Tic Tac Toe - Invent Your Own Computer Games with Python - page 178

index 0 , which is just a placeholder that we ignore). If there is at least one space in
board that is set to a single space ' ' then it will return False .
The for loop will let us check spaces 1 through 9 on the Tic Tac Toe board. (Remember
that range(1, 10) will make the for loop iterate over the integers 1 , 2 , 3 , 4 , 5 , 6 , 7 ,
8 , and 9 .) As soon as it finds a free space in the board (that is, when isSpaceFree
(board, i) returns True ), the isBoardFull() function will return False .
If execution manages to go through every iteration of the loop, we will know that none of
the spaces are free. So at that point, we will execute return True .
The Start of the Game
140. print('Welcome to Tic Tac Toe!')
Line 140 is the first line that isn't inside of a function, so it is the first line of code that is
executed when we run this program.
142. while True:
143. # Reset the board
144. theBoard = [' '] * 10
This while loop has True for the condition, so that means we will keep looping in this
loop until we encounter a break statement. Line 144 sets up the main Tic Tac Toe board
that we will use, named theBoard . It is a 10-string list, where each string is a single
space ' '. Remember the little trick using the multiplication operator with a list to replicate
it: [' '] * 10 . That evaluates to [' ', ' ', ' ', ' ', ' ', ' ', ' ',
' ', ' ', ' '] , but is shorter for us to type [' '] * 10 .
Deciding the Player's Mark and Who Goes First
145. playerLetter, computerLetter = inputPlayerLetter()
The inputPlayerLetter() function lets the player type in whether they want to be
X or O. The function returns a 2-string list, either ['X', 'O'] or ['O', 'X'] . We use
the multiple assignment trick here that we learned in the Hangman chapter. If
inputPlayerLetter() returns ['X', 'O'] , then playerLetter is set to 'X' and
computerLetter is set to 'O'. If inputPlayerLetter() returns ['O', 'X'] ,
then playerLetter is set to 'O' and computerLetter is set to 'X'.
146. turn = whoGoesFirst()
147. print('The ' + turn + ' will go first.')