Backport PR #37198 on branch 1.1.x (BUG: Regression in Resample.apply raised error when apply affected only a Series)

[meeseeksmachine]

Backport PR #37198: BUG: Regression in Resample.apply raised error when apply affected only a Series

[simonjayhawkins]

DO NOT MERGE - failures are not false positives

[phofl]

Was this introduced through my fix or did I use something, which is only working on master and not 1.1.4?

[simonjayhawkins]

Was this introduced through my fix or did I use something, which is only working on master and not 1.1.4?

not got to the bottom of it yet. see #36951 (comment)

[jbrockmendel]

any ideas where changes in master could have corrected this

Not off the top of my head, no.

[simonjayhawkins]

any ideas where changes in master could have corrected this

Not off the top of my head, no.

appears to be #35792

https://github.com/simonjayhawkins/pandas/runs/1305349161?check_suite_focus=true

[simonjayhawkins]

looks like #35792 can be backported without any conflicts (except release note). added here for ci checking. would backport independently if moving forward with this. (whatsnew would need to be changed on master)

depending on tests passing, 3 options.

1. don't backport BUG: Regression in Resample.apply raised error when apply affected only a Series #37198 (see REGR: resample apply fails with KeyError/AttributeError for function that works on DataFrame but fails on Series #36951 (comment) this regression is from 0.25.3 to 1.0.0, we could decide not to backport.)

2. backport BUG: Regression in Resample.apply raised error when apply affected only a Series #37198 and change tests to not check frequency of index on backport only.

3. backport BUG: Regression in Resample.apply raised error when apply affected only a Series #37198 and CLN/BUG: Clean/Simplify _wrap_applied_output #35792 (which fixes bug for Categoricalndex and preserves freq in DatetimeIndex following DataFrameGroupby)

[simonjayhawkins]

@jreback OK with option 3 #37305 (comment)

will start on a separate backport PR #35792 and PR to move the whatsnew