One button, many drinks

A coffee machine has a button called Latte. You press it. You do not need to know how it works inside.

Programmers make buttons like this. A button is a piece of code with a name. You write it once. Then you use it as often as you like.

Key words

Word	What it means	Example
Function	A named piece of code that does a job and gives back a result.	len("cat") gives back 3
Procedure	A named piece of code that does a job and gives back no result.	print("hi") shows text
Subroutine	The general word for a function or a procedure.
Parameter	A name in the brackets ( ) of a definition. It receives a value.	def area(width, height)
Argument	The real value you send in when you call it.	area(3, 4)
Return value	The result a function gives back.
Call	To use a function or a procedure by writing its name.	area(3, 4)
Variable	A name that holds a value.
def	The Python word that starts a definition.

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Why use them?

Without them you copy the same lines again and again. A mistake means fixing every copy.

With a subroutine you write the lines once and give them a name. You fix the mistake in one place. The program is also shorter and easier to read.

Try it: a procedure

Code: python

def welcome():
    print("Welcome to the bus!")
    print("Please tap your card.")

welcome()
welcome()

# Call it a third time. Then change a message.

Sending values in

A subroutine may take parameters. They are names in the brackets. When you call it, you send arguments: the real values.

It may also take none. welcome() has empty brackets.

Try it: a parameter

Code: python

def welcome(name):
    print("Welcome,", name)

welcome("Sam")
welcome("Aisha")

# name is the parameter. "Sam" is the argument.
# Call it with your own name.

Giving a result back

A function must give back a result. It uses the word return. The code that called it can keep the result in a variable.

A procedure does a job, such as printing. It gives back no result.

Try it: a function

Code: python

def ticket_cost(adults, children):
    return adults * 2.5 + children * 1.0

total = ticket_cost(2, 3)
print("The total is", total)
print("Double it:", total * 2)

# Try 1 adult and 4 children.

Function or procedure?

	Function	Procedure
Does a named job	Yes	Yes
May take parameters	Yes, or none	Yes, or none
Gives back a result	Yes, it must	No, it must not
Example	len(), max(), ticket_cost()	print(), welcome()

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print is not return

data-callouts.warning.label

print shows text on the screen. return sends a value back to the code that called the function. A function that only prints gives back nothing. Python calls that value None.

Fix it: the missing return

Code: python

def double(number):
    print(number * 2)

answer = double(4)
print("The answer is", answer)

# It prints None. Change print to return. Then run it again.

Fix it: a missing argument

Code: python

def greet(name, time_of_day):
    print("Good", time_of_day + ",", name)

greet("Sam")

# Run it and read the message. The procedure needs two arguments.
# Add the second one, for example "morning".