Computer Science for STEM The PreTeXt Interactive Edition

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 pattern:

We’ve already seen the while and for loops which follow 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 lists of numeric data and a common operation we need is to find the size of the range of numbers in a list. It would make sense if we did not have to duplicate all the steps each time we want to find the size of the range. "printRange" can be thought of as an abstraction of a number of smaller steps. We will need to provide one piece of information for the function to do its work: the list we need the range of.

Here is a program containing a function to capture this idea. Give it a try.

This function is named printRange. It has one parameter --- a variable storing a list of numbers. In the function definition this parameter is called lst. Make sure you know where the body of the function ends — it depends on the indentation and the blank lines don’t count for this purpose!

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 print, range and 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 last line of the program, we call the function, and pass my_list1 as the list to be analyzed.

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 the size of the range of multiple lists. Make sure you can identify all three invocations of the printRange function.

Check your understanding

def printSquare(size):
    """Print a square of asterices with side size."""
    for i in range(size):
        print("*"*size)

def printSquare(size):
    """Print a square of asterices with side size."""
    for i in range(size):
        print("*"*size)

def printSquare(size):
  """Print a square of asterices with side size."""
  for i in range(size):
      print("*"*size)