Checking membership in `itertools.count()` will either return True in linear time or enter an infinite loop that cannot be terminated with Ctrl-c. This

```
import itertools
1 in itertools.count(0,2)
```

It is expected that the above code will use an efficient (O(1)) membership algorithm like range objects.

This seem like a misdirected expectation. count returns a regular iterator and the purpose behind it is not to enable membership checks, this only works because it's a regular iterators and normal rules for checking membership are applied.

In any case, it is possible to write an infinite loop in Python either by accident or intentionally, this doesn't seem like a reasonable rationale for making this sort changes.

Problems with long-running iterators are already discussed in:

Issue 31815: rejected proposal to check for interrupts
Issue 33939: proposal to flag iterators as being infinite

Reopening. While my behavioral expectation may be wrong, the current behavior is inappropriate. It would make more sense for count to have a `__contains__` method which raises a TypeError.

Adding the __contains__() method to the count iterator would not solve the general problem with infinite iterators. For example with the following expressions:

    -1 in filter(None, itertools.count())
    -1 in map(float, itertools.count())

It is not worth to add a method just to handle a single case of misusing. You should not use "in" with infinite iterators.

The general problem with infinite iterators is indeed a bigger issue and its resolution would probably resolve this issue too. With the examples you gave at least the user can ctrl-c to interrupt. Entering an infinite, *uninterruptible* loop is a consequence so bad that it deserves a guard rail.

> On May 25, 2019, at 11:48, Serhiy Storchaka <report@bugs.python.org> wrote:
> 
> 
> Serhiy Storchaka <storchaka+cpython@gmail.com> added the comment:
> 
> Adding the __contains__() method to the count iterator would not solve the general problem with infinite iterators. For example with the following expressions:
> 
>    -1 in filter(None, itertools.count())
>    -1 in map(float, itertools.count())
> 
> It is not worth to add a method just to handle a single case of misusing. You should not use "in" with infinite iterators.
> 
> ----------
> nosy: +serhiy.storchaka
> 
> _______________________________________
> Python tracker <report@bugs.python.org>
> <https://bugs.python.org/issue37040>
> _______________________________________

Oops you are right. These enter uninterruptible loops too. They are exceptional situations and should do the expected thing - throw exceptions as well.

Dan, please leave this closed.  It duplicates #31815 and should be closed on that grounds alone, regardless of the status of the latter.  #31815 was closed in favor of #33939, which is open, with ongoing discussion.  Resolution either way is blocked because different core developers disagree.