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 (sucess

Answer 1

There are cases where len(someObj) is not the same as someObj.__len__() since len() validates __len__()'s return value. Here are the possible errors in Python 3.6.9:

• Too small, i.e. less than 0

  ValueError: __len__() should return >= 0
  

• Too big, i.e. greater than sys.maxsize (CPython-specific, per the docs)

  OverflowError: cannot fit 'int' into an index-sized integer
  

• An invalid type, e.g float

  TypeError: 'float' object cannot be interpreted as an integer
  

• Missing, e.g. len(object)

  TypeError: object of type 'type' has no len()
  

  I mention this because object.__len__() raises a different exception, AttributeError.

It's also worth noting that range(sys.maxsize+1) is valid, but its __len__() raises an exception:

OverflowError: Python int too large to convert to C ssize_t