Is there any case where len(someObj) does not call someObj's __len__ function?

Question

Is there any case where len(someObj) does not call someObj's __len__ function?

I recently replaced the former with the latter in a (sucessful) effort to speed up some code. I want to make sure there's not some edge case somewhere where len(someObj) is not the same as someObj.__len__().

Answer 1

If __len__ returns a length over sys.maxsize, len() will raise an exception. This isn't true of calling __len__ directly. (In fact you could return any object from __len__ which won't be caught unless it goes through len().)