Python Cheat Sheet

1. Basics

print("Hello World")

# Single line comment
'''This is 
a multi-line comment'''

x = 5
y = "Hello"

int      # Integer
float    # Decimal
str      # String
list     # [1,2,3]
tuple    # (1,2,3)
dict     # {"key": "value"}
set      # {1,2,3}
bool     # True / False

int("5")     # Convert string to int
str(5)       # Convert int to string
float("3.14")# Convert string to float

2. Operators

+   Addition        (5 + 3 = 8)
-   Subtraction     (5 - 3 = 2)
*   Multiplication  (5 * 3 = 15)
/   Division        (5 / 2 = 2.5)
//  Floor Division  (5 // 2 = 2)
%   Modulus         (5 % 2 = 1)
**  Exponentiation  (2 ** 3 = 8)

==   Equal
!=   Not equal
>    Greater than
<    Less than
>=   Greater or equal
<=   Less or equal

and   Returns True if both are true
or    Returns True if one is true
not   Reverses the result

x in list       # True if x exists in list
x not in list   # True if x does not exist

x is y         # True if same object
x is not y     # True if not same object

3. Control Flow

x = 10
if x > 5:
    print("Greater than 5")
elif x == 5:
    print("Equal to 5")
else:
    print("Less than 5")

for i in range(5):
    print(i)

count = 0
while count < 5:
    print(count)
    count += 1

for i in range(5):
    if i == 3:
        continue   # Skips 3
    if i == 4:
        break      # Stops loop
    print(i)

4. Functions

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

def add(a, b):
    return a + b
print(add(3, 5))

def greet(name="User"):
    print("Hello", name)

def example(*args, **kwargs):
    print(args)
    print(kwargs)

example(1,2,3, name="Aditya", age=16)

square = lambda x: x**2
print(square(5))

x = 10
def func():
    global x
    x = 20

5. Data Structures

s = "Python"
print(s.upper())   # "PYTHON"
print(s.lower())   # "python"
print(s[0:3])      # "Pyt"

mylist = [1, 2, 3]
mylist.append(4)     # [1, 2, 3, 4]
mylist.remove(2)     # [1, 3, 4]
print(mylist[0])     # 1

mytuple = (1, 2, 3)
print(mytuple[1])    # 2

mydict = {"name": "Aditya", "age": 16}
print(mydict["name"])     # Aditya
mydict["age"] = 17

myset = {1, 2, 3}
myset.add(4)        # {1, 2, 3, 4}
myset.remove(2)     # {1, 3, 4}

6. File Handling

with open("file.txt", "r") as f:
    data = f.read()
    print(data)

with open("file.txt", "w") as f:
    f.write("Hello, World!")

with open("file.txt", "a") as f:
    f.write("\\nNew Line")

with open("file.txt", "r") as f:
    for line in f:
        print(line.strip())

import json
data = {"name": "Aditya", "age": 16}
with open("data.json", "w") as f:
    json.dump(data, f)

with open("data.json", "r") as f:
    print(json.load(f))

7. Error & Exception Handling

try:
    x = 10 / 0
except ZeroDivisionError:
    print("Cannot divide by zero")

try:
    x = 5 / 1
except:
    print("Error occurred")
else:
    print("No error, result:", x)

try:
    f = open("file.txt")
except FileNotFoundError:
    print("File not found")
finally:
    print("This will always run")

x = -1
if x < 0:
    raise ValueError("Negative value not allowed")

8. Modules & Packages

import math
print(math.sqrt(16))

from random import randint
print(randint(1, 10))

import numpy as np
arr = np.array([1, 2, 3])

import os
print(os.getcwd())

import datetime
print(datetime.datetime.now())

# Install with pip
pip install requests

# Use installed package
import requests
res = requests.get("https://example.com")
print(res.status_code)

# mymodule.py
def greet(name):
    return f"Hello {name}"

# main.py
import mymodule
print(mymodule.greet("Aditya"))

9. Object-Oriented Programming (OOP)

class Person:
    def __init__(self, name, age):
        self.name = name
        self.age = age

p1 = Person("Aditya", 16)
print(p1.name)

class MyClass:
    count = 0

    def __init__(self):
        MyClass.count += 1

    def instance_method(self):
        return "Instance method"

    @classmethod
    def class_method(cls):
        return f"Objects created: {cls.count}"

    @staticmethod
    def static_method():
        return "Static method"

class Animal:
    def speak(self):
        print("I am an animal")

class Dog(Animal):
    def speak(self):
        print("Woof!")

d = Dog()
d.speak()

class Cat:
    def sound(self):
        return "Meow"

class Dog:
    def sound(self):
        return "Woof"

for animal in (Cat(), Dog()):
    print(animal.sound())

class Book:
    def __init__(self, title):
        self.title = title

    def __str__(self):
        return f"Book: {self.title}"

b = Book("Python 101")
print(b)

10. Advanced Python

squares = [x**2 for x in range(5)]
print(squares)

def gen():
    for i in range(3):
        yield i

for val in gen():
    print(val)

def decorator(func):
    def wrapper():
        print("Before function")
        func()
        print("After function")
    return wrapper

@decorator
def say_hello():
    print("Hello")

say_hello()

with open("file.txt", "w") as f:
    f.write("Hello World")

import re
text = "My number is 12345"
result = re.findall(r"\d+", text)
print(result)   # ['12345']

# Create venv
python -m venv myenv

# Activate (Windows)
myenv\Scripts\activate

# Activate (Linux/Mac)
source myenv/bin/activate

11. Python for Automation

import os
os.system("echo Hello World")

import subprocess
result = subprocess.run(["ls", "-l"], capture_output=True, text=True)
print(result.stdout)

from openpyxl import Workbook
wb = Workbook()
ws = wb.active
ws["A1"] = "Hello Excel"
wb.save("sample.xlsx")

from PyPDF2 import PdfReader
reader = PdfReader("file.pdf")
print(reader.pages[0].extract_text())

import requests
from bs4 import BeautifulSoup

res = requests.get("https://example.com")
soup = BeautifulSoup(res.text, "html.parser")
print(soup.title.text)

import requests
res = requests.get("https://jsonplaceholder.typicode.com/todos/1")
print(res.json())

12. Python for Web Development

from flask import Flask
app = Flask(__name__)

@app.route("/")
def home():
    return "Hello, Flask!"

if __name__ == "__main__":
    app.run(debug=True)

# Install Django
pip install django

# Start project
django-admin startproject myproject

# Run server
python manage.py runserver

from flask import Flask, jsonify
app = Flask(__name__)

@app.route("/api")
def api():
    return jsonify({"message": "Hello API"})

if __name__ == "__main__":
    app.run()

13. Data Handling

import numpy as np
arr = np.array([1, 2, 3])
print(arr.mean())

import pandas as pd
data = {"name": ["Aditya", "Raj"], "age": [16, 18]}
df = pd.DataFrame(data)
print(df.head())

import matplotlib.pyplot as plt
x = [1,2,3]
y = [2,4,6]
plt.plot(x, y)
plt.show()

14. Python for Cybersecurity

import hashlib
print(hashlib.md5(b"password").hexdigest())
print(hashlib.sha256(b"password").hexdigest())

import socket
for port in range(20, 25):
    s = socket.socket(socket.AF_INET, socket.SOCK_STREAM)
    if s.connect_ex(("127.0.0.1", port)) == 0:
        print(f"Port {port} open")
    s.close()

import itertools
chars = "abc123"
for attempt in itertools.product(chars, repeat=3):
    print("".join(attempt))

import os
os.system("nmap -sV 127.0.0.1")

15. Testing & Debugging

import pdb
x = 5
pdb.set_trace()
print(x * 2)

import unittest

def add(a, b):
    return a + b

class TestAdd(unittest.TestCase):
    def test_sum(self):
        self.assertEqual(add(2, 3), 5)

unittest.main()

import logging
logging.basicConfig(level=logging.INFO)
logging.info("This is an info message")