ENH: Unify API of groupby.sum and sum

Is your feature request related to a problem?

The level keyword in sum is marked as deprecated, but the suggested replacement with groupby.sum does not support skipna, therefore, some behaviours are impossible to replicate easily.

Describe the solution you’d like

grouby.sum should accept the skipna=False keyword,

API breaking implications

Currently the API is inconsistent, since df.sum(axis=1, level=0, skipna=False) has no equivalent since df.groupby(axis=1, level=0).sum(skipna=False) is not available.

Describe alternatives you’ve considered

One can of course do: df.groupby(axis=1, level=0).apply(lambda x : x.sum(axis=1, skipna=False)) but this cannot be the intended use.

Additional context

# Your code here, if applicable

[jreback]

i am +1 in unifying signatures between aggregators

this is a footgun that we can easily prevent

being efficient for a non default keyword doesn’t matter that much. api consistency is much more important

[simonjayhawkins]

and if you could add some references to support the discussion and other readers that’ll be great too. e.g. the deprecation and alternatives, where this was discussed etc.

the deprecation was discussed in #39983 and implemented in #40869 and introduced in pandas 1.3

the issue here has already been communicated (with a copy and paste-able code sample) in https://github.com/pandas-dev/pandas/issues/39983#issuecomment-916549939

It is too late now that we are on pandas 1.4.x to undo the deprecation until an appropriate less verbose alternative is available.

there is already an issue to enable skipna on groupby reduction ops #15675, so I think safe to close this as a duplicate.

@phil20686 Thanks for the report. please continue to contribute to the discussion at #15675 instead.