pandas.DataFrame.groupby.nunique（）不会删除groupby列。 这是错误吗？

Question

Although I set the parameter as_index to True, pandas.DataFrame.groupby.nunique() keeps the columns I am grouping by in the result.

The pandas version is: 0.24.1

df = pd.DataFrame(
    {'a': [1, 1, 2, 3, 2],
     'b': [1, 2, 3, 4, 4]}
)
df.groupby('a', as_index=True).nunique()

The output is:

#    a  b
# a      
# 1  1  2
# 2  1  2
# 3  1  1

I expected:

#    b
# a   
# 1  2
# 2  2
# 3  1

As a counterexample that behaves as expected:

df.groupby('a', as_index=True).max()

results in:

#    b
# a   
# 1  2
# 2  4
# 3  4

Answer 1

If you run [print(df.to_string() + '\\n') for i, df in df.groupby('a', as_index=True)] , you get printed:

   a  b
0  1  1
1  1  2

   a  b
2  2  3
4  2  4

   a  b
3  3  4

The a column isn't set as the index for each data frame group. It is the output of the groupby which has its index set to the group indices when as_index=True (which also is the default), not the data frame groups themselves.