Should I convert dict.keys() into list(dict.keys()) for iteration in Python3?, 2to3 suggests converting that

I am doing a migration, Python2 to Pytnon3 with 2to3. (Python2.7.12 and Python3.5.2 exactly)

While doing the migration, 2to3 suggests I use type-cast like the below.

a = {1: 1, 2: 2, 3: 3}

for i in a.keys():  ----> for i in list(a.keys()):
    print(i)

After that, I try to check what difference there is in a script.

$ python3
>>> a = {1: 1, 2: 2, 3: 3}
>>> a.keys()
dict_keys([1, 2, 3])
>>> for i in a.keys(): print(i)
1
2
3

It apparently returns different type dict_keys not being list but dict_keys still seems to work with loop like list without type-cast in the above simple code.

I wonder If I use without type-cast, there would be some side-effect or not. If there is nothing, it looks unnecessary operation.

Why does 2to3 suggest that?

Solution

Generally it doesn’t matter for iterating, but it does matter if you try to take an index, because keys() isn’t a list in py3, so you can’t take an index of it, it is generally a safe operation, know the cost of the list call, generally if it was ok in py2 it will be ok in py3.