Non-Programmer's Tutorial for Python 2.6/Defining Functions

Creating Functions
To start off this chapter I am going to give you an example of what you could do but shouldn't (so don't type it in):

with the output being:

The absolute values of 23 and 23 are equal

The program seems a little repetitive. Programmers hate to repeat things -- that's what computers are for, after all! (Note also that finding the absolute value changed the value of the variable, which is why it is printing out 23, and not -23 in the output.) Fortunately Python allows you to create functions to remove duplication. Here is the rewritten example:

with the output being:

The absolute values of 23 and -23 are equal

The key feature of this program is the  statement. The  keyword (short for "define") starts a function definition. " " is followed by the name of the function " ". Next, comes the single function parameter named, " ". A parameter holds a value passed into the function from the program that "calls" the function. Parameters of a function in the  statement, must be enclosed within a parenthesis. The value that is passed to a function parameter is called an argument. So for now, a parameter and argument points to the same thing. The block of indented statements after the " " are then executed whenever the function is used. The statements within the function continue to be run until either the indented statements end, or a " " statement is encountered. The  statement returns a value back to the place where the function was called in the calling program.

Notice how the values of  and   are not changed. Functions can be used to repeat tasks that don't return values. Here are some examples:

with output being:

Hello Hello Welcome Fred width = 4 height = 5 area = 20

That example shows some more stuff that you can do with functions. Notice that you can use one or more parameters, or none at all. Notice also that a function doesn't necessarily need to "return" a value, so a  statement is optional.

Variables in functions
When eliminating repeated code, you often notice that variables are repeated in the code. In Python, these are dealt with in a special way. So far, all variables we have seen are global variables. Functions work with a special type of variables called local variables. These variables only exist within the function and only while the function is running. When a local variable has the same name as another variable (such as a global variable), the local variable hides the other. Sound confusing? Well, these next examples (which are a bit contrived) should help clear things up.

When run, we will receive an output of:

in print_func a = 17 a = 4 which is global variable assigned prior to the function print_func

Variable assignments inside a function do not override global variables, they exist only inside the function. Even though  was assigned a new value inside the function, this newly assigned value exists only within the   function. After the function finishes running and the value of an  variable is printed again, we see the value assigned to the global   variable being printed.

Complex example
The output is: in a_func a_var = 15 in a_func b_var = 115 in a_func d_var = 30 in a_func c_var = 25 a_var = 10 b_var = 15 c_var = 125 d_var = Traceback (most recent call last): File "C:\Python24\def2", line 19, in -toplevel- print "d_var = ", d_var NameError: name 'd_var' is not defined In this example the variables,  , and   are all local variables when they are inside the function. After the statement  is run, they all cease to exist. The variable  is "automatically" a local variable since it is a parameter named by the function definition. The variables  and   are local variables since they appear on the left of an equals sign within the function in the statements:   and.

Inside of the function  has no value assigned to it. When the function is called with, 15 is assigned to   since at that point in time   is 15, making the call to the function. This ends up setting the value of  to 15 when it is inside of   function.

As you can see, once the function finishes running, the local variables  and   that had hidden the global variables of the same name are gone. Then the statement  prints the value  rather than the value   since the local variable that hid the global variable is gone.

Another thing to notice is the  that happens at the end. This appears since the variable  no longer exists since   finished. All the local variables are deleted when the function exits. If you want to get something back from a function, then you will have to use  statement within the function.

One last thing to notice is that the value of  remains unchanged inside   since it is not a parameter and it never appears on the left of an equals sign inside of the function. When a global variable is accessed inside a function, the function uses only value of the global variable but it cannot change the value assigned to the global variable outside the function.

Functions allow local variables that exist only inside the function and can hide other variables that are outside the function.

Examples
temperature2.py

Sample Run:

Options: 'p' print options 'c' convert from celsius 'f' convert from fahrenheit 'q' quit the program option: c Celsius temperature: 30 Fahrenheit: 86.0 option: f Fahrenheit temperature: 60 Celsius: 15.5555555556 option: q

area2.py

Sample Run:

Your Name: Josh Hello! Welcome, Josh To find the area of a rectangle, enter the width and height below. Width: -4 Must be a positive number Width: 4 Height: 3 Width = 4 Height = 3 so Area = 12

Exercises
Rewrite the area2.py program from the Examples above to have a separate function for the area of a square, the area of a rectangle, and the area of a circle. This program should include a menu interface.