Writing your own functions

A function that ends without return gives back None, which is why print(print("hi")) shows hi and then None.

Two functions that look the same on screen

Most beginners believe that showing a value and handing it back are the same thing. Here are two functions that both seem to "give" the area of a rectangle. def starts a function definition: a name, the inputs in brackets, a colon, and an indented body. Run the program and both lines of output look alike.

def show_area(w, h):
    print(w * h)

def area(w, h):
    return w * h

show_area(4, 3)
print(area(4, 3))

12
12

Now try to use the results in a calculation. The difference appears at once.

def show_area(w, h):
    print(w * h)

def area(w, h):
    return w * h

a = show_area(4, 3)
b = area(4, 3)
print(a, b)
print(b * 2)

12
None 12
24

show_area prints 12 to the screen and then ends. It hands nothing back, and a function that hands nothing back returns the special value None. So a is None. area prints nothing; it returns 12 to the line that called it, and that 12 can be stored, printed, or doubled. Printing is for people looking at the screen. Returning is for the rest of the program. If you want to use a result later, return it.

Parameters are the names, arguments are the values

In def area(w, h):, w and h are parameters: names that exist only inside the function. In area(4, 3), 4 and 3 are arguments: the values that fill those names for that one call. They are matched by position, so the first argument goes to the first parameter. Call the function with the wrong number of arguments and Python stops with a TypeError that names the missing parameter.

A parameter can have a default value, written with = in the definition. Callers can leave that argument out, and the default is used. The Python tutorial shows defaults in its section on defining functions, and they are everywhere in the built-in functions. print itself has one: its end parameter defaults to a new line.

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

print(greet("Mina"))
print(greet("Jun", "Hi"))
print("no new line", end=" ")
print("here")

Hello, Mina!
Hi, Jun!
no new line here

return ends the function straight away

A function stops at the first return it reaches, even if there are more lines below. That makes early returns a tidy way to handle special cases. The function below uses the ticket rule from the unit on if, now packaged so any part of a program can ask for a price.

def ticket_price(age):
    if age < 7:
        return 0
    if age <= 18:
        return 3000
    return 5000

family = [40, 15, 5]
total = 0
for age in family:
    total += ticket_price(age)
print(total)

8000

Variables created inside a function are local: they exist while the function runs and disappear when it ends. A variable named total inside one function has nothing to do with a total outside it. This is what lets you reuse a function without worrying about which names the rest of the program happens to use.