Python Functions Explained — Define, Call, and Reuse Code Like a Pro

# --- Defining a function ---
# 'def' tells Python we're creating a reusable block of code.
# 'greet_user' is the name we're giving it.
# The colon starts the function body.
def greet_user():
    # Everything indented here belongs to the function.
    # This code does NOT run yet — we're just defining it.
    print("Hey there! Welcome to TheCodeForge.")
    print("Let's learn Python together.")

# --- Calling the function ---
# Now we actually RUN the code inside the function.
# Just write the function name followed by parentheses.
greet_user()   # First call
greet_user()   # Second call — same result, zero extra effort
greet_user()   # Third call — see how reuse works?

# --- Function with parameters ---
# 'customer_name' and 'item_ordered' are PARAMETERS.
# They're placeholders — they only get real values when the function is called.
def confirm_order(customer_name, item_ordered):
    print(f"Order confirmed for {customer_name}!")
    print(f"You ordered: {item_ordered}")
    print("---")

# When we call the function, we pass ARGUMENTS — the actual values.
# 'Alice' replaces 'customer_name', 'Python Book' replaces 'item_ordered'.
confirm_order("Alice", "Python Book")
confirm_order("Bob", "Mechanical Keyboard")


# --- Default parameter values ---
# If 'discount_percent' isn't provided, it defaults to 10.
# This makes it optional for the caller.
def calculate_final_price(original_price, discount_percent=10):
    # Calculate the discount amount
    discount_amount = original_price * (discount_percent / 100)
    # Subtract the discount from the original price
    final_price = original_price - discount_amount
    print(f"Original: ${original_price:.2f}")
    print(f"Discount: {discount_percent}%  (saves ${discount_amount:.2f})")
    print(f"You pay:  ${final_price:.2f}")
    print("---")

# Calling without a discount — uses the default 10%
calculate_final_price(100)

# Calling WITH a custom discount — overrides the default
calculate_final_price(100, 25)

# --- Basic return ---
# This function CALCULATES something and GIVES IT BACK to the caller.
# Without 'return', the result would be trapped inside the function forever.
def calculate_area_of_rectangle(width, height):
    area = width * height  # Do the calculation
    return area            # Hand the result back to whoever called us

# The returned value lands in 'living_room_area' — we can use it anywhere.
living_room_area = calculate_area_of_rectangle(5, 4)
print(f"Living room area: {living_room_area} square metres")

# We can also use the return value directly in an expression.
total_area = calculate_area_of_rectangle(5, 4) + calculate_area_of_rectangle(3, 3)
print(f"Combined area: {total_area} square metres")


# --- Multiple return paths ---
# The function returns different values based on a condition.
# It stops the moment it hits any 'return' statement.
def classify_temperature(celsius):
    if celsius < 0:
        return "freezing"        # Function exits here if celsius < 0
    elif celsius < 15:
        return "cold"            # Function exits here if 0 <= celsius < 15
    elif celsius < 25:
        return "comfortable"     # Function exits here if 15 <= celsius < 25
    else:
        return "hot"             # Function exits here for everything else

# Each call follows a different path through the function.
print(classify_temperature(-5))   # freezing
print(classify_temperature(10))   # cold
print(classify_temperature(20))   # comfortable
print(classify_temperature(35))   # hot


# --- What happens without return ---
# This function prints but does NOT return anything.
def print_greeting(name):
    print(f"Hello, {name}!")

# Storing the 'result' of a function with no return statement...
result = print_greeting("Maya")
print(f"The return value was: {result}")  # ...gives you None

# --- Local scope demonstration ---
def brew_coffee(bean_type):
    # 'brewing_temperature' is a LOCAL variable.
    # It only exists inside this function.
    brewing_temperature = 93  # degrees Celsius
    result = f"Brewing {bean_type} at {brewing_temperature}°C"
    return result

cup = brew_coffee("Ethiopian Yirgacheffe")
print(cup)

# Trying to access 'brewing_temperature' outside the function causes an error.
# Uncomment the next line to see: NameError: name 'brewing_temperature' is not defined
# print(brewing_temperature)


# --- Global vs local ---
# 'shop_name' is a GLOBAL variable — defined outside any function.
shop_name = "TheCodeForge Coffee"

def display_shop_info():
    # We can READ global variables from inside a function.
    print(f"Welcome to {shop_name}")

display_shop_info()


# --- The right way: pass in, return out ---
# Instead of modifying globals, we pass data in and get new data back.
def apply_loyalty_discount(original_price, loyalty_points):
    # 'discount_rate' is local — it can't leak out and cause side effects.
    discount_rate = min(loyalty_points * 0.01, 0.30)  # cap at 30%
    discounted_price = original_price * (1 - discount_rate)
    return round(discounted_price, 2)

final_price = apply_loyalty_discount(50.00, 15)
print(f"Price after loyalty discount: ${final_price}")