PythonCheatsheet

1# Variables are created on assignment
2name = "Alice"
3age = 30
4price = 19.99
5is_active = True
6 
7# Multiple assignment
8x, y, z = 1, 2, 3
9a = b = c = 0
10 
11# Constants (by convention, uppercase)
12MAX_SIZE = 100
13PI = 3.14159

1# Numeric types
2integer = 42
3floating = 3.14
4complex_num = 3 + 4j
5 
6# Text
7string = "Hello, World"
8 
9# Boolean
10flag = True  # or False
11 
12# None (null equivalent)
13empty = None
14 
15# Check type
16type(integer)      # <class 'int'>
17isinstance(42, int)  # True

1text = "hello world"
2 
3# Methods
4text.upper()          # "HELLO WORLD"
5text.lower()          # "hello world"
6text.title()          # "Hello World"
7text.split()          # ["hello", "world"]
8text.replace("o", "0") # "hell0 w0rld"
9text.strip()          # Remove whitespace
10 
11# f-strings (Python 3.6+)
12name = "Alice"
13age = 30
14f"Hello, {name}! You are {age}."
15 
16# Slicing
17text[0:5]   # "hello"
18text[-5:]   # "world"
19text[::2]   # "hlowrd"

1nums = [1, 2, 3, 4, 5]
2 
3# Access and slice
4nums[0]       # 1
5nums[-1]      # 5
6nums[1:4]     # [2, 3, 4]
7 
8# Modify
9nums.append(6)      # Add to end
10nums.insert(0, 0)   # Insert at index
11nums.pop()          # Remove last
12nums.remove(3)      # Remove by value
13 
14# Useful methods
15len(nums)           # Length
16sum(nums)           # Sum
17sorted(nums)        # Sorted copy
18nums.reverse()      # Reverse in place
19nums.index(2)       # Find index

1person = {
2    "name": "Alice",
3    "age": 30,
4    "city": "NYC"
5}
6 
7# Access
8person["name"]           # "Alice"
9person.get("age")        # 30
10person.get("job", "N/A") # Default value
11 
12# Modify
13person["email"] = "a@b.com"
14del person["city"]
15 
16# Iterate
17person.keys()    # dict_keys
18person.values()  # dict_values
19person.items()   # key-value pairs
20 
21for key, value in person.items():
22    print(f"{key}: {value}")

1# Conditionals
2age = 18
3if age >= 21:
4    print("Adult")
5elif age >= 18:
6    print("Young adult")
7else:
8    print("Minor")
9 
10# For loop
11for i in range(5):
12    print(i)  # 0, 1, 2, 3, 4
13 
14for item in ["a", "b", "c"]:
15    print(item)
16 
17# While loop
18count = 0
19while count < 3:
20    print(count)
21    count += 1
22 
23# Loop control
24# break - exit loop
25# continue - skip iteration

1# Basic function
2def greet(name):
3    return f"Hello, {name}!"
4 
5# Default parameters
6def greet(name="World"):
7    return f"Hello, {name}!"
8 
9# Multiple return values
10def get_stats(nums):
11    return min(nums), max(nums)
12 
13low, high = get_stats([1, 2, 3])
14 
15# *args and **kwargs
16def func(*args, **kwargs):
17    print(args)    # Tuple
18    print(kwargs)  # Dictionary
19 
20# Lambda (anonymous function)
21square = lambda x: x ** 2
22square(5)  # 25

1class Person:
2    def __init__(self, name, age):
3        self.name = name
4        self.age = age
5 
6    def greet(self):
7        return f"Hi, I'm {self.name}"
8 
9    def __str__(self):
10        return f"Person({self.name})"
11 
12# Create instance
13alice = Person("Alice", 30)
14alice.greet()  # "Hi, I'm Alice"
15 
16# Inheritance
17class Student(Person):
18    def __init__(self, name, age, grade):
19        super().__init__(name, age)
20        self.grade = grade

1# List comprehension
2squares = [x**2 for x in range(10)]
3# [0, 1, 4, 9, 16, 25, 36, 49, 64, 81]
4 
5# With condition
6evens = [x for x in range(10) if x % 2 == 0]
7# [0, 2, 4, 6, 8]
8 
9# Dictionary comprehension
10squares_dict = {x: x**2 for x in range(5)}
11# {0: 0, 1: 1, 2: 4, 3: 9, 4: 16}
12 
13# Set comprehension
14unique = {x % 3 for x in range(10)}
15# {0, 1, 2}
16 
17# Nested comprehension
18matrix = [[i*j for j in range(3)]
19          for i in range(3)]

1# Reading a file
2with open("file.txt", "r") as f:
3    content = f.read()       # Entire file
4    # or
5    lines = f.readlines()    # List of lines
6 
7# Writing to a file
8with open("file.txt", "w") as f:
9    f.write("Hello, World!")
10 
11# Appending to a file
12with open("file.txt", "a") as f:
13    f.write("\nNew line")
14 
15# Read line by line (memory efficient)
16with open("large.txt") as f:
17    for line in f:
18        process(line)

1try:
2    result = 10 / 0
3except ZeroDivisionError:
4    print("Cannot divide by zero")
5except Exception as e:
6    print(f"Error: {e}")
7else:
8    print("Success!")  # Runs if no error
9finally:
10    print("Always runs")
11 
12# Raising exceptions
13def validate(age):
14    if age < 0:
15        raise ValueError("Age cannot be negative")
16    return age
17 
18# Common exceptions:
19# ValueError, TypeError, KeyError
20# IndexError, FileNotFoundError

1# Import entire module
2import math
3math.sqrt(16)  # 4.0
4 
5# Import specific items
6from math import sqrt, pi
7sqrt(16)  # 4.0
8 
9# Import with alias
10import numpy as np
11import pandas as pd
12 
13# Import all (avoid in production)
14from math import *
15 
16# Useful standard library modules
17import os        # Operating system
18import sys       # System-specific
19import json      # JSON parsing
20import datetime  # Date and time
21import random    # Random numbers
22import re        # Regular expressions