Question

In the following code:

all_subsets = []
subsets = [[], [2]]
left_most = 1 
for subset in subsets:
    print subset
    all_subsets.append(subset)
    all_subsets.append(subset.append(left_most))
print all_subsets

I suppose the return should be [[], [1], [2], [2, 1]], but I actually got the return of [[1], None, [2, 1], None]. i.e., [] has been converted to None.

I suppose it’s the problem of modifying subset in the loop twice, which pointing to the same physical address. However, I can’t fix the problem even if I make a local copy before modify the list, such as the following:

all_subsets = []
subsets = [[], [2]]
left_most = 1
for subset in subsets:
    print subset
    subset_orig = list(subset)
    subset_plus = list(subset).append(left_most)
    all_subsets.append(subset_orig)
    all_subsets.append(subset_plus)
print all_subsets

Any idea? Thanks!

Answer 1

The problem is that append() modifies the list in-place, and returns None.

Here is one way to fix the code:

all_subsets = []
subsets = [[], [2]]
left_most = 1 
for subset in subsets:
    print subset
    all_subsets.append(subset)
    all_subsets.append(subset + [left_most])
print all_subsets

This produces [[], [1], [2], [2, 1]] as you expect.