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I've got a problem. In python I've got an function, which should print the longest path. The function works, if I use a list like [[1,2,3,4,5][1,2,3,4,5]] but if I use list like [x] for x in range(5)]*2, or any other list like that. (I don't know what is this called in python, but if I have list like [vale]*something), the code doesn't work. So, my question is, how can I make new list, without any references. Deepcopy or [:] doesn't work, because references inside list, are still there. But I don't know how to copy just values, without references. :) Thanks. 

def naj_narascajoca(vrtek):


    visina = len(vrtek)
    sirina = len(vrtek[0])

    vrt = []
    vrt.append([-1]*(sirina+2))
    for greda in vrtek:
        greda.insert(0,-1)
        greda.append(-1)
        vrt.append(greda)

    vrt.append([-1]*(sirina+2))


    for i in vrt:
        print(i)


    pot = ""
    maximal = 0
    je_vecja = True
    x = 1
    y = 1
    max_odlocitev = 0
    while je_vecja == True:

        print("--------------",vrt[x][y],"-----------------")

        vre_left = vrt[x][y-1]
        vre_right = vrt[x][y+1]
        vre_up = vrt[x-1][y]
        vre_down = vrt[x+1][y]
        sez = sorted([vre_down,vre_left, vre_right, vre_up])
        for i in sez:
            if i > maximal:
                max_odlocitev = i;
                break
        #max_odlocitev = max(vrt[x][y-1],vrt[x][y+1], vrt[x+1][y], vrt[x-1][y])
        if maximal < max_odlocitev:
                maximal = max_odlocitev
                if vrt[x][y-1] == maximal:
                    pot += "L"
                    y -=1
                elif vrt[x][y+1] == maximal:
                    pot += "R"
                    y+=1
                elif vrt[x+1][y] == maximal:
                    pot += "D"
                    x+=1
                elif vrt[x-1][y] == maximal:
                    pot += "U"
                    x-=1


        else:
            je_vecja = False

    print(pot)

vrt = [[1,3,3,8,5,4,2,1,5,6],
    [2,4,3,3,6,8,1,3,5,6],
    [4,5,6,4,7,4,3,6,4,7],
    [2,8,7,0,0,7,4,7,8,0],
    [2,3,4,7,0,8,7,6,3,8],
    [3,7,9,0,8,5,3,2,3,4],
    [1,5,7,7,6,4,2,3,5,6],
    [0,6,3,3,6,8,0,6,7,7],
    [0,1,3,2,8,0,0,0,0,0],
    [3,1,0,3,6,7,0,5,3,1],
    [1,3,5,7,0,8,6,5,3,1],
    [3,6,3,1,3,5,8,7,5,1],
    [4,3,6,0,0,8,4,7,5,3],
    [3,5,6,8,6,3,1,3,5,2]]

#naj_narascajoca(vrt)
naj_narascajoca([[x] for x in range(5)]*2)
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2 Answers 2

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1

Anything like this:

[any mutable objects] * 2

… is explicitly making two references to each mutable object. It doesn't matter how you create the original list, it's the same problem, with the same solution explained in the FAQ: you have to define the list twice. So, instead of this:

[[x] for x in range(5)]*2

… do this:

[[x] for _ in range(2) for x in range(5)]

The reason deepcopy doesn't seem to work is that you're (presumably) using it like this:

copy.deepcopy([[x] for x in range(5)]*2)

And deepcopy is smarter than you think. It's making a deep copy of your structure of 2 references to the same list of 5 lists. So you end up with 5 new lists, a new list of 5 lists, and 2 new references to that new list.

But it can do what you want. You just need to copy the list before putting the two copies together:

>>> a = [[x] for x in range(5)]
>>> b = [copy.deepcopy(a), copy.deepcopy(a)]
>>> # or [copy.deepcopy(a) for _ in range(2)]
>>> b[0] is b[1]
False
>>> b[0][0] is b[1][0]
False

Or, alternatively, map the copy over the list of two references, so they each get copied individually:

>>> b = [copy.deepcopy(x) for x in a*2]
>>> b[0][0] is b[1][0]
False
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4 Comments

Thanks for this, but the problem is, that I don't know what type of list I will get. The list may be given like that [any mutable objects] * 2 or, like [[1,2,3,4][1,2,2,2]]... or anything else. So, I would like to make new list, but copy just values, not references. Is that possible?
I can't edit last comment. I will add some more explanation. And, if I get a list, let's call it abc and abc[0] is abc[1] I want new list, for example defg[], that will be defg[0] is not defg[1] but abc[0] == defg[0] and same for [1]. So, same values, but no references, even in inner lists.
@MatejMohar: I think I know what you mean. Let me edit the answer.
Thank you! This is what I was lookig for! Uh, now I understand how copying works:)
0

When you use *2 on a list, you're not making copies of the list as you've discovered - you're making two references to the exact same list. You can use another level of list comprehension to get what you need.

[[[x] for x in range(5)] for y in range(2)]

Or perhaps the problem is that you were getting a list of lists in the inner part when you wanted just a single list 1-5:

[list(range(1, 6)) for x in range(2)]
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4 Comments

The comprehension of comprehensions doesn't give you the right list; you get [[[0], [1], [2], [3], [4]], [[0], [1], [2], [3], [4]]] instead of [[0], [1], [2], [3], [4], [0], [1], [2], [3], [4]]. You need a single nested comprehension (as in my answer), unless you want to flatten it after the fact (e.g., list(chain.from_iterable(…))).
@abarnert, that's why I presented the second solution because I thought that's probably closer to what was intended.
Hi! Thanks for answer. I answer on @abarnet answer, so please, take a look :)
@MarkRansom: The second solution gives [[1, 2, 3, 4, 5], [1, 2, 3, 4, 5]], which is also a different shape than the OP's original.

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