Python Cheatsheet

Python Cheatsheet

Data Types & Variables

# Numbers
x = 10          # int
y = 3.14        # float
z = 2 + 3j      # complex

# Strings
s = "hello"
s2 = 'world'
multi = """multi
line"""

# Boolean
t = True
f = False

# None
n = None

# Type checking / conversion
type(x)         # <class 'int'>
int("42")       # 42
float("3.14")   # 3.14
str(100)        # "100"
bool(0)         # False

String Operations

s = "Hello, World!"

s.upper()           # "HELLO, WORLD!"
s.lower()           # "hello, world!"
s.strip()           # trim whitespace
s.replace("o","0")  # "Hell0, W0rld!"
s.split(", ")       # ["Hello", "World!"]
s.startswith("He")  # True
s.endswith("!")     # True
s.find("World")     # 7
len(s)              # 13

# f-strings (Python 3.6+)
name = "Alice"
age = 30
print(f"{name} is {age} years old")

# Format
"{:.2f}".format(3.14159)  # "3.14"
f"{3.14159:.2f}"          # "3.14"

Collections

# List — ordered, mutable
lst = [1, 2, 3, 4]
lst.append(5)       # [1,2,3,4,5]
lst.insert(0, 0)    # [0,1,2,3,4,5]
lst.remove(3)       # removes first 3
lst.pop()           # removes & returns last
lst.sort()
lst.reverse()
lst[1:3]            # slice [1,2]

# Tuple — ordered, immutable
t = (1, 2, 3)
t[0]                # 1

# Set — unordered, unique
s = {1, 2, 3, 2}    # {1, 2, 3}
s.add(4)
s.discard(1)
a | b               # union
a & b               # intersection
a - b               # difference

# Dictionary — key-value
d = {"name": "Alice", "age": 30}
d["name"]           # "Alice"
d.get("missing", "default")
d.keys()
d.values()
d.items()
d.update({"city": "NYC"})
del d["age"]

Control Flow

# if / elif / else
x = 10
if x > 0:
    print("positive")
elif x == 0:
    print("zero")
else:
    print("negative")

# Ternary
result = "yes" if x > 0 else "no"

# for loop
for i in range(5):          # 0..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, v in enumerate(["a","b","c"]):
    print(i, v)             # 0 a, 1 b, 2 c

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

# break / continue / else on loop
for i in range(10):
    if i == 3: continue
    if i == 7: break
else:
    print("loop finished without break")

Functions

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

# Default args
def power(base, exp=2):
    return base ** exp

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

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

# Lambda
square = lambda x: x ** 2

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

# Generators
def countdown(n):
    while n > 0:
        yield n
        n -= 1

# Decorators
def my_decorator(func):
    def wrapper(*args, **kwargs):
        print("Before")
        result = func(*args, **kwargs)
        print("After")
        return result
    return wrapper

@my_decorator
def say_hi():
    print("Hi!")

List Comprehensions

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

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

# Nested
matrix = [[i*j for j in range(1,4)] for i in range(1,4)]

# Dict comprehension
d = {k: v for k, v in zip("abc", [1,2,3])}

# Set comprehension
unique = {x % 3 for x in range(10)}

# Generator expression (lazy)
gen = (x**2 for x in range(100))

Classes & OOP

class Animal:
    species = "Unknown"          # class variable

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

    def speak(self):
        return f"{self.name} makes a sound"

    @classmethod
    def get_species(cls):
        return cls.species

    @staticmethod
    def is_animal():
        return True

    def __str__(self):
        return f"Animal({self.name})"

    def __repr__(self):
        return f"Animal({self.name!r}, {self.age!r})"

# Inheritance
class Dog(Animal):
    def speak(self):
        return f"{self.name} says Woof!"

    def __init__(self, name, age, breed):
        super().__init__(name, age)
        self.breed = breed

# Dataclass (Python 3.7+)
from dataclasses import dataclass

@dataclass
class Point:
    x: float
    y: float
    z: float = 0.0

Exception Handling

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

# Raise
def divide(a, b):
    if b == 0:
        raise ValueError("Cannot divide by zero")
    return a / b

# Custom exception
class AppError(Exception):
    def __init__(self, msg, code):
        super().__init__(msg)
        self.code = code

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("file.txt", "w") as f:
    f.write("Hello\n")

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

# JSON
import json
data = {"key": "value"}
json_str = json.dumps(data, indent=2)
obj = json.loads(json_str)

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

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

Useful Built-ins & Libraries

# Built-ins
abs(-5)             # 5
round(3.567, 2)     # 3.57
max([1,2,3])        # 3
min([1,2,3])        # 1
sum([1,2,3])        # 6
sorted([3,1,2])     # [1,2,3]
reversed([1,2,3])   # iterator
zip([1,2],[3,4])    # [(1,3),(2,4)]
map(str, [1,2,3])   # ["1","2","3"]
filter(None, [0,1,2,False,3])  # [1,2,3]
any([False, True])  # True
all([True, True])   # True

# os / pathlib
import os
from pathlib import Path
os.getcwd()
Path("dir/file.txt").exists()
Path("dir").mkdir(parents=True, exist_ok=True)

# datetime
from datetime import datetime, timedelta
now = datetime.now()
today = datetime.today().date()
delta = timedelta(days=7)
next_week = now + delta
fmt = now.strftime("%Y-%m-%d %H:%M")

# random
import random
random.random()         # 0.0 – 1.0
random.randint(1, 10)
random.choice([1,2,3])
random.shuffle(lst)