BUG: Attributes of DataFrameGroupBy when subset to a series

[rhshadrach]

xref #9959

When subsetting the columns of a dataframe after a groupby to a series:

df = pd.DataFrame({'a': [1], 'b': [2], 'c': [3]}).set_index('a')
g = df.groupby('a')['b']

the public-facing groupby arguments axis and as_index are not passed on to the resulting SeriesGroupBy.

I think the following two examples should probably behave the same:

g = df.groupby('a', as_index=False)['b']
g2 = df['b'].groupby('a', as_index=False)

but the first produces

   b
0  2

And the 2nd deliberately raises TypeError: as_index=False only valid with DataFrame.

Note that the arguments axis and as_index are only meant for DataFrames, and as such, I think a line such as

g = df.groupby('a', as_index=False)['b']

should raise.

[rhshadrach]

I updated the OP to only include the two attributes that would cause a potential API change.

[rhshadrach]

After rethinking this, perhaps instead

df['b'].groupby('a', as_index=False)

should not raise, but rather be the same as df.groupby('a', as_index=False)['b'] I don't yet know how difficult this would be.

[jbrockmendel]

@rhshadrach you've done a lot of work in this area since the OP. any chance this has been resolved?

[rhshadrach]

For

df = pd.DataFrame({'a': [1], 'b': [2], 'c': [3]}).set_index('a')
g = df.groupby([0, 1], axis=1)['b']
g2 = df['b'].groupby('a', axis=1)

both lines g and g2 raise correctly (I think this has always been the case). For as_index, SeriesGroupBy now fully (I believe) supports it but we raise in Series.groupby if you try to pass as_index=False. I'm in favor of allowing as_index=False here, but that's #36507. Closing in favor of that issue.

[jbrockmendel]

Looks like test_subsetting_columns_keeps_attrs has an xfail for SeriesGroupBy with axis=1, not bc the attribute is failing to be inherited but bc the subsetting operation is raising when it tries to do result = expected["b"]

[rhshadrach]

I think this is expected behavior and shouldn't be an xfail. I'll move it to it's own test.