Group a multi-indexed pandas dataframe by one of its levels?

Question

Is it possible to groupby a multi-index (2 levels) pandas dataframe by one of the multi-index levels ?

The only way I know of doing it is to reset_index on a mult

Answer 1

Yes, use the level parameter. Take a look here. Example:

In [26]: s

first  second  third
bar    doo     one      0.404705
               two      0.577046
baz    bee     one     -1.715002
               two     -1.039268
foo    bop     one     -0.370647
               two     -1.157892
qux    bop     one     -1.344312
               two      0.844885
dtype: float64

In [27]: s.groupby(level=['first','second']).sum()

first  second
bar    doo       0.981751
baz    bee      -2.754270
foo    bop      -1.528539
qux    bop      -0.499427
dtype: float64

Answer 2

If there are already multiple index available, then simply position number can be used instead of column name:

df = df.groupby(level=[0,1]).size()