Missing __length_hint__ in __getitem__ iterator

[Peter Otten]

Giovanni Bajo wrote:

> Hello,
> 
> given the following object:
> 
>  >>> class A(object):
> ...     def __getitem__(self, idx):
> ...             if idx >= 10: raise IndexError
> ...             return idx
> ...     def __len__(self):
> ...             return 10
> ...
> 
> I noticed that the iterator that Python constructs:
> 
>  >>> a = A()
>  >>> i = iter(a)
>  >>> dir(i)
> ['__class__', '__delattr__', '__doc__', '__getattribute__', '__hash__',
> '__init__', '__iter__', '__len__', '__new__', '__reduce__',
> '__reduce_ex__', '__repr__', '__setattr__', '__str__', 'next']
> 
> does not have a __length_hint__ method.
> 
> Is this just a missing optimization, or there is a deep semantic reason
> for which a __length_hint__ could not be constructed out of the __len__
> result?

It's there, just not doctored into the dir() output:

>>> class A(object):
...     def __getitem__(self, index): return index
...     def __len__(self): return 42
...
>>> iter(A()).__length_hint__
<built-in method __length_hint__ of iterator object at 0x401d558c>
>>> iter(A()).__length_hint__()
42

Peter