View Full Version : [SOLVED] Python Lists behaving CRAZY !!

January 2nd, 2009, 11:45 PM
Hey ,
am having a bit of a problem and I dont know how to get around it . This is the first time I am using python and am having some trouble with lists .

please have a look at this code

openList = [0,1]
prop = [0,0]

cellList = [prop,prop]
print "part 1"
for x in openList:
print x,"*",cellList[x]
print x,"#",cellList[x]

print "part 2"
for x in openList:
print x," ",cellList[x]

I would have expected the Output to be :

part 1
part 2
0 [1,0]
1 [2,0]

But for some reason beyond my comprehension the output comes as :

part 1
0 * [0, 0]
0 # [1, 0]
1 * [1, 0]
1 # [2, 0]
part 2
0 [2, 0]
1 [2, 0]

Can someone explain why this is so ?

Am using python 2.5.2 , the one that comes with Ubuntu , Hardy Heron x64.

Thanks ,

Tony Flury
January 2nd, 2009, 11:54 PM
your code has

cellList = [prop,prop]

What this actually does is puts two references to the prop list into cellList - which means when you change one copy of prop, you will change both instances, because they are the same object.

If you do this

cellList =[[0.0][0.0]]

your script will do what you expect.

January 2nd, 2009, 11:56 PM
a = [0, 1]
b = [a, a]

print "original:",b # prints [[0, 1], [0, 1]]

b[0][0] = 1

print "modified:",b # prints [[1, 1], [1, 1]]

Python doesn't actually copy the values from one list to another, it stores a reference, so it happens as Tony Flury pointed out.

January 3rd, 2009, 12:35 AM
A list is an object. Think about it, you put the same object in the containing list twice, then you modified it and expected the same object to be different just because it was referenced in two different locations. Copy a list if you need two different instances, but remember that they ARE mutable objects.

January 3rd, 2009, 01:17 AM
As a beginner, get used to the fact that problem is usually that you do not exactly understand what you asked computer to do - and it alsways obeys without failure, even if you don't know what you asked for.

Wyb explained what happened - uoi used same object twice.

to make (shalow) copy, just use slice of a list:

cell_list = [prop[:], prop[:]]

If you need deeper copy, use deepcopy()

But more likely, if you need more than 1-dimensional list, dictionary with tuples as key is what you want instead:

matrix = {}
matrix[1,2] = 42 # or even another dict

January 3rd, 2009, 07:35 AM
Hey guys ,

thanks problem solved . You guys were right , I was copying references when I should have been using copy.deepcopy()

Its working fine now , thank you :)