Python List Values being assigned to each other

Question

I'm having a problem writing the following code:

D = [
        [0,3,8,999,-4],
        [999,0,999,1,7],
        [999,4,0,999,999],
        [2,999,-5,0,999],
        [999,999,999,6,0]
    ]

def FloydWarshall (W):
    matrices = [W[:]]
    pred = [W[:]]

    print (matrices is pred)
    print (pred is matrices)
    print (pred is W)
    print (matrices is W)

    for i in range(0,len(pred[0])):
        for j in range(len(pred[0][i])):

            if pred[0][i][j] != 999 and pred[0][i][j] != 0:
                pred[0][i][j] = i +1
            else:
                pred[0][i][j] = False 

    return (matrices,pred)
FloydWarshall(D)

the values that are being returned are the exact same matrices, why is this? The print statement say that they are not pointers to the same spot in memory, correct?

The is keyword checks the id's of the two variables. The id's corresponds to the emplacements of the variables in memory (in CPython implementation). As said in the documentation: docs.python.org/3.3/library/functions.html#id — aldeb
– aldeb, Commented Apr 18, 2013 at 0:00
@segfolt: That's what OP wanted. a is b is True if they're the same object. — Blender
– Blender, Commented Apr 18, 2013 at 0:01
@Blender: Yes. That was just a precision because of the OP's last question. — aldeb
– aldeb, Commented Apr 18, 2013 at 0:23

jamylak · Accepted Answer · 2013-04-18 00:00:17Z

5

You are only making a shallow copy of the nested lists, so mutating them will still affect both matrices. You might want to use copy.deepcopy

answered Apr 18, 2013 at 0:00

jamylak

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1 Comment

Lilluda 5 Over a year ago

Correct you are. Thank you!

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