f"Name: {name}, Age: {age}")    # f-string (Python 3.6+)

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

# Comments
# Single line comment

"""
Not a true comment -- this is a string literal.
When first in a module/class/function, it becomes a docstring.
"""

# Type checking
type(name)      # <class 'str'>
isinstance(name, str)  # True

Data Types

Type Example Mutable
int 42 No
float 3.14 No
str "hello" No
bool True, False No
list [1, 2, 3] Yes
tuple (1, 2, 3) No
dict {"a": 1} Yes
set {1, 2, 3} Yes
None None No 
# Type conversion
int("42")       # 42
float("3.14")   # 3.14
str(42)         # "42"
list("abc")     # ['a', 'b', 'c']
bool(0)         # False
bool(1)         # True

Strings

s = "Hello World"

# Access
s[0]            # 'H'
s[-1]           # 'd'
s[0:5]          # 'Hello'
s[6:]           # 'World'

# Methods
s.lower()       # 'hello world'
s.upper()       # 'HELLO WORLD'
s.strip()       # Remove whitespace from both ends
s.lstrip()      # Remove from left
s.rstrip()      # Remove from right
s.split()       # ['Hello', 'World']
s.split(",")    # Split by comma
s.replace("World", "Python")  # 'Hello Python'
s.startswith("Hello")  # True
s.endswith("World")    # True
s.find("World")        # 6 (index, -1 if not found)
s.count("l")           # 3
s.isdigit()            # False
s.isalpha()            # False (has space)
s.title()              # 'Hello World'

# Join
", ".join(["a", "b", "c"])  # 'a, b, c'

# F-strings (formatting)
name = "Sam"
age = 25
f"Name: {name}, Age: {age}"
f"Price: {19.99:.2f}"       # 'Price: 19.99'
f"{'hello':>10}"            # '     hello' (right-align)
f"{'hello':<10}"            # 'hello     ' (left-align)
f"{'hello':^10}"            # '  hello   ' (center)
f"{1000000:,}"              # '1,000,000'

# Multi-line strings
text = """Line 1
Line 2
Line 3"""

# Raw strings (ignore escape sequences)
path = r"C:\Users\Sam\file.txt"

Lists

lst = [1, 2, 3, 4, 5]

# Access
lst[0]          # 1
lst[-1]         # 5
lst[1:3]        # [2, 3]

# Modify
lst.append(6)           # [1, 2, 3, 4, 5, 6]
lst.insert(0, 0)        # [0, 1, 2, 3, 4, 5, 6]
lst.extend([7, 8])      # [0, 1, 2, 3, 4, 5, 6, 7, 8]
lst.remove(0)           # Remove first occurrence of 0
lst.pop()               # Remove and return last item
lst.pop(0)              # Remove and return item at index 0

# Search
3 in lst                # True
lst.index(3)            # Index of first occurrence
lst.count(3)            # Count occurrences

# Sort
lst.sort()              # Sort in place
lst.sort(reverse=True)  # Sort descending
sorted(lst)             # Return new sorted list (doesn't modify original)

# Other
lst.reverse()           # Reverse in place
lst.copy()              # Shallow copy
len(lst)                # Length
min(lst)                # Minimum
max(lst)                # Maximum
sum(lst)                # Sum

# List comprehension
squares = [x**2 for x in range(10)]            # [0, 1, 4, 9, 16, ...]
evens = [x for x in range(20) if x % 2 == 0]  # [0, 2, 4, 6, ...]

Dictionaries

d = {"name": "Sam", "age": 25, "city": "LA"}

# Access
d["name"]               # 'Sam'
d.get("name")           # 'Sam'
d.get("phone", "N/A")   # 'N/A' (default if key missing)

# Modify
d["email"] = "[email protected]"   # Add/update
del d["city"]                     # Delete key
d.pop("age")                      # Remove and return value

# Methods
d.keys()                # dict_keys(['name', 'email'])
d.values()              # dict_values(['Sam', '[email protected]'])
d.items()               # dict_items([('name', 'Sam'), ...])
d.update({"age": 26})   # Merge another dict

# Check
"name" in d             # True

# Loop
for key, value in d.items():
    print(f"{key}: {value}")

# Dict comprehension
squares = {x: x**2 for x in range(5)}  # {0: 0, 1: 1, 2: 4, 3: 9, 4: 16}

Tuples and Sets

# Tuples (immutable lists)
t = (1, 2, 3)
t[0]            # 1
x, y, z = t     # Unpacking: x=1, y=2, z=3

# Sets (unique values, unordered)
s = {1, 2, 3, 3, 3}    # {1, 2, 3}
s.add(4)
s.remove(1)
s.discard(99)           # Remove if exists (no error if missing)

# Set operations
a = {1, 2, 3}
b = {2, 3, 4}
a | b           # Union: {1, 2, 3, 4}
a & b           # Intersection: {2, 3}
a - b           # Difference: {1}
a ^ b           # Symmetric difference: {1, 4}

Control Flow

# If/elif/else
if x > 10:
    print("big")
elif x > 5:
    print("medium")
else:
    print("small")

# Ternary
result = "even" if x % 2 == 0 else "odd"

# Match (Python 3.10+)
match status:
    case 200:
        print("OK")
    case 404:
        print("Not Found")
    case _:
        print("Unknown")

Loops

# For loop
for i in range(5):          # 0, 1, 2, 3, 4
    print(i)

for i in range(2, 10, 2):   # 2, 4, 6, 8
    print(i)

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

for i, item in enumerate(["a", "b", "c"]):
    print(f"{i}: {item}")    # 0: a, 1: b, 2: c

for key, val in {"a": 1, "b": 2}.items():
    print(f"{key}={val}")

# While loop
while x > 0:
    x -= 1

# Break and continue
for i in range(10):
    if i == 5:
        break       # Stop the loop
    if i % 2 == 0:
        continue    # Skip to next iteration
    print(i)        # 1, 3

# Zip (loop over multiple lists)
names = ["Alice", "Bob"]
ages = [25, 30]
for name, age in zip(names, ages):
    print(f"{name} is {age}")

Functions

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

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

# *args (variable positional arguments)
def add(*args):
    return sum(args)
add(1, 2, 3)   # 6

# **kwargs (variable keyword arguments)
def info(**kwargs):
    for key, val in kwargs.items():
        print(f"{key}: {val}")
info(name="Sam", age=25)

# Lambda (anonymous function)
square = lambda x: x**2
square(5)   # 25

# Map, filter, reduce
list(map(lambda x: x**2, [1, 2, 3]))       # [1, 4, 9]
list(filter(lambda x: x > 2, [1, 2, 3]))   # [3]

from functools import reduce
reduce(lambda a, b: a + b, [1, 2, 3])      # 6

# Type hints (optional, for documentation)
def greet(name: str, age: int) -> str:
    return f"{name} is {age}"

Classes

class Person:
    # Class variable
    species = "Human"

    # Constructor
    def __init__(self, name, age):
        self.name = name        # Instance variable
        self.age = age

    # Method
    def greet(self):
        return f"Hi, I'm {self.name}"

    # String representation
    def __repr__(self):
        return f"Person('{self.name}', {self.age})"

# Create instance
p = Person("Sam", 25)
p.greet()       # "Hi, I'm Sam"
p.name          # "Sam"

# Inheritance
class Student(Person):
    def __init__(self, name, age, grade):
        super().__init__(name, age)
        self.grade = grade

    def study(self):
        return f"{self.name} is studying"

File I/O

# Read entire file
with open("file.txt") 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 list
with open("file.txt") as f:
    lines = f.readlines()

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

# Append to file
with open("file.txt", "a") as f:
    f.write("More text\n")

# Read/write JSON
import json

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

with open("data.json", "w") as f:
    json.dump(data, f, indent=2)

# Read/write CSV
import csv

with open("data.csv") as f:
    reader = csv.DictReader(f)
    for row in reader:
        print(row["name"])

Error Handling

# Try/except
try:
    result = 10 / 0
except ZeroDivisionError:
    print("Cannot divide by zero")
except (TypeError, ValueError) as e:
    print(f"Error: {e}")
except Exception as e:
    print(f"Unexpected: {e}")
else:
    print("No errors occurred")
finally:
    print("Always runs")

# Raise exceptions
if age < 0:
    raise ValueError("Age cannot be negative")

# Custom exception
class MyError(Exception):
    pass

Common Modules

# OS and paths
import os
os.getcwd()                    # Current directory
os.listdir(".")                # List directory
os.path.exists("file.txt")    # Check if exists
os.path.join("dir", "file")   # Join paths
os.makedirs("a/b/c", exist_ok=True)
os.environ.get("HOME")        # Environment variable

# pathlib (modern alternative)
from pathlib import Path
p = Path("dir/file.txt")
p.exists()
p.read_text()
p.write_text("content")
list(Path(".").glob("*.py"))   # Find Python files

# Datetime
from datetime import datetime, timedelta
now = datetime.now()
now.strftime("%Y-%m-%d %H:%M:%S")
datetime.strptime("2026-04-07", "%Y-%m-%d")
tomorrow = now + timedelta(days=1)

# Regular expressions
import re
re.search(r"\d+", "abc123")        # Match object
re.findall(r"\d+", "a1b2c3")       # ['1', '2', '3']
re.sub(r"\d+", "X", "a1b2c3")      # 'aXbXcX'

# HTTP requests (install: pip install requests)
import requests
r = requests.get("https://api.example.com/data")
r.status_code      # 200
r.json()            # Parse JSON response

# Subprocess
import subprocess
result = subprocess.run(["ls", "-la"], capture_output=True, text=True)
print(result.stdout)

# System
import sys
sys.argv            # Command-line arguments
sys.exit(1)         # Exit with code

Virtual Environments

# Create virtual environment
python3 -m venv myenv

# Activate
source myenv/bin/activate        # Linux/macOS
myenv\Scripts\activate           # Windows

# Install packages
pip install requests flask

# Save dependencies
pip freeze > requirements.txt

# Install from requirements
pip install -r requirements.txt

# Deactivate
deactivate

One-Liners

# Swap variables
a, b = b, a

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

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

# Reverse a string
s[::-1]

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

# Merge dictionaries (Python 3.9+)
merged = d1 | d2

# Find most common element
max(set(lst), key=lst.count)

# Read file into list of lines
lines = open("file.txt").read().splitlines()

# Simple HTTP server
# python3 -m http.server 8000

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