4.6: Sanity Checks with assert Statements

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# 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
BOARDWIDTH = 10 # number of columns of icons
BOARDHEIGHT = 7 # number of rows of icons
assert (BOARDWIDTH * BOARDHEIGHT) % 2 == 0, 'Board needs to have an even number of boxes for pairs of matches.'
XMARGIN = int((WINDOWWIDTH - (BOARDWIDTH * (BOXSIZE + GAPSIZE))) / 2)
YMARGIN = int((WINDOWHEIGHT - (BOARDHEIGHT * (BOXSIZE + GAPSIZE))) / 2)

The assert statement on line 15 ensures that the board width and height we’ve selected will result in an even number of boxes (since we will have pairs of icons in this game). There are three parts to an assert statement: the assert keyword, an expression which, if False, results in crashing the program. The third part (after the comma after the expression) is a string that appears if the program crashes because of the assertion.

The assert statement with an expression basically says, "The programmer asserts that this expression must be True, otherwise crash the program." This is a good way of adding a sanity check to your program to make sure that if the execution ever passes an assertion we can at least know that that code is working as expected.