Problem with objects copying each other in memory

[Cameron Pulsford]

Thanks, that did it! Why is that the case though? Or rather, why do  
the assignments to temp.x and temp.y not effect the self.x and self.y?  
How come I only run into the problem with the list?


On Feb 12, 2009, at 5:15 PM, andrew cooke wrote:

>
> you're setting the new knight's "sl" to the value self.sl and then  
> adding
> values to it.  that's the same list - so you are adding values to the
> self.sl list when you add them to the knight's sl.
>
> this is easier to understand just by seeing the fix, which is to use:
>
> temp = Knight(self.x, self.y, self.g, self.h, self.gp, list(self.sl))
>
> which will make a new copy of the list, so you are only adding to  
> the sl
> in the (new) knight.
>
> andrew
>
>
>
> dlocpuwons wrote:
>> Using Python 2.6.1...
>>
>> I am (attempting) to make an A* search for a chess problem, but I am
>> running into a really annoying shared memory issue in my successor
>> function. Here it is stripped down to the important parts that relate
>> to my problem.
>>
>> def successors(self):
>> 		result = []
>> 		moves = [[2, 1], [2, -1], [-2, 1], [-2, -1], [1, 2], [1, -2], [-1,
>> 2], [-1, -2]] #possible moves for a knight
>>
>> 		for i in moves:
>> 			temp = Knight(self.x, self.y, self.g, self.h, self.gp, self.sl)
>> 			temp.x += i[0]
>> 			temp.y += i[1]
>> 			temp.sl.append([temp.x, temp.y]) #Adds the new current state to  
>> the
>> visited states list
>> 			result.append(temp)
>> 		return result
>>
>> The method creates a temporary Knight object, increments it to the  
>> new
>> position and then adds this new position to its list of visited
>> states. Then it returns a list of these 8 new objects. The problem
>> seems to be with the "result.sl.append()" line. As the method chugs
>> along and creates the new objects the "temp.sl" lines seems to stay  
>> in
>> memory, so when the method is done all the new objects have all the
>> new states that were made over the course of the method instead of
>> just their own created in the loop. For example when I try to get
>> successors for a piece that is initially at (2,2) with no previously
>> visited states, the method prints this out for the sl (state list)
>> value
>>
>> [[2, 2], [4, 3], [4, 1], [0, 3], [0, 1], [3, 4], [3, 0], [1, 4], [1,
>> 0]]
>> [[2, 2], [4, 3], [4, 1], [0, 3], [0, 1], [3, 4], [3, 0], [1, 4], [1,
>> 0]]
>> [[2, 2], [4, 3], [4, 1], [0, 3], [0, 1], [3, 4], [3, 0], [1, 4], [1,
>> 0]]
>> [[2, 2], [4, 3], [4, 1], [0, 3], [0, 1], [3, 4], [3, 0], [1, 4], [1,
>> 0]]
>> [[2, 2], [4, 3], [4, 1], [0, 3], [0, 1], [3, 4], [3, 0], [1, 4], [1,
>> 0]]
>> [[2, 2], [4, 3], [4, 1], [0, 3], [0, 1], [3, 4], [3, 0], [1, 4], [1,
>> 0]]
>> [[2, 2], [4, 3], [4, 1], [0, 3], [0, 1], [3, 4], [3, 0], [1, 4], [1,
>> 0]]
>> [[2, 2], [4, 3], [4, 1], [0, 3], [0, 1], [3, 4], [3, 0], [1, 4], [1,
>> 0]]
>>
>> but what it should print out is
>>
>> [[2, 2], [4, 3]]
>> [[2, 2], [4, 1]]
>> [[2, 2], [0, 3]]
>> [[2, 2], [0, 1]]
>> [[2, 2], [3, 4]]
>> [[2, 2], [3, 0]]
>> [[2, 2], [1, 4]]
>> [[2, 2], [1, 0]]
>>
>> It sort of seems like python is trying to be too smart and is trying
>> to keep things in memory. Is there anyway to work around this?
>> --
>> http://mail.python.org/mailman/listinfo/python-list
>>
>>
>
>