Why does defining __getitem__ on a class make it iterable in python?

Question

Why does defining __getitem__ on a class make it iterable?

For instance if I write:

class b:
  def __getitem__(self, k):
    return k

cb = b()

for k in cb:
  print k

I get the output:

0
1
2
3
4
5
6
7
8
...

I would really expect to see an error returned from "for k in cb:"

Answer 1

Iteration's support for __getitem__ can be seen as a "legacy feature" which allowed smoother transition when PEP234 introduced iterability as a primary concept. It only applies to classes without __iter__ whose __getitem__ accepts integers 0, 1, &c, and raises IndexError once the index gets too high (if ever), typically "sequence" classes coded before __iter__ appeared (though nothing stops you from coding new classes this way too).

Personally, I would rather not rely on this in new code, though it's not deprecated nor is it going away (works fine in Python 3 too), so this is just a matter of style and taste ("explicit is better than implicit" so I'd rather explicitly support iterability rather than rely on __getitem__ supporting it implicitly for me -- but, not a bigge).

Answer 2

If you take a look at PEP234 defining iterators, it says:

1. An object can be iterated over with "for" if it implements
   __iter__() or __getitem__().

2. An object can function as an iterator if it implements next().