Pandas groupby().apply() - 从应用函数返回 None 会弄乱结果

Question

I perform groupby() and apply() on a few data frames with the same structure:

d = d.groupby( 'groupby_col', as_index = False ).apply( some_function )

For some it works as expected, for some it fails. The way it fails is that the dataframe becomes a series where each element contains just column names. It looks like this:

In [18]: d.head()
Out[18]:
groupby_col
134663372801          some_col_1 some_col_2 some_col_3 some_col_4...
134663372802          some_col_1  some_col_2  some_col_3  some_col_4...
134663372803          some_col_1  some_col_2  some_col_3  some_col_4...
134663372804          some_col_1  some_col_2  some_col_3  some_col_4...
134663372805          some_col_1  some_col_2  some_col_3  some_col_4...
dtype: object

BTW, the applied function returns either a data frame with a correct number of columns or None.

What might be the reason for this and how to debug it?

Answer 1

The problem goes away if instead of returning None from the applied function I alway return a frame - replaced

if some_condition:
    return

with

if some_condition:
    return d[:0]        # return the empty frame so that the columns match