Functions allowed in DataFrameGroupby.transform are not all tested

[charlesdong1991]

e.g. fillna, tshiftand corrwith are all allowed in group.transform(), but looking at pandas/tests/groupby/test_transform.py, they are not tested. There are more, and I will add to here and make a PR to tests these cases.

[TomAugspurger]

Looks like #27974 was closed as stale. Marking as a good first issue, if anyone wants to pick it up based on that PR.

[lfiedler]

Hi, I would like to work on this.

[echozzy629]

Hi, I would like to work on this.

[simonjayhawkins]

@echozzy629 Thanks for the interest. #32069 has been submitted to address this issue.

[simonjayhawkins]

superseded by #32293 , #31269 , #27472 and #32344, see #32069 (comment)