1.3 Functions

Big ideas

• def name(parameters): defines a function; the indented body runs only when the function is called with name(arguments).
• return value hands a result back to the caller and ends the function. A function with no return returns None. Printing is not returning.
• Parameters are local: variables created inside a function vanish when it ends. A function can read a global variable but should take what it needs as parameters.
• Default values (def greet(name, punct="!")) make arguments optional. Keyword arguments (greet(punct="?", name="Al")) can come in any order.
• Design: one job per function, a docstring that says what it does, and small functions that call each other. Test each one alone before wiring them together.

Code you should know

def area(w, h):
    """Area of a rectangle."""
    return w * h

def show(w, h):
    print("area is", w * h)

x = area(3, 4)      # 12 is stored
y = show(3, 4)      # prints, then y is None
print(x, y)
print(area(2, 5) + 1)

area is 12
12 None
11

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

print(greet("Sam"))
print(greet("Sam", "Hi"))
print(greet(punct="?", name="Sam"))

Hello, Sam!
Hi, Sam!
Hello, Sam?

total = 0

def add(n):
    total = n + 1      # a new local variable
    return total

print(add(5))
print(total)

6
0

Worked example

def double(n):
    return n * 2

def quad(n):
    return double(double(n))

print(quad(3) + double(1))

Common mistakes

• Printing inside a function and then trying to use the value: you get None.
• Code after return never runs.
• Forgetting to call the function at all, or calling it before it is defined.
• Changing a global variable inside a function without global creates a local copy instead.