Iterators membership testing

[Chris Angelico]

On Sun, Aug 9, 2015 at 7:06 PM, Pierre Quentel <pierre.quentel at gmail.com> wrote:
> "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."
>
> ...
> I get an assertion error. Setting a trace on __next__ suggests that for
> membership testing, the interpreter consumes the iterator until the searched
> value is found (or until exhaustion), then it resumes iteration at this point.

That's exactly right. The only way for the interpreter to handle 'in'
on an iterator is something like this:

def contains(iter, obj):
    for val in iter:
        if val == obj: return True
    return False

That's what the docs describe. So what you have is something like this:

for i in iterator:
    for j in iterator:
        if i == j: break
    else:
        assert False, '%s not found' %i

You're dragging values from the same iterator, so you're consuming it
as part of your membership test. You can do this kind of thing:

>>> 5 in A(10)
True

but if you've already consumed a few values, those won't be in the
iterator any more:

>>> x = A(10)
>>> next(x)
0
>>> next(x)
1
>>> next(x)
2
>>> next(x)
3
>>> 2 in x
False

This is simply how iterators work. They're very different from
repeatable iterables like lists or range objects, where you _can_ test
for membership that way:

>>> x = [10,20,30]
>>> for i in x: assert i in x
...
>>> x = iter([10,20,30])
>>> for i in x: assert i in x
...
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
AssertionError

Note that I _could_ create a list that would pass this assertion,
simply by duplicating every value:

>>> x = iter([10,10,20,20,30,30])
>>> for i in x: assert i in x
...

But it's iterating only three times here, and the 'in' check is
consuming the other three values. Once your A(10) has yielded some
value, it will never yield it again, so the assertion can never pass.

Does that explain matters?

ChrisA