Skip to main content
Engineering LibreTexts

3.05: Magic Numbers are Bad

  • Page ID
    13580
  • # Memory Puzzle
    # By Al Sweigart al@inventwithpython.com
    # http://inventwithpython.com/pygame
    # Released under a "Simplified BSD" license
    
    import random, pygame, sys
    from pygame.locals import *
    
    FPS = 30 # frames per second, the general speed of the program
    WINDOWWIDTH = 640 # size of window's width in pixels
    WINDOWHEIGHT = 480 # size of windows' height in pixels
    REVEALSPEED = 8 # speed boxes' sliding reveals and covers
    BOXSIZE = 40 # size of box height & width in pixels
    GAPSIZE = 10 # size of gap between boxes in pixels
    

    The game programs in this book use a lot of constant variables. You might not realize why they’re so handy. For example, instead of using the BOXSIZE variable in our code we could just type the integer 40 directly in the code. But there are two reasons to use constant variables.

    First, if we ever wanted to change the size of each box later, we would have to go through the entire program and find and replace each time we typed 40. By just using the BOXSIZE constant, we only have to change line 13 and the rest of the program is already up to date. This is much better, especially since we might use the integer value 40 for something else besides the size of the white boxes, and changing that 40 accidentally would cause bugs in our program.

    Second, it makes the code more readable. Go down to the next section and look at line 18. This sets up a calculation for the XMARGIN constant, which is how many pixels are on the side of the entire board. It is a complicated looking expression, but you can carefully piece out what it means. Line 18 looks like this:

    XMARGIN = int((WINDOWWIDTH - (BOARDWIDTH * (BOXSIZE + GAPSIZE))) / 2) 
    

     But if line 18 didn’t use constant variables, it would look like this:

    XMARGIN = int((640 – (10 * (40 + 10))) / 2) 
    

    Now it becomes impossible to remember what exactly the programmer intended to mean. These unexplained numbers in the source code are often called magic numbers. Whenever you find yourself entering magic numbers, you should consider replacing them with a constant variable instead. To the Python interpreter, both of the previous lines are the exact same. But to a human programmer who is reading the source code and trying to understand how it works, the second version of line 18 doesn’t make much sense at all! Constants really help the readability of source code.

    Of course, you can go too far replacing numbers with constant variables. Look at the following code:

    ZERO = 0
    ONE = 1
    TWO = 99999999
    TWOANDTHREEQUARTERS = 2.75 
    

    Don’t write code like that. That’s just silly.