Python Cheatsheet

Setup & Environment

  • Install Python (macOS)
brew install python
  • Install Python (Linux)
# Ubuntu/Debian
sudo apt update
sudo apt install python3 python3-pip

# CentOS/RHEL
sudo yum install python3 python3-pip
  • Install Python (Windows)
  1. Download from python.org
  2. Run installer and check "Add to PATH"
  3. Or use Microsoft Store
  • Check Python version
python --version
python3 --version
  • Create virtual environment
python -m venv myenv
source myenv/bin/activate  # Linux/macOS
myenv\Scripts\activate     # Windows
  • Deactivate virtual environment
deactivate

Package Management

  • Install package
pip install requests
  • Install specific version
pip install requests==2.28.1
  • Install from requirements file
pip install -r requirements.txt
  • Generate requirements file
pip freeze > requirements.txt
  • List installed packages
pip list
  • Show package info
pip show requests
  • Upgrade package
pip install --upgrade requests
  • Uninstall package
pip uninstall requests

Running Python

  • Run Python file
python script.py
  • Run Python interactively
python
  • Run module as script
python -m http.server 8000
  • Run with specific interpreter
python3.9 script.py

Basic Python Structure

#!/usr/bin/env python3

def main():
    print("Hello, World!")

if __name__ == "__main__":
    main()

Variables and Data Types

  • Numbers
integer = 42
float_num = 3.14
complex_num = 1 + 2j
  • Strings
text = "Hello"
multiline = """This is a
multiline string"""
f_string = f"Hello {name}"
  • Booleans
is_true = True
is_false = False
  • Lists
numbers = [1, 2, 3, 4, 5]
mixed = [1, "hello", 3.14, True]
  • Tuples
coordinates = (10, 20)
single_item = (42,)
  • Dictionaries
person = {"name": "Alice", "age": 30}
empty_dict = {}
  • Sets
unique_numbers = {1, 2, 3, 4, 5}
empty_set = set()

Control Flow

  • If/elif/else
if x > 0:
    print("Positive")
elif x < 0:
    print("Negative")
else:
    print("Zero")
  • For loop
for i in range(10):
    print(i)

for item in [1, 2, 3]:
    print(item)

for key, value in {"a": 1, "b": 2}.items():
    print(key, value)
  • While loop
i = 0
while i < 10:
    print(i)
    i += 1
  • List comprehension
squares = [x**2 for x in range(10)]
evens = [x for x in range(20) if x % 2 == 0]
  • Match statement (Python 3.10+)
match status:
    case "active":
        print("System is active")
    case "inactive":
        print("System is inactive")
    case _:
        print("Unknown status")

Functions

  • Define function
def greet(name, greeting="Hello"):
    return f"{greeting}, {name}!"

def add_numbers(*args):
    return sum(args)

def create_person(**kwargs):
    return kwargs
  • Lambda functions
square = lambda x: x**2
add = lambda x, y: x + y
  • Decorators
def my_decorator(func):
    def wrapper(*args, **kwargs):
        print("Before function call")
        result = func(*args, **kwargs)
        print("After function call")
        return result
    return wrapper

@my_decorator
def say_hello():
    print("Hello!")

Classes and Objects

  • Define class
class Person:
    def __init__(self, name, age):
        self.name = name
        self.age = age
    
    def greet(self):
        return f"Hello, I'm {self.name}"
    
    def __str__(self):
        return f"Person(name='{self.name}', age={self.age})"
  • Inheritance
class Student(Person):
    def __init__(self, name, age, student_id):
        super().__init__(name, age)
        self.student_id = student_id
    
    def study(self):
        return f"{self.name} is studying"
  • Properties
class Circle:
    def __init__(self, radius):
        self._radius = radius
    
    @property
    def radius(self):
        return self._radius
    
    @radius.setter
    def radius(self, value):
        if value < 0:
            raise ValueError("Radius cannot be negative")
        self._radius = value
    
    @property
    def area(self):
        return 3.14159 * self._radius ** 2

Exception Handling

  • Try/except/finally
try:
    result = 10 / 0
except ZeroDivisionError as e:
    print(f"Error: {e}")
except Exception as e:
    print(f"Unexpected error: {e}")
finally:
    print("This always executes")
  • Raise exceptions
def check_age(age):
    if age < 0:
        raise ValueError("Age cannot be negative")
    return age

def custom_exception():
    raise Exception("Something went wrong")
  • Custom exceptions
class CustomError(Exception):
    def __init__(self, message):
        self.message = message
        super().__init__(self.message)

raise CustomError("This is a custom error")

File I/O

  • Read file
with open("file.txt", "r") as f:
    content = f.read()

with open("file.txt", "r") as f:
    lines = f.readlines()

with open("file.txt", "r") as f:
    for line in f:
        print(line.strip())
  • Write file
with open("output.txt", "w") as f:
    f.write("Hello, World!")

with open("output.txt", "a") as f:
    f.write("Appended text")
  • JSON files
import json

# Write JSON
data = {"name": "Alice", "age": 30}
with open("data.json", "w") as f:
    json.dump(data, f)

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

Common Built-in Functions

  • Type and conversion
type(42)           # <class 'int'>
int("42")          # 42
float("3.14")      # 3.14
str(42)            # "42"
bool(1)            # True
  • Math functions
abs(-5)            # 5
round(3.14159, 2)  # 3.14
min(1, 2, 3)       # 1
max(1, 2, 3)       # 3
sum([1, 2, 3])     # 6
  • Sequence functions
len([1, 2, 3])     # 3
sorted([3, 1, 2])  # [1, 2, 3]
reversed([1, 2, 3]) # [3, 2, 1]
enumerate(["a", "b", "c"])  # [(0, 'a'), (1, 'b'), (2, 'c')]
zip([1, 2, 3], ["a", "b", "c"])  # [(1, 'a'), (2, 'b'), (3, 'c')]
  • Filter and map
list(filter(lambda x: x > 0, [-1, 0, 1, 2]))  # [1, 2]
list(map(lambda x: x**2, [1, 2, 3, 4]))       # [1, 4, 9, 16]

String Methods

text = "Hello World"
text.upper()           # "HELLO WORLD"
text.lower()           # "hello world"
text.strip()           # Remove whitespace
text.replace("World", "Python")  # "Hello Python"
text.split()           # ["Hello", "World"]
text.startswith("Hello")  # True
text.endswith("World")    # True
text.find("World")        # 6
"Python" in text          # False

List Methods

numbers = [1, 2, 3]
numbers.append(4)      # [1, 2, 3, 4]
numbers.insert(0, 0)   # [0, 1, 2, 3, 4]
numbers.remove(2)      # [0, 1, 3, 4]
numbers.pop()          # Returns and removes last item
numbers.sort()         # Sort in place
numbers.reverse()      # Reverse in place
numbers.count(1)       # Count occurrences
numbers.index(3)       # Find index of item

Dictionary Methods

person = {"name": "Alice", "age": 30}
person.keys()          # dict_keys(['name', 'age'])
person.values()        # dict_values(['Alice', 30])
person.items()         # dict_items([('name', 'Alice'), ('age', 30)])
person.get("name")     # "Alice"
person.get("email", "N/A")  # "N/A" (default value)
person.update({"city": "New York"})  # Add/update items
person.pop("age")      # Remove and return value

Modules and Imports

  • Import module
import math
import os
from datetime import datetime
from collections import Counter
import json as js
  • Create module (save as mymodule.py)
def greet(name):
    return f"Hello, {name}!"

PI = 3.14159
  • Use module
import mymodule
print(mymodule.greet("Alice"))
print(mymodule.PI)

Common Standard Library Modules

  • datetime
from datetime import datetime, timedelta

now = datetime.now()
tomorrow = now + timedelta(days=1)
formatted = now.strftime("%Y-%m-%d %H:%M:%S")
  • os and pathlib
import os
from pathlib import Path

os.getcwd()            # Current directory
os.listdir(".")        # List files
os.path.exists("file.txt")  # Check if file exists

path = Path("folder/file.txt")
path.parent            # folder
path.name              # file.txt
path.suffix            # .txt
  • random
import random

random.randint(1, 10)  # Random integer
random.choice([1, 2, 3, 4, 5])  # Random choice
random.shuffle([1, 2, 3, 4, 5])  # Shuffle list
  • collections
from collections import Counter, defaultdict

counter = Counter([1, 2, 2, 3, 3, 3])
# Counter({3: 3, 2: 2, 1: 1})

dd = defaultdict(list)
dd["key"].append("value")

Virtual Environment Best Practices

  • Create project-specific environment
python -m venv project_env
source project_env/bin/activate
  • Install packages for project
pip install requests flask
pip freeze > requirements.txt
  • Recreate environment
pip install -r requirements.txt

Testing

  • Simple test with assert
def test_add():
    assert add(2, 3) == 5
    assert add(-1, 1) == 0
  • unittest module
import unittest

class TestMath(unittest.TestCase):
    def test_add(self):
        self.assertEqual(add(2, 3), 5)
    
    def test_subtract(self):
        self.assertEqual(subtract(5, 3), 2)

if __name__ == "__main__":
    unittest.main()
  • pytest (install with pip install pytest)
def test_add():
    assert add(2, 3) == 5

def test_subtract():
    assert subtract(5, 3) == 2

Common Patterns

  • Context managers
class MyContext:
    def __enter__(self):
        print("Entering context")
        return self
    
    def __exit__(self, exc_type, exc_val, exc_tb):
        print("Exiting context")

with MyContext() as ctx:
    print("Inside context")
  • Generators
def count_up_to(n):
    count = 1
    while count <= n:
        yield count
        count += 1

for num in count_up_to(5):
    print(num)
  • Iterator protocol
class Counter:
    def __init__(self, max_count):
        self.max_count = max_count
        self.count = 0
    
    def __iter__(self):
        return self
    
    def __next__(self):
        if self.count < self.max_count:
            self.count += 1
            return self.count
        raise StopIteration

Useful Third-Party Packages

  • requests — HTTP library
pip install requests
  • pandas — Data manipulation
pip install pandas
  • numpy — Numerical computing
pip install numpy
  • flask — Web framework
pip install flask
  • pytest — Testing framework
pip install pytest

Resources