Tic Tac Toe - Invent Your Own Computer Games with Python - page 174

How the Code Works: Lines 83 to 94
Choosing a Move from a List of Moves
83. def chooseRandomMoveFromList(board, movesList):
84. # Returns a valid move from the passed list on the
passed board.
85. # Returns None if there is no valid move.
86. possibleMoves = []
87. for i in movesList:
88. if isSpaceFree(board, i):
89. possibleMoves.append(i)
The chooseRandomMoveFromList() function will be of use to us when we are
implementing the code for our AI. The first parameter board is the 10-string list that
represents a Tic Tac Toe board. The second parameter movesList is a list of integers that
represent possible moves. For example, if movesList is [1, 3, 7, 9] , that means
we should return the number for one of the corner spaces on the board.
The chooseRandomMoveFromList() function will then choose one of those moves
from the possibleMoves list. It also makes sure that the move that it chooses is not
already taken. To do this, we create a blank list and assign it to possibleMoves . The
for loop will go through the list of moves passed to this function in movesList . If that
move is available (which we figure out with a call to isSpaceFree() ), then we add it to
possibleMoves with the append() method.
91. if len(possibleMoves) != 0:
92. return random.choice(possibleMoves)
93. else:
94. return None
At this point, the possibleMoves list has all of the moves that were in movesList
that are also free spaces on the board represented by board . If the list is not empty, then
there is at least one possible move that can be made on the board.
This list might be empty. For example, if movesList was [1, 3, 7, 9] but the
board represented by the board parameter had all the corner spaces already taken, the
possibleMoves list would have been empty.
If possibleMoves is empty, then len(possibleMoves) will evaluate to 0 and
the code in the else-block will execute. Notice that it returns something called None .