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

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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).

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