Python Basics Cheatsheet

Variables & Types

# Dynamic typing — no declarations needed
name = "Alice"
age = 30
score = 98.6
active = True
nothing = None

# Type hints (optional but recommended)
name: str = "Alice"
age: int = 30

Strings

# f-strings (prefer over .format() or %)
greeting = f"Hello, {name}! You are {age} years old."

# Common string methods
s = "  hello world  "
s.strip()          # "hello world"
s.upper()          # "  HELLO WORLD  "
s.split()          # ["hello", "world"]
", ".join(["a", "b", "c"])  # "a, b, c"
"hello".startswith("he")    # True

Lists

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

items.append(6)          # add to end
items.insert(0, 0)       # insert at index
items.pop()              # remove & return last
items.pop(0)             # remove & return at index
items.remove(3)          # remove first occurrence of value
len(items)               # length
items[0]                 # first element
items[-1]                # last element
items[1:3]               # slice [2, 3]
items[::-1]              # reversed copy

# List comprehension
squares = [x**2 for x in range(10)]
evens = [x for x in range(20) if x % 2 == 0]

Dictionaries

person = {"name": "Alice", "age": 30}

person["name"]                   # "Alice"
person.get("missing", "default") # safe access
person["city"] = "NYC"           # add/update key
del person["age"]                # delete key
"name" in person                 # True
person.keys()                    # dict_keys
person.values()                  # dict_values
person.items()                   # dict_items (key, val tuples)

# Dict comprehension
squares = {x: x**2 for x in range(5)}

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"

# for loop
for item in items:
    print(item)

for i, item in enumerate(items):   # with index
    print(i, item)

for k, v in person.items():        # dict iteration
    print(k, v)

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

Functions

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

# *args and **kwargs
def log(*args, **kwargs):
    print(args, kwargs)

# Lambda
double = lambda x: x * 2

# Type annotations
from typing import Optional, List

def find(items: List[str], target: str) -> Optional[int]:
    try:
        return items.index(target)
    except ValueError:
        return None

Error Handling

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

File I/O

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

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

# Append
with open("log.txt", "a") as f:
    f.write("new line\n")

Useful Built-ins

sorted([3, 1, 2])                      # [1, 2, 3]
sorted(items, key=lambda x: x["age"]) # sort by key
reversed([1, 2, 3])                    # iterator
zip([1, 2], ["a", "b"])               # [(1, "a"), (2, "b")]
map(str, [1, 2, 3])                    # map object
filter(lambda x: x > 2, [1, 2, 3])   # filter object
any([False, True, False])              # True
all([True, True, False])               # False
sum([1, 2, 3])                         # 6
min([3, 1, 2])                         # 1
max([3, 1, 2])                         # 3