Python Cheat Sheet: Syntax, Commands and Built-in Functions in Tables

This Python cheat sheet gathers on one page everything you need to keep at hand during your first months: Python syntax, loops, built-in functions, string, list and dictionary methods, and terminal commands. Each table is a short reference with working examples you can paste straight into the REPL.

Python syntax basics

The main feature of the language is that blocks are defined by indentation, not by braces. After a line ending with a colon (if, for, def, class), the following lines are indented by 4 spaces. Do not mix tabs and spaces, or you will get an IndentationError.

Data types

Operators

Conditions and pattern matching

if score >= 90:
    grade = "A"
elif score >= 70:
    grade = "B"
else:
    grade = "C"

status = "adult" if age >= 18 else "minor"   # conditional expression

match command:
    case "start":
        print("Starting")
    case "stop" | "exit":
        print("Stopping")
    case _:
        print("Unknown command")

The following count as false: False, None, 0, the empty string and empty collections, so instead of if len(items) > 0 people simply write if items.

Python loops: for and while syntax

The for loop

The Python for loop syntax is for variable in sequence: followed by a body indented under it. The loop iterates over any iterable object: a string, a list, a dictionary, a file or a range.

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

for i in range(10, 0, -2):      # 10 8 6 4 2
    print(i)

for i, fruit in enumerate(["apple", "pear"], start=1):
    print(i, fruit)

for name, score in zip(names, scores):
    print(name, score)

for key, value in prices.items():
    print(key, value)

The while loop

attempts = 3
while attempts > 0:
    password = input("Password: ")
    if password == "secret":
        print("Logged in")
        break
    attempts -= 1
else:
    print("No attempts left")   # runs if the loop ended without break

List comprehensions

squares = [x ** 2 for x in range(10)]
evens = [x for x in numbers if x % 2 == 0]
lengths = {word: len(word) for word in words}
unique = {c.lower() for c in text}
total = sum(x * x for x in range(100))   # generator expression

Functions

def greet(name: str, greeting: str = "Hello") -> str:
    """Returns a greeting."""
    return f"{greeting}, {name}!"

def total(*args, **kwargs):
    return sum(args), kwargs

square = lambda x: x * x
print(greet("Anna"))                 # Hello, Anna!
print(greet("Anna", greeting="Hi"))  # Hi, Anna!

Do not use a mutable default value such as def f(items=[]): the list is created once and accumulates between calls. The correct way is items=None and, inside, if items is None: items = [].

Python built-in functions

Python built-in functions are available without any import. These are the core functions that appear in almost every script.

The most convenient Python reference is always at hand: the help() function in the REPL. help(dict) shows all dictionary methods with descriptions, and dir(obj) lists the available names.

Python string methods

Python string basics: a string is immutable, and each method returns a new string. Slices work the same way as for lists: s[1:4], and s[::-1] reverses the string.

Python list methods

A list is a mutable ordered collection. Most methods change the list in place and return None, so a = a.sort() is a common mistake.

Python dictionary methods

Tuples and sets

Exceptions and files

try:
    number = int(input("Number: "))
except ValueError as err:
    print("This is not a number:", err)
else:
    print("Success")
finally:
    print("Always runs")

with open("data.txt", "w", encoding="utf-8") as f:
    f.write("a line\n")

with open("data.txt", encoding="utf-8") as f:
    for line in f:
        print(line.rstrip())

Classes

from dataclasses import dataclass

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

    def bark(self) -> str:
        return f"{self.name}: Woof!"

@dataclass
class Point:
    x: float
    y: float

print(Dog("Rex").bark())   # Rex: Woof!
print(Point(1, 2))         # Point(x=1, y=2)

Built-in Python modules

The standard library comes with the interpreter — a single import is enough.

Python 3.15 added lazy imports (PEP 810): lazy import pandas as pd postpones loading the module until it is first used, so a script with heavy dependencies starts noticeably faster.

Python commands in the terminal

These Python commands launch the interpreter, create virtual environments and install libraries. On Windows, people usually write python or py instead of python3.

Common beginner mistakes

• Comparing with = — a condition needs ==; a single = is assignment.
• A missing colon after if, for or def causes a SyntaxError.
• Adding a string and a number: "Age: " + 25 is an error; write f"Age: {25}".
• Changing a list while looping over it — iterate over a copy for x in a[:] or build a new list.
• input() returns a string — wrap it in int() or float() for arithmetic.

Summary

Python syntax rests on indentation, colons and a small set of constructs: conditions, for and while loops, functions and classes. Add two dozen built-in functions and the methods of strings, lists and dictionaries, and you will cover most everyday tasks. Save this cheat sheet, and once the tables feel familiar, move on to modules and asynchronous code. All of this is walked through in practice in the course "Python 3.15: from beginner to pro".