Tic Tac Toe - Invent Your Own Computer Games with Python - page 175

10 - Tic Tac Toe
The None Value
None is a special value that you can assign to a variable. The None value represents the
lack of a value. None is the only value of the data type NoneType. (Just like the boolean
data type has only two values, the NoneType data type has only one value, None .) It can be
very useful to use the None value when you have not set a variables value yet. For
example, say you had a variable named quizAnswer which holds the user's answer to
some True-False pop quiz question. You could set quizAnswer to None if the user
skipped the question or did not answer it. Using None would be better because if you set it
to True or False before assigning the value of the user's answer, it may look like the user
gave an answer the question even though they didn't.
Calls to functions that do not return anything (that is, they exit by reaching the end of the
function and not from a return statement) will evaluate to None . The None value is written
without quotes and with a capital "N" and lowercase "one".
How the Code Works: Lines 96 to 187
Creating the Computer's Artificial Intelligence
96. def getComputerMove(board, computerLetter):
97. # Given a board and the computer's letter, determine
where to move and return that move.
98. if computerLetter == 'X':
99. playerLetter = 'O'
100. else:
101. playerLetter = 'X'
The getComputerMove() function is where our AI will be coded. The parameters
are a Tic Tac Toe board (in the board parameter) and which letter the computer is (either
'X' or 'O' ). The first few lines simply assign the other letter to a variable named
playerLetter . This lets us use the same code, no matter who is X and who is O. This
function will return the integer that represents which space the computer will move.
Remember how our algorithm works:
First, see if there is a move the computer can make that will win the game. If there is,
take that move. Otherwise, go to the second step.
Second, see if there is a move the player can make that will cause the computer to lose
the game. If there is, we should move there to block the player. Otherwise, go to the third
step.
Third, check if any of the corner spaces (spaces 1, 3, 7, or 9) are free. (We always want
to take a corner piece instead of the center or a side piece.) If no corner piece is free, then