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Python Cheat Sheet

Welcome to your ultimate Python Cheat Sheet — a fun and easy guide for quick reference! Dive in and let’s make coding in Python even more enjoyable.

1. Variables

  • Declaration: Assign values using =.
x = 10
y = 'hello'
z = [1, 2, 3]
  • Dynamic Typing: Variables can change types.
x = 10
x = 'changed!'
  • Naming Conventions:
    • Use snake_case for variable names (e.g., user_age).
    • Avoid using reserved keywords (e.g., class, def, if).
    • Variable names should be descriptive but concise (e.g., total_price, not x).

2. Strings

  • Manipulation:
s = 'hello world'
print(s.upper())  # HELLO WORLD
print(s.lower())  # hello world
print(s.capitalize())  # Hello world
  • Count Characters:
print(s.count('l'))  # 3
  • Concatenation & Repetition:
x = 'hi'
y = 'bye'
print(x + y)  # hibye
print(x * 3)  # hihihi

3. Lists

  • Mutable: Change values, append, extend, pop.
x = [1, 2, 3]
x[0] = 'changed'
x.append(4)
x.extend([5, 6])
x.pop(1)  # Removes the second item
  • Copy by Reference:
y = x  # Points to the same object
y = x[:]  # Creates a new copy

4. Tuples

  • Immutable: Cannot change elements.
t = (1, 2, 3)
# t[0] = 5  # Error: Tuples cannot be modified

5. Dictionaries

  • Key-Value Pairs:
d = {'key1': 'value1', 'key2': 'value2'}
print(d['key1'])  # value1
d['key3'] = 'value3'
del d['key1']
  • Iteration:
for key, value in d.items():
    print(key, value)

6. Sets

  • No Duplicates:
s = {1, 2, 3, 3}
print(s)  # {1, 2, 3}
  • Operations:
s1 = {1, 2, 3}
s2 = {3, 4, 5}
print(s1.union(s2))  # {1, 2, 3, 4, 5}
print(s1.intersection(s2))  # {3}
print(s1.difference(s2))  # {1, 2}

7. Conditionals

  • If-Else:
if x > 0:
    print('Positive')
elif x == 0:
    print('Zero')
else:
    print('Negative')

8. Loops

  • For Loop:
for i in range(5):
    print(i)
  • While Loop:
while x < 5:
    print(x)
    x += 1
  • Break & Continue:
for i in range(10):
    if i == 5:
        break  # Stops the loop
    if i % 2 == 0:
        continue  # Skips to the next iteration
    print(i)

9. Functions

  • Basic Syntax:
def greet(name):
    return f'Hello {name}!'
  • Default Parameters:
def greet(name, age=30):
    print(f'{name} is {age} years old')
  • Higher-Order Functions:
def apply_function(func, value):
    return func(value)
print(apply_function(lambda x: x**2, 5))  # 25
  • Lambda Functions:
add = lambda x, y: x + y
print(add(2, 3))  # 5

10. Comprehensions

  • Lists, Dictionaries, Tuples:
squares = [x**2 for x in range(5)]
filtered = [x for x in range(10) if x % 2 == 0]
  • Set Comprehensions:
unique_squares = {x**2 for x in range(5)}
  • Generator Expressions:
lazy_squares = (x**2 for x in range(5))

11. Exception Handling

  • Try-Except:
try:
    x = 1 / 0
except ZeroDivisionError as e:
    print(e)
finally:
    print('Always runs')

12. File I/O

  • Read/Write:
with open('file.txt', 'w') as f:
    f.write('Hello, File!')

with open('file.txt', 'r') as f:
    print(f.read())

13. Advanced Concepts

  • Unpacking:
def my_func(*args, **kwargs):
    print(args, kwargs)
my_func(1, 2, three=3, four=4)
  • Map & Filter:
nums = [1, 2, 3, 4]
doubled = list(map(lambda x: x*2, nums))
evens = list(filter(lambda x: x%2 == 0, nums))
  • Decorators:
def decorator(func):
    def wrapper():
        print("Before function call")
        func()
        print("After function call")
    return wrapper

@decorator
def say_hello():
    print("Hello")
say_hello()
  • Context Managers:
with open('file.txt', 'w') as f:
    f.write('Using a context manager')

14. F-Strings

name = 'Alice'
age = 30
print(f'{name} is {age} years old')
# With Format Specifiers
pi = 3.14159
print(f'Pi rounded to two decimals: {pi:.2f}')