The doc reference/expressions.srt says that

> For user-defined classes which do not define __contains__() but do 
> define __iter__(), x in y is True if some value z with x == z is 
> produced while iterating over y. If an exception is raised during the 
> iteration, it is as if in raised that exception.

and

> Lastly, the old-style iteration protocol is tried: if a class defines 
> __getitem__(), x in y is True if and only if there is a non-negative 
> integer index i such that x == y[i], and all lower integer indices do 
> not raise IndexError exception. (If any other exception is raised, it 
> is as if in raised that exception).

The documentation doesn't match the implementation, which clearly does `x is y or x == y` to check if `x` is the element `y` from a container. Both the `__iter__` and the index-iteration method test the elements using `is` first. While the document says that `x is x` means that `x == x` should be true, it is not true for example in the case of `nan`:

I think you change is appropriate given that the "equivalent to" for the built in iterators contains the 'is' expression.

However, I also think we should drop that second whole paragraph, and in the previous paragraph say "...but are :term:`iterable`...".  Because conceptually what we do is iterate whatever we're given if iter doesn't return a TypeError, and it is iter that handles the fallback to the older __getitem__ protocol.  I don't see why we should re-explain the iteration protocol in the docs for 'in'.  (I haven't looked at how this is actually implemented, but the implementation ought to be equivalent to that or we will eventually run into bugs.)  I don't think this would result in any loss of precision or understandability, and in fact I think it would make it easier to understand.

See also issue 27605, which also wants to edit this section slightly.

New changeset 2f5b9dcc0a89cbde1499c76df81c36bfd5ef9aa8 by Cheryl Sabella (Antti Haapala) in branch 'master':
bpo-30969: Fix docs about the comparison in absence of __contains__ (GH-2761)
https://github.com/python/cpython/commit/2f5b9dcc0a89cbde1499c76df81c36bfd5ef9aa8

New changeset e8661c1dabc206bf7fe6e302e38fa426aa734dd0 by Miss Islington (bot) in branch '3.7':
bpo-30969: Fix docs about the comparison in absence of __contains__ (GH-2761)
https://github.com/python/cpython/commit/e8661c1dabc206bf7fe6e302e38fa426aa734dd0