Functions

Creating reusable code blocks with functions, parameters, and return values.

Defining Functions

Use 'def' keyword to define a function.
Functions can take parameters (inputs).
Use 'return' to send back a value.
Call function by name with parentheses.

# Simple function
def greet():
    print("Hello!")

greet()  # Call the function

# Function with parameters
def greet_person(name):
    print(f"Hello, {name}!")

greet_person("Alice")

# Function with return value
def add(a, b):
    return a + b

result = add(5, 3)
print(result)  # 8

Default Parameters

Set default values for parameters.
Optional parameters must come after required ones.
Caller can override default values.

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

print(greet("Alice"))              # Hello, Alice!
print(greet("Bob", "Hi"))          # Hi, Bob!

def power(base, exponent=2):
    return base ** exponent

print(power(5))      # 25 (5^2)
print(power(5, 3))   # 125 (5^3)

Multiple Return Values

Return multiple values as a tuple.
Unpack returned values into variables.
Useful for functions that compute multiple results.

def get_stats(numbers):
    total = sum(numbers)
    count = len(numbers)
    average = total / count
    return total, count, average

nums = [10, 20, 30, 40]
total, count, avg = get_stats(nums)
print(f"Total: {total}, Count: {count}, Avg: {avg}")

Lambda Functions

Small anonymous functions defined with 'lambda'.
Useful for short, simple operations.
Often used with map(), filter(), sorted().

# Regular function
def square(x):
    return x ** 2

# Lambda equivalent
square = lambda x: x ** 2
print(square(5))  # 25

# With map
numbers = [1, 2, 3, 4]
squared = list(map(lambda x: x ** 2, numbers))
print(squared)  # [1, 4, 9, 16]

# With filter
evens = list(filter(lambda x: x % 2 == 0, numbers))
print(evens)  # [2, 4]

Scope

Local variables exist only inside functions.
Global variables exist outside functions.
Use 'global' keyword to modify global variables inside functions.

# Global variable
count = 0

def increment():
    global count
    count += 1

increment()
print(count)  # 1

# Local variable
def calculate():
    result = 10 * 5  # Local to this function
    return result

print(calculate())  # 50
# print(result)     # Error: result not defined