4.15: The global Statement, and Why Global Variables are Evil

def main():
    global FPSCLOCK, DISPLAYSURF
    pygame.init()
    FPSCLOCK = pygame.time.Clock()
    DISPLAYSURF = pygame.display.set_mode((WINDOWWIDTH, WINDOWHEIGHT))

    mousex = 0 # used to store x coordinate of mouse event
    mousey = 0 # used to store y coordinate of mouse event
    pygame.display.set_caption('Memory Game')

This is the start of the main() function, which is where (oddly enough) the main part of the game code is. The functions called in the main() function will be explained later in this chapter.

Line 2 [49] is a global statement. The global statement is the global keyword followed by a comma-delimited list of variable names. These variable names are then marked as global variables. Inside the main() function, those names are not for local variables that might just happen to have the same name as global variables. They are the global variables. Any values assigned to them in the main() function will persist outside the main() function. We are marking the FPSCLOCK and DISPLAYSURF variables as global because they are used in several other functions in the program. (More info is at http://invpy.com/scope.)

There are four simple rules to determine if a variable is local or global:

1. If there is a global statement for a variable at the beginning of the function, then the variable is global.
2. If the name of a variable in a function has the same name as a global variable and the function never assigns the variable a value, then that variable is the global variable.
3. If the name of a variable in a function has the same name as a global variable and the function does assign the variable a value, then that variable is a local variable.
4. If there isn't a global variable with the same name as the variable in the function, then that variable is obviously a local variable.

You generally want to avoid using global variables inside functions. A function is supposed to be like a mini-program inside your program with specific inputs (the parameters) and an output (the return value). But a function that reads and writes to global variables has additional inputs and output. Since the global variable could have been modified in many places before the function was called, it can be tricky to track down a bug involving a bad value set in the global variable.

Having a function as a separate mini-program that doesn’t use global variables makes it easier to find bugs in your code, since the parameters of the function are clearly known. It also makes changing the code in a function easier, since if the new function works with the same parameters and gives the same return value, it will automatically work with the rest of the program just like the old function.

Basically, using global variables might make it easier to write your program but they generally make it harder to debug.

In the games in this book, global variables are mostly used for variables that would be global constants that never change, but need the pygame.init() function called first. Since this happens in the main() function, they are set in the main() function and must be global for other functions to see them. But the global variables are used as constants and don’t change, so they are less likely to cause confusing bugs.

If you don’t understand this, don’t worry. Just write your code so that you pass in values to functions rather than have the functions read global variables as a general rule.