README.md

🐍 Python 3 Cheatsheet

Table of Contents

1. Variables and Data Types
2. Control Flow
3. Functions
4. Collections
5. List Comprehensions
6. File Handling
7. Modules and Packages
8. Error Handling
9. Classes and Objects
10. Useful Libraries
11. Decorators
12. Generators

---

Variables and Data Types 📊

Basic Data Types

# Integer
x = 10

# Float
pi = 3.14

# String
name = "Python"

# Boolean
is_cool = True

# NoneType
nothing = None

Type Conversion

# Convert to string
str_x = str(x)

# Convert to integer
int_pi = int(pi)

# Convert to float
float_x = float(x)

---

Control Flow 🔄

If Statements

if x > 5:
    print("x is greater than 5")
elif x == 5:
    print("x is 5")
else:
    print("x is less than 5")

Loops

For Loop

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

# Iterating over a list
fruits = ["apple", "banana", "cherry"]
for fruit in fruits:
    print(fruit)

While Loop

count = 5
while count > 0:
    print(count)
    count -= 1

Break and Continue

for i in range(10):
    if i == 5:
        break  # Exit the loop
    if i % 2 == 0:
        continue  # Skip the rest of the code in this iteration
    print(i)

---

Functions 🛠️

Defining Functions

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

print(greet("Alice"))

Default Arguments

def greet(name="Guest"):
    return f"Hello, {name}!"

print(greet())

Variable-Length Arguments

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

print(add(1, 2, 3, 4))

Keyword Arguments

def display_info(**kwargs):
    for key, value in kwargs.items():
        print(f"{key}: {value}")

display_info(name="Alice", age=25)

---

Collections 📚

Lists

fruits = ["apple", "banana", "cherry"]
fruits.append("date")
print(fruits)

# Accessing elements
print(fruits[1])  # Output: banana

# Slicing
print(fruits[1:3])  # Output: ['banana', 'cherry']

Tuples

coordinates = (10, 20)
print(coordinates)

# Unpacking
x, y = coordinates
print(x, y)

Sets

unique_numbers = {1, 2, 3, 4, 4}
print(unique_numbers)

# Set operations
a = {1, 2, 3}
b = {3, 4, 5}
print(a & b)  # Intersection
print(a | b)  # Union
print(a - b)  # Difference

Dictionaries

person = {"name": "Alice", "age": 25}
print(person["name"])

# Adding and updating
person["city"] = "New York"
person["age"] = 26

# Iterating over keys and values
for key, value in person.items():
    print(f"{key}: {value}")

---

List Comprehensions 📝

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

# With condition
even_squares = [x**2 for x in range(10) if x % 2 == 0]
print(even_squares)

---

File Handling 📂

Writing to a File

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

Reading from a File

with open("example.txt", "r") as file:
    content = file.read()
    print(content)

Appending to a File

with open("example.txt", "a") as file:
    file.write("\nAppend this line.")

Reading Line by Line

with open("example.txt", "r") as file:
    for line in file:
        print(line.strip())  # strip() removes trailing newline

---

Modules and Packages 📦

Importing Modules

import math
print(math.sqrt(16))

from math import pi
print(pi)

Creating a Module

Create a file mymodule.py:

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

Use the module:

import mymodule
print(mymodule.greet("Alice"))

Installing Packages

pip install requests

---

Error Handling ⚠️

Try-Except Block

try:
    result = 10 / 0
except ZeroDivisionError:
    print("Cannot divide by zero!")
finally:
    print("This will always run.")

Custom Exceptions

class CustomError(Exception):
    pass

try:
    raise CustomError("An error occurred")
except CustomError as e:
    print(e)

---

Classes and Objects 🧩

Defining a Class

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

    def bark(self):
        return f"{self.name} says woof!"

my_dog = Dog("Rex", 5)
print(my_dog.bark())

Inheritance

class Animal:
    def __init__(self, name):
        self.name = name

    def speak(self):
        raise NotImplementedError("Subclass must implement abstract method")

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

my_dog = Dog("Rex")
print(my_dog.speak())

Class and Static Methods

class MyClass:
    @classmethod
    def class_method(cls):
        return "Class method called"

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

print(MyClass.class_method())
print(MyClass.static_method())

---

Useful Libraries 📚

Requests

import requests
response = requests.get("https://api.github.com")
print(response.status_code)
print(response.json())

NumPy

import numpy as np
array = np.array([1, 2, 3, 4])
print(array * 2)

matrix = np.array([[1, 2], [3, 4]])
print(matrix)
print(matrix.T)  # Transpose

Pandas

import pandas as pd
data = {"name": ["Alice", "Bob"], "age": [25, 30]}
df = pd.DataFrame(data)
print(df)

# Basic operations
print(df.describe())
print(df[df["age"] > 25])

Matplotlib

import matplotlib.pyplot as plt

x = [1, 2, 3, 4]
y = [10, 20, 25, 30]

plt.plot(x, y)
plt.xlabel("X axis")
plt.ylabel("Y axis")
plt.title("Simple Plot")
plt.show()

Scikit-Learn

from sklearn.linear_model import LinearRegression
import numpy as np

# Sample data
X = np.array([[1], [2], [3], [4]])
y = np.array([1, 2, 3, 4])

# Create and train the model
model = LinearRegression()
model.fit(X, y)

# Make predictions
predictions = model.predict(np.array([[5]]))
print(predictions)

---

Decorators 🎨

Function Decorators

def my_decorator(func):
    def wrapper():
        print("Something is happening before the function is called.")
        func()
        print("Something is happening after the function is called.")
    return wrapper

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

say_hello()

Class Decorators

def decorator(cls):
    class Wrapped(cls):
        def __init__(self, *args, **kwargs):
            print("Wrapped class")
            super().__init__(*args, **kwargs)
    return Wrapped

@decorator
class MyClass:
    def __init__(self):
        print("Original class")

instance = MyClass()

---

Generators 🚀

Generator Function

def countdown

(n):
    while n > 0:
        yield n
        n -= 1

for number in countdown(5):
    print(number)

Generator Expression

squares = (x**2 for x in range(10))
for square in squares:
    print(square)