MultiIndex: support isna, fixes #34019
- [x] closes #34019
- [ ] tests added / passed
- [ ] passes `black pandas`
- [ ] passes `git diff upstream/master -u -- "*.py" | flake8 --diff`
- [ ] whatsnew entry

Note sure what to do with the (smoke) tests here. I suppose an explicit test comparing input and output would be good to have.

Tested on top of 1.1.5, but seems there are additional failures on master. Before putting in any additional time I'd like to know if the chosen approach is acceptable from a functional point of view, that is

>  If there is within a level an NA, the entire label is considered NA

is considered the correct approach. > Before putting in any additional time I'd like to know if the chosen approach is acceptable from a functional point of view, that is
> 
> > If there is within a level an NA, the entire label is considered NA
> 
> is considered the correct approach.

Well ofc it's completely up to you! Bear in mind that this will need to go through multiple reviews and you will likely get feedback faster if you provide concrete examples of the implementation.

FWIW this seems not to break anything which is good. The one failure is an xfailed test which now doesn't raise:
``` 
pandas/tests/indexes/multi/test_integrity.py::test_isna_behavior
```
if you remove the xfail the checks should be green.
 > this is not a good soln here. rather defining nunique on a MI is better

In the cython class? > > this is not a good soln here. rather defining nunique on a MI is better
> 
> In the cython class?

no on MultiIndex This pull request is stale because it has been open for thirty days with no activity. Please update or respond to this comment if you're still interested in working on this. closing as stale. if you want to continue, pls ping and can re-open.