BUG: groupby drops columns if the column name is the same as the grouper #8112
Description
>>> df = pd.DataFrame(np.random.randint(1, 9, (2, 3)), columns=['jim', 'joe', 'jolie'])
>>> df
jim joe jolie
0 4 7 8
1 7 8 3
>>> ts = df['joe'] * 0
>>> ts.name
'joe'
>>> gr = df.groupby(ts)
>>> gr.nth(0) # this invokes _set_selection_from_grouper internally
jim jolie
joe
7 4 8
>>> gr.apply(sum) # joe column is gone
jim jolie
joe
0 11 11
whereas:
>>> df.groupby(ts).apply(sum)
jim joe jolie
joe
0 11 15 11
What happens is that this line removes the column from selection if it has the same name as the grouper.