Question

I’m trying to update values in a list of lists using the following source code:

target_agreement = get_object_or_404(TargetAgreement.objects, pk=agreement_id)
target_category_set = TargetCategory.objects.filter(company=target_agreement.company, is_active=True).order_by('position')

category_targets = []
if target_category_set:
    totals = [[0]*3]*len(target_category_set) #list of lists with list on level 2 having length of 3
else:
    totals = [[0]*3]*1

for (index1,target_category) in enumerate(target_category_set):
        category_targets_temp = []

        for (index2,target) in enumerate(target_category.category_targets.filter(user=request.user, agreement=target_agreement)):
            category_targets_temp.append(target)
            print "*******"
            print "index is: " 
            print index1
            print totals[index1][0]
            totals[index1][0] = totals[index1][0] + target.weight
            print totals[index1][0]
            print "totals are"
            print totals
            print "*******"
        print "final result"
        print totals[index1][0]
        print totals
        print "-----"
        category_targets.append(category_targets_temp)
    print totals

The behavior I do not understand is that totals[index1][0] = totals[index1][0] + target.weight is not only updating the first element in the list referenced by index1, but all first elements in all lists.

The result is like the following:

[[88, 0, 0], [88, 0, 0], [88, 0, 0], [88, 0, 0]]

But I would have expected:

[[36, 0, 0], [50, 0, 0], [1, 0, 0], [1, 0, 0]]

Can somebody clarify what I did wrong. Thanks for your help.

Answer 1

The reason is the way you created that totals list. By multiplying the list –

totals = [[0]*3]*len(target_category_set) 

You create a list whose elements are references to the same list. Thus, when you modify one element, all of them are modified. Consider:

In [1]: l = [[1, 2]]*3

In [2]: l
Out[2]: [[1, 2], [1, 2], [1, 2]]

In [3]: l[0].append(3)

In [4]: l
Out[4]: [[1, 2, 3], [1, 2, 3], [1, 2, 3]]

But you could avoid this by changing the list definition:

In [5]: l = [[1, 2] for _ in range(3)]

In [6]: l[0].append(3)

In [7]: l
Out[7]: [[1, 2, 3], [1, 2], [1, 2]]