Course 5.2Lesson 1 of 6

Defining and calling a function

A function is a group of lines with a name. The word def starts it, then comes the name, brackets and a colon, and the indented lines are its body. Defining a function only stores it, and calling it with its name and brackets runs the lines. A function can be called as often as you like, so you write the lines once. Functions avoid repeating, make a fix easy to do in one place, and make a program clear to read. A function that is defined but never called does nothing.

about 30 min1 simulation2 practice questions

Lesson 1 of 6Show all lessons

Try the code and the simulations beside the lesson.

What you will be able to do

After this lesson you can

define a function with def,
call it, as often as you like,
say what happens when a function is defined and when it is called, and
explain why functions are useful.

A recipe with a name

You have used functions made by others: print(), len(), range(). You only had to know their names. You did not need to know how len() counts. A function is a group of lines with a name. You write the lines once. Then you use the name whenever you need them.

Think of a recipe in a cookery book. The book has the steps of "tea" once. When you want tea, you do not write the steps again. You say "make tea". A function is the recipe, and calling it is saying "make tea".

A function definition taken apart. The line def area of length and width, with a colon, starts the function and gives its name and its parameters. A line in triple quotation marks is a short note. The indented lines are the body. The word return sends the answer back. Below, area of 5 and 3 gives 15. A note says that defining a function does not run it, and calling it does.

The name of a function follows the rules for the names of variables: small letters, words joined with _, and a name that says what the function does. greet is a good name. g1 is not.

Defining is not calling

This is the first thing to understand, and the most common mistake.

The function say_goodbye is defined, but the program does not show Goodbye!. Why? Because the function was never called. Only the last line of the program runs, and it is a print. To see Goodbye!, add the line say_goodbye() at the end. A function that is not called is like a recipe that nobody cooks.

Notice the brackets. say_goodbye alone, without brackets, is the name of the function, and does not call it. With brackets, say_goodbye() calls it.

The order matters

Python reads from the top. A function must be defined before the line that calls it runs. If you call greet() on line 1 and define it on line 5, Python stops with NameError: name 'greet' is not defined. The usual style is: all the def blocks at the top, and the main program below them.

def only stores the recipe under a name. The brackets in greet() are what make it run.

Why use functions?

Three reasons, and each will be useful in your own programs.

No repeating. Write the lines once, use them many times. A program that repeats the same five lines in ten places is long and boring to read.
Easy to fix. If the lines are wrong, you change them in one place, and every call is fixed.
Clear. A good name tells what happens. A main program that reads print_report() is easier to understand than thirty lines.

There is a fourth reason, which you will feel later: you can test a function alone, and trust it afterwards.

Common mistakes

Defining and not calling. Nothing happens.
Forgetting the brackets in the call: greet does nothing, greet() runs.
Forgetting the colon in the def line.
Forgetting the indent of the body (an IndentationError).
Calling the function above its def. A NameError.

Practice

Start with two quick questions. Each New numbers gives you a fresh one, so you can practise as long as you like.

Then the labs and the problems:

Warm-up. Open the lab Say it with a function. You define a function and call it twice.
Build. Open the problem Say hello three times and the problem Greet by name.
Stretch. Open the problem Spot the missing call and say, for four programs, whether they show anything.

Remember these

def name(): followed by indented lines defines a function. Defining does not run it.
name() calls it and runs the lines.
A function must be defined before the line that calls it.
Functions avoid repeating, are easy to fix, and make a program clear.