Functions — What & Why

Functions are reusable blocks of code you give a name to, so you can run them whenever you want without rewriting the same code.

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Without a Function (Repetitive)

print("Welcome, Alex!")
print("Your adventure begins!")

print("Welcome, Sam!")
print("Your adventure begins!")

print("Welcome, Jordan!")
print("Your adventure begins!")

That's a lot of copy-paste! What if you want to change the message? You'd have to edit it 3 times.

With a Function (Reusable)

def welcome(name):
    print(f"Welcome, {name}!")
    print("Your adventure begins!")

welcome("Alex")
welcome("Sam")
welcome("Jordan")

Now if you want to change the message, you only change it once inside the function.

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Anatomy of a Function

def function_name(parameter1, parameter2):
    # code goes here (indented!)
    return result

• def — keyword that says "I'm creating a function"
• Function name — what you call it (use snake_case)
• Parameters — inputs the function receives (in parentheses)
• Body — the indented code that runs when you call it
• return — sends a value back (optional)

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Return vs Print

This is the most important thing to understand:

# This function PRINTS (shows text, but gives back nothing)
def greet(name):
    print(f"Hi {name}!")

# This function RETURNS (gives back a value you can use)
def double(number):
    return number * 2

Why does it matter?

# With return — you can save and use the result
result = double(5)
print(result)        # 10
print(double(5) + 3) # 13

# With print — the value is gone after printing
result = greet("Alex")  # Prints "Hi Alex!" but...
print(result)            # None! There's nothing to save

Rule of thumb:

• Use print when you just want to display something
• Use return when you need to use the answer later

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Calling a Function

def add(a, b):
    return a + b

# Calling the function:
answer = add(3, 5)
print(answer)  # 8

# You can also use it directly:
print(add(10, 20))  # 30

The values you pass in (3, 5) are called arguments. They get matched to the parameters (a, b) in order.