Python Basics Cheat Sheet

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Python syntax, data types, control flow, functions, and file I/O

Data Types

Numeric Types

int, float, complex — arithmetic and conversions

x = 42          # int
y = 3.14        # float
z = 2 + 3j      # complex
int("10")       # convert string to int

Strings

Immutable sequences of characters

name = "Alice"
greeting = f"Hello, {name}!"  # f-string
multi = """line1
line2"""

Booleans & None

True/False and the null value None

is_active = True
result = None
bool(0)    # False
bool("hi") # True

String Methods

Common Methods

Frequently used string transformation methods

s = "  Hello, World!  "
s.strip()         # "Hello, World!"
s.lower()         # "  hello, world!  "
s.upper()         # "  HELLO, WORLD!  "
s.replace("Hello", "Hi")  # "  Hi, World!  "
s.split(", ")     # ["  Hello", "World!  "]

Slicing & Formatting

Slice strings and format output

s = "Python"
s[0:3]    # "Pyt"
s[-3:]    # "hon"
f"{42:.2f}"   # "42.00"
f"{"hi":>10}"  # "        hi"

Control Flow

if / elif / else

Conditional execution

if score >= 90:
    grade = "A"
elif score >= 80:
    grade = "B"
else:
    grade = "C"

for loop

Iterate over sequences and ranges

for i in range(5):      # 0,1,2,3,4
    print(i)
for item in my_list:
    print(item)
for i, v in enumerate(lst):
    print(i, v)

while loop

Loop while condition is true

while count < 10:
    count += 1
    if count == 5:
        continue   # skip to next
    if count == 8:
        break      # exit loop

List Comprehensions

Concise way to create filtered/transformed lists

squares = [x**2 for x in range(10)]
evens = [x for x in range(20) if x % 2 == 0]
matrix = [[r*c for c in range(3)] for r in range(3)]

Functions

def & return

Define a function with typed hints

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

*args & **kwargs

Accept variable positional and keyword arguments

def total(*args: int) -> int:
    return sum(args)

def display(**kwargs: str) -> None:
    for k, v in kwargs.items():
        print(f"{k}: {v}")

Lambda Functions

Anonymous one-line functions

square = lambda x: x ** 2
sorted_data = sorted(items, key=lambda x: x["age"])

List Operations

List Methods

Modify and query lists in-place or return new values

lst = [3, 1, 4, 1, 5]
lst.append(9)       # [3,1,4,1,5,9]
lst.extend([2, 6])  # extends in-place
lst.insert(0, 0)    # insert at index
lst.remove(1)       # removes first 1
lst.pop()           # removes & returns last
lst.sort()          # sort in-place
lst.reverse()       # reverse in-place

Dictionary Operations

Dict Methods

Common dictionary operations

d = {"name": "Alice", "age": 30}
d.get("email", "N/A")     # safe get with default
d.update({"city": "NYC"})  # merge/update
list(d.keys())             # ["name", "age", "city"]
list(d.values())           # ["Alice", 30, "NYC"]
list(d.items())            # [("name","Alice"), ...]

File I/O

Read & Write Files

Open files safely using context managers

# Read a file
with open("data.txt", "r") as f:
    content = f.read()
    lines = f.readlines()

# Write a file
with open("output.txt", "w") as f:
    f.write("Hello\n")
    f.writelines(["line1\n", "line2\n"])

Common Patterns

List Comprehension

Concise data transformation with filtering

numbers = range(1, 21)

# Basic transformation
squares = [n**2 for n in numbers]

# With filter
even_squares = [n**2 for n in numbers if n % 2 == 0]

# Nested
pairs = [(x, y) for x in range(3) for y in range(3) if x != y]

# Dict comprehension
word_lengths = {word: len(word) for word in ["python", "is", "great"]}

Dictionary Comprehension

Build and transform dictionaries concisely

inventory = {"apple": 5, "banana": 0, "orange": 3}

# Invert a dict
inverted = {v: k for k, v in inventory.items()}

# Filter dict
in_stock = {k: v for k, v in inventory.items() if v > 0}
# {"apple": 5, "orange": 3}

# Transform values
doubled = {k: v * 2 for k, v in inventory.items()}

Context Manager

Use with statement for safe resource handling

import json

# Read JSON file
with open("config.json", "r") as f:
    config = json.load(f)

# Write JSON file
with open("output.json", "w") as f:
    json.dump({"key": "value"}, f, indent=2)

# Multiple context managers
with open("input.txt") as src, open("output.txt", "w") as dst:
    dst.write(src.read().upper())