# How to Think Like a Computer Scientist: The PreTeXt Interactive Edition

## Section6.8Using a Main Function

Using functions is a good idea. It helps us to modularize our code by breaking a program into logical parts where each part is responsible for a specific task. For example, in one of our first programs there was a function called drawSquare that was responsible for having some turtle draw a square of some size. The actual turtle and the actual size of the square were defined to be provided as parameters. Here is that original program.
import turtle

def drawSquare(t, sz):
"""Make turtle t draw a square of with side sz."""

for i in range(4):
t.forward(sz)
t.left(90)

wn = turtle.Screen()          # Set up the window and its attributes
wn.bgcolor("lightgreen")

alex = turtle.Turtle()        # create alex
drawSquare(alex, 50)          # Call the function to draw the square

wn.exitonclick()

If you look closely at the structure of this program, you will notice that we first perform all of our necessary import statements, in this case to be able to use the turtle module. Next, we define the function drawSquare. At this point, we could have defined as many functions as were needed. Finally, there are five statements that set up the window, create the turtle, perform the function invocation, and wait for a user click to terminate the program.
These final five statements perform the main processing that the program will do. Notice that much of the detail has been pushed inside the drawSquare function. However, there are still these five lines of code that are needed to get things done.
In many programming languages (e.g. Java and C++), it is not possible to simply have statements sitting alone like this at the bottom of the program. They are required to be part of a special function that is automatically invoked by the operating system when the program is executed. This special function is called main. Although this is not required by the Python programming language, it is actually a good idea that we can incorporate into the logical structure of our program. In other words, these five lines are logically related to one another in that they provide the main tasks that the program will perform. Since functions are designed to allow us to break up a program into logical pieces, it makes sense to call this piece main.
The following activecode shows this idea. In line 11 we have defined a new function called main that doesn’t need any parameters. The five lines of main processing are now placed inside this function. Finally, in order to execute that main processing code, we need to invoke the main function (line 20). When you push run, you will see that the program works the same as it did before.
Now our program structure is as follows. First, import any modules that will be required. Second, define any functions that will be needed. Third, define a main function that will get the process started. And finally, invoke the main function (which will in turn call the other functions as needed).

### Note6.8.1.

In Python there is nothing special about the name main. We could have called this function anything we wanted. We chose main just to be consistent with some of the other languages.
Before the Python interpreter executes your program, it defines a few special variables. One of those variables is called __name__ and it is automatically set to the string value "__main__" when the program is being executed by itself in a standalone fashion. On the other hand, if the program is being imported by another program, then the __name__ variable is set to the name of that module. This means that we can know whether the program is being run by itself or whether it is being used by another program and based on that observation, we may or may not choose to execute some of the code that we have written.
For example, assume that we have written a collection of functions to do some simple math. We can include a main function to invoke these math functions. It is much more likely, however, that these functions will be imported by another program for some other purpose. In that case, we would not want to execute our main function.
The activecode below defines two simple functions and a main.
Line 12 uses an if statement to ask about the value of the __name__ variable. If the value is "__main__", then the main function will be called. Otherwise, it can be assumed that the program is being imported into another program and we do not want to call main because that program will invoke the functions as needed. This ability to conditionally execute our main function can be extremely useful when we are writing code that will potentially be used by others. It allows us to include functionality that the user of the code will not need, most often as part of a testing process to be sure that the functions are working correctly.

### Note6.8.2.

In order to conditionally execute the main function, we used a structure called an if statement to create what is known as selection. This topic will be studied in much more detail later.