TST: Groupby first/last/nth nan column test

[jamescobonkerr]

• xref BUG: groupby first()/last() change datatype of all NaN columns to float; nth() preserves datatype #33591 (comment)
• tests added / passed
• passes black pandas
• passes git diff upstream/master -u -- "*.py" | flake8 --diff
• whatsnew entry

The first two asserts (first()/last()) fail on 1.0.3, as expected.

[WillAyd]

Thanks - just a minor nit to make things more readable otherwise lgtm

[simonjayhawkins]

Thanks @jamescobonkerr for the PR. does this test follow the same code path as the code sample in the issue OP?, see #33591 (comment)

[simonjayhawkins]

The first two asserts (first()/last()) fail on 1.0.3, as expected

this was changed in #32949

be86b65 is the first new commit
commit be86b65
Author: jbrockmendel jbrockmendel@gmail.com
Date: Fri Apr 10 09:18:06 2020 -0700

CLN: remove unnecessary Series._convert calls (#32949)

[jamescobonkerr]

Thanks @jamescobonkerr for the PR. does this test follow the same code path as the code sample in the issue OP?, see #33591 (comment)

Ah, you're right — pardon the oversight.

The first two asserts (first()/last()) fail on 1.0.3, as expected

this was changed in #32949

be86b65 is the first new commit
commit be86b65
Author: jbrockmendel jbrockmendel@gmail.com
Date: Fri Apr 10 09:18:06 2020 -0700

CLN: remove unnecessary Series._convert calls (#32949)

Removing the _convert call from core.groupby.generic.DataFrameGroupBy._wrap_aggregated_output seems to fix the reported issue, however, breaks a range of other tests. I don't have a great understanding of how GroupBy works. I'll close this PR.

[simonjayhawkins]

@jamescobonkerr Thanks for working on this.

The test you have added here does have value since it is testing a change in behaviour against regression.

As you pointed out this test fails on 1.0.3 and is fixed on master. AFAICT this is working since #32949

I'll reopen this PR, but once merged will not close #33591 (rather than open a new issue at this point, i'll reference you code sample in #33591)

[simonjayhawkins]

Thanks @jamescobonkerr lgtm pending green ci.

[jreback]

thanks @jamescobonkerr always happy to take these