Of course! Here is a comprehensive Python cheatsheet covering the most essential concepts, from basic syntax to more advanced features. It's structured to be a quick reference.

Python Cheatsheet

Basics & Syntax

• Comments: # This is a single-line comment
  • Multiline: """This is a multiline comment (docstring)"""
• Variables: No declaration needed. Dynamic typing.

  name = "Alice"
  age = 30
  height = 5.6
  is_student = True

• Print:

  print("Hello, World!")
  print(name, age) # Output: Alice 30
  print(f"Hello, {name}") # f-string (recommended)
  print("Hello, " + name) # Concatenation

• Input:

  user_input = input("Enter your name: ") # Returns a string
  print(f"Hello, {user_input}")

Data Types

• Numbers: int, float, complex

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

• Strings: str

  s = "Hello"
  s = 'Hello'
  s = """This is a
  multiline string."""

  • Indexing/Slicing: s[0] is 'H', s[1:4] is 'ell', s[-1] is 'o'.
• Booleans: True, False
• Lists: Ordered, mutable collections. [ ]

  my_list = [1, "apple", 3.14, True]
  my_list.append("banana") # Add to end
  my_list[0] = 10         # Change element
  my_list.remove("apple") # Remove by value

• Tuples: Ordered, immutable collections.

  my_tuple = (1, "apple", 3.14)
  # my_tuple[0] = 10 # This will cause an error

• Dictionaries: Key-value pairs. Unordered (in Python < 3.7), mutable.

  my_dict = {"name": "Alice", "age": 30}
  my_dict["city"] = "New York" # Add or update
  print(my_dict["name"])      # Access value

• Sets: Unordered collections of unique items.

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

Operators

• Arithmetic: , , , (float division), (floor division), (modulo), (exponent)
• Comparison: , , >, <, >=, <=
• Logical: and, or, not
• Membership: in, not in

  print("apple" in ["apple", "banana"]) # True
  print("apple" not in {"orange", "kiwi"}) # True

• Assignment: , , , , , etc.

  x = 5
  x += 3 # x is now 8

Control Flow

• If/Elif/Else:

  if age < 18:
      print("Minor")
  elif age >= 18 and age < 65:
      print("Adult")
  else:
      print("Senior")

• For Loops:

  # Iterate over a list
  for item in my_list:
      print(item)
  # Iterate with index
  for i in range(len(my_list)):
      print(f"Index {i}: {my_list[i]}")
  # Using enumerate (more Pythonic)
  for index, item in enumerate(my_list):
      print(f"Index {index}: {item}")

• While Loops:

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

• Break & Continue:

  • break: Exits the loop immediately.
  • continue: Skips the rest of the current iteration and moves to the next.

Functions

• Definition:

  def greet(name, greeting="Hello"): # name is required, greeting is optional
      """This function greets a person."""
      return f"{greeting}, {name}!"
  message = greet("Bob")
  print(message) # Output: Hello, Bob!

• Arguments:

  • Positional: greet("Charlie")
  • Keyword: greet(name="Charlie", greeting="Hi")
  • *Arbitrary `args` (variable positional):**

    def sum_all(*args):
        return sum(args)
    print(sum_all(1, 2, 3, 4)) # 10

  • Arbitrary `kwargs` (variable keyword):**

    def print_details(**kwargs):
        for key, value in kwargs.items():
            print(f"{key}: {value}")
    print_details(name="David", age=40, city="London")

• Lambda Functions (Anonymous):

  # Single expression, anonymous
  square = lambda x: x**2
  print(square(5)) # 25

Data Structures (Advanced Operations)

• List Comprehensions: (Create lists concisely)

  # Create a list of squares
  squares = [x**2 for x in range(10)]
  # Create a list of squares for even numbers only
  even_squares = [x**2 for x in range(10) if x % 2 == 0]

• Dictionary Comprehensions:

  # Create a dictionary mapping numbers to their squares
  square_dict = {x: x**2 for x in range(5)}

• Set Comprehensions:

  # Create a set of unique lengths of words
  word_lengths = {len(word) for word in ["hello", "world", "python"]}

File I/O

• Reading a file:

  # 'r' for read mode (default)
  with open("my_file.txt", "r") as f:
      content = f.read() # Reads entire file
      # for line in f: # Reads line by line
      #     print(line)

• Writing to a file:

  # 'w' for write mode (overwrites file)
  # 'a' for append mode (adds to end of file)
  with open("my_file.txt", "w") as f:
      f.write("Hello, this is a new line.\n")
      f.write("This is another line.")

Error Handling

• Try/Except/Finally:

  try:
      result = 10 / 0
  except ZeroDivisionError:
      print("Cannot divide by zero!")
  except Exception as e:
      print(f"An error occurred: {e}")
  finally:
      print("This block always runs.")

Object-Oriented Programming (OOP)

• Class Definition:

  class Dog:
      # Class attribute (shared by all instances)
      species = "Canis familiaris"
      # Constructor/Initializer
      def __init__(self, name, age):
          self.name = name
          self.age = age
      # Instance method
      def bark(self):
          return f"{self.name} says Woof!"
      # String representation
      def __str__(self):
          return f"{self.name} is {self.age} years old."

• Creating an Object (Instance):

  my_dog = Dog("Rex", 5)
  print(my_dog) # Uses __str__
  print(my_dog.bark())
  print(my_dog.species)

Modules & Packages

• Importing a Module:

  import math
  print(math.sqrt(16)) # 4.0
  from math import pi, sqrt
  print(sqrt(25)) # 5.0
  import math as m
  print(m.pi) # 3.14159...

• Installing a Package (using pip):

  pip install requests

Useful Built-in Functions

• len(obj): Length of a sequence.
• type(obj): Type of an object.
• isinstance(obj, type): Checks if an object is of a certain type.
• str(obj), int(obj), float(obj): Type conversion.
• max(iterable), min(iterable): Maximum/minimum value.
• sum(iterable): Sum of numbers in an iterable.
• sorted(iterable): Returns a new sorted list.
• range(start, stop, step): Generates a sequence of numbers.
• enumerate(iterable): Returns index and value pairs.
• zip(iterable1, iterable2): Combines iterables into tuples.

  names = ["Alice", "Bob"]
  ages = [30, 25]
  for name, age in zip(names, ages):
      print(f"{name} is {age} years old.")

Popular Libraries

• requests: For making HTTP requests.

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

• pandas: For data analysis and manipulation.

  import pandas as pd
  df = pd.read_csv("data.csv")
  print(df.head())

• numpy: For numerical computing.

  import numpy as np
  arr = np.array([1, 2, 3, 4])
  print(arr * 2) # [2, 4, 6, 8]

• matplotlib & seaborn: For data visualization.

  import matplotlib.pyplot as plt
  plt.plot([1, 2, 3, 4], [1, 4, 9, 16])
  plt.show()