Creating a new object from a function in python

Question

class SlidePuzzle(object):

    def __init__(self, state = None):
        if state == None: state = [[1,2,3],[4,'N',5],[6,7,8]]
        self.puzzle_state = list(state)

    def isValidMove(self, move):
        x,y = self.getBlank()

        if move == 'up':
            return y != 0
        elif move == 'down':
            return y != 2
        elif move == 'left':
            return x != 0
        elif move == 'right':
            return x != 2
        else:
            print "Invalid Move"
            return 0
    def getValidMoves(self):
        validMoves = ['up', 'down', 'left', 'right']

        if not self.isValidMove('down'):
            validMoves.remove('down')
        if not self.isValidMove('up'):
            validMoves.remove('up')
        if not self.isValidMove('right'):
            validMoves.remove('right')
        if not self.isValidMove('left'):
            validMoves.remove('left')

        return validMoves

    def generateChildren(self):
        return [SlidePuzzle(self.puzzle_state).move(m) for m in self.getValidMoves()]

If I run these commands:

- p = SlidePuzzle()
- print p
- p.move('up')
- print p
- print p.generateChildren()
- print p

This is the output. I have not included all of my source but as you can see the the move function works as needed. What I dont understand is why the generateChildren function not only doesnt seem to be creating any new Slide Puzzle Objects but it also messes with the puzzle state of the calling object.

- [1, 2, 3]
- [4, 'N', 5]
- [6, 7, 8]

- [1, 'N', 3]
- [4, 2, 5]
- [6, 7, 8]

- [None, None, None]

- [1, 2, 3]
- [4, 'N', 5]
- [6, 7, 8]

chepner · Accepted Answer · 2012-09-12 15:07:02Z

2

generateChildren returns a list of return values from SlidePuzzle.move(), not a list of SlidePuzzle objects. You don't show move, but I suspect it returns None.

answered Sep 12, 2012 at 15:07

chepner

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3 Comments

mgilson Over a year ago

You don't need to "suspect". it does return None (since you see the result in the output list).

mtaliaf Over a year ago

Thanks for the feedback. That was a dumb mistake after alot of banging my head against the wall. I fixed generate children to look like this\n code def generateChildren(self): validMoves = self.getValidMoves() children = [SlidePuzzle(self.puzzle_state[:]) for m in validMoves] for i in range(len(children)): children[i].move(validMoves[i]) return children code \n but the move command still doesnt appear to be affecting the newly created slide puzzle objects. They are simply in the default state

chepner Over a year ago

Please add the definition of move() to your question.

dav1d · Accepted Answer · 2012-09-12 15:34:50Z

-1

Lists are passed by "reference", so you have to copy the list at first, otherwise your new Puzzle will operate on the same list:

def generateChildren(self):
    return [SlidePuzzle(self.puzzle_state[:]).move(m) for m in self.getValidMoves()]

Note the [:] which copies the list.

Furthermore .move returns None, that's why it ends up beeing None.

edited Sep 12, 2012 at 15:34

answered Sep 12, 2012 at 15:02

dav1d

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2 Comments

chepner Over a year ago

SlidePuzzle.__init__ makes the copy of the given list.

Andy Hayden Over a year ago

Does this explain why it currently returns None?

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