In Python, a function is a named sequence of statements that belong together. Their primary purpose is to help us organize programs into chunks that match how we think about the solution to the problem.
The syntax for a function definition is:
def name( parameters ): statements
You can make up any names you want for the functions you create, except that you can’t use a name that is a Python keyword, and the names must follow the rules for legal identifiers that were given previously. The parameters specify what information, if any, you have to provide in order to use the new function. Another way to say this is that the parameters specify what the function needs to do its work.
There can be any number of statements inside the function, but they have to be
indented from the
def. In the examples in this book, we will use the
standard indentation of four spaces. Function definitions are the second of
several compound statements we will see, all of which have the same
A header line which begins with a keyword and ends with a colon.
A body consisting of one or more Python statements, each indented the same amount – 4 spaces is the Python standard – from the header line.
We’ve already seen the
for loop which follows this pattern.
In a function definition, the keyword in the header is
def, which is
followed by the name of the function and some parameters enclosed in
parentheses. The parameter list may be empty, or it may contain any number of
parameters separated from one another by commas. In either case, the parentheses are required.
We need to say a bit more about the parameters. In the definition, the parameter list is more specifically known as the formal parameters. This list of names describes those things that the function will need to receive from the user of the function. When you use a function, you provide values to the formal parameters.
The figure below shows this relationship. A function needs certain information to do its work. These values, often called arguments or actual parameters, are passed to the function by the user.
This type of diagram is often called a black-box diagram because it only states the requirements from the perspective of the user. The user must know the name of the function and what arguments need to be passed. The details of how the function works are hidden inside the “black-box”.
Suppose we’re working with turtles and a common operation we need is to draw squares. It would make sense if we did not have to duplicate all the steps each time we want to make a square. “Draw a square” can be thought of as an abstraction of a number of smaller steps. We will need to provide two pieces of information for the function to do its work: a turtle to do the drawing and a size for the side of the square. We could represent this using the following black-box diagram.
Here is a program containing a function to capture this idea. Give it a try.
This function is named
drawSquare. It has two parameters — one to tell
the function which turtle to move around and the other to tell it the size
of the square we want drawn. In the function definition they are called
sz respectively. Make sure you know where the body of the function
ends — it depends on the indentation and the blank lines don’t count for
If the first thing after the function header is a string (some tools insist that it must be a triple-quoted string), it is called a docstring and gets special treatment in Python and in some of the programming tools.
Another way to retrieve this information is to use the interactive
interpreter, and enter the expression
<function_name>.__doc__, which will retrieve the
docstring for the function. So the string you write as documentation at the start of a function is
retrievable by python tools at runtime. This is different from comments in your code,
which are completely eliminated when the program is parsed.
By convention, Python programmers use docstrings for the key documentation of their functions.
Defining a new function does not make the function run. To do that we need a
function call. This is also known as a function invocation. We’ve already seen how to call some built-in functions like
int. Function calls contain the name of the function to be
executed followed by a list of values in parentheses, called arguments, which are assigned
to the parameters in the function definition.
So in the second to the last line of
the program, we call the function, and pass
alex as the turtle to be manipulated,
and 50 as the size of the square we want.
Once we’ve defined a function, we can call it as often as we like and its
statements will be executed each time we call it. In this case, we could use it to get
one of our turtles to draw a square and then we can move the turtle and have it draw a different square in a
different location. Note that we lift the tail so that when
alex moves there is no trace. We put the tail
back down before drawing the next square. Make sure you can identify both invocations of the
In the next example, we’ve changed the
function a little and we get
tess to draw 15 squares with some variations. Once the function has
been defined, we can call it as many times as we like with whatever actual parameters we like.
Even if a function call needs no arguments,
( ) after the function name are required. This
can lead to a difficult bug: A function name without the
parenthesis is a legal expression referring to the function; for example,
alex.penup, but they do
not call the associated functions.
This workspace is provided for your convenience. You can use this activecode window to try out anything you like.
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