Python 3 — Functions & Reusable Code

# Define function
def greet():
    print("Welcome back, coder!")
    print("Let's write some Python code.")

# Call the function (can call multiple times)
greet()
greet()

# Function with docstring (documentation)
def show_info():
    """Displays program information to the user."""
    print("=== Python Tutorial v1.0 ===")
    print("Developed with Our Compiler")

show_info()
print(show_info.__doc__)  # Access docstring

# Function with parameters
def calculate_area(length, width):
    area = length * width
    return area

result = calculate_area(5, 10)
print(f"Area: {result}")   # Area: 50

# Multiple return values (returns a tuple)
def min_max(numbers):
    return min(numbers), max(numbers)

low, high = min_max([3, 7, 1, 9, 4])
print(f"Min: {low}, Max: {high}")   # Min: 1, Max: 9

# Returning early
def divide(a, b):
    if b == 0:
        return None  # Early return
    return a / b

print(divide(10, 2))   # 5.0
print(divide(10, 0))   # None

def greet_user(name, greeting="Hello", punctuation="!"):
    print(f"{greeting}, {name}{punctuation}")

greet_user("Balaji")               # Hello, Balaji!
greet_user("Alice", "Hi")          # Hi, Alice!
greet_user("Bob", "Hey", ".")      # Hey, Bob.

# Power function with default exponent
def power(base, exp=2):
    return base ** exp

print(power(5))       # 25 (5 squared by default)
print(power(5, 3))    # 125 (5 cubed)

# *args — variable positional arguments
def add_all(*numbers):
    print(f"Numbers received: {numbers}")  # It's a tuple
    return sum(numbers)

print(add_all(1, 2))          # 3
print(add_all(1, 2, 3, 4, 5)) # 15

# **kwargs — variable keyword arguments
def display_profile(**info):
    print(f"Profile info: {info}")   # It's a dict
    for key, value in info.items():
        print(f"  {key}: {value}")

display_profile(name="Balaji", age=25, city="Hyderabad")

# Combining all types
def full_example(required, *args, default="ok", **kwargs):
    print(f"required={required}, args={args}, default={default}, kwargs={kwargs}")

app_name = "Our Compiler"   # Global variable

def display():
    local_msg = "Inside function"  # Local variable
    print(app_name)   # Can READ global variables
    print(local_msg)

display()
# print(local_msg)  # ❌ NameError: not accessible outside

# Modifying a global variable inside a function
counter = 0

def increment():
    global counter    # Declare intent to modify global
    counter += 1

increment()
increment()
print(f"Counter: {counter}")  # Counter: 2

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

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

# Lambda with multiple parameters
add = lambda a, b: a + b
print(add(3, 7))   # 10

# Lambdas shine when used with sorted(), map(), filter()
students = [("Alice", 92), ("Bob", 78), ("Charlie", 85)]

# Sort by score (second element)
students.sort(key=lambda s: s[1])
print(students)  # Sorted by score ascending

# Filter students who scored above 80
top = list(filter(lambda s: s[1] > 80, students))
print(top)

# Double all scores
doubled = list(map(lambda s: (s[0], s[1] * 2), students))
print(doubled)

# Factorial: 5! = 5 × 4 × 3 × 2 × 1 = 120
def factorial(n):
    if n == 0 or n == 1:   # Base case
        return 1
    return n * factorial(n - 1)  # Recursive case

print(factorial(5))   # 120
print(factorial(10))  # 3628800

# Fibonacci sequence
def fib(n):
    if n <= 1:             # Base case
        return n
    return fib(n-1) + fib(n-2)  # Recursive case

for i in range(10):
    print(fib(i), end=" ")  # 0 1 1 2 3 5 8 13 21 34