6.8 Using a main Function

• 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

Before:

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()

After:

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)

def main():                      # Define the main function
    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()

main()                           # Invoke the main function

Note

• 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.