this is clearly documented: https://pandas.pydata.org/pandas-docs/dev/reference/api/pandas.DataFrame.groupby.html?highlight=groupby#pandas.DataFrame.groupby

I don't see any reason to expect this to align.

this is clearly documented: https://pandas.pydata.org/pandas-docs/dev/reference/api/pandas.DataFrame.groupby.html?highlight=groupby#pandas.DataFrame.groupby

I don't see any reason to expect this to align.

Hi. Would you clarify how the documentation indicates that the series should not be aligned? As I read the excerpt below on the by parameter, when grouping by a Series, pandas should first align the series and then use its values to assign groups.

If a dict or Series is passed, the Series or dict VALUES will be used to determine the groups (the Series’ values are first aligned; see .align() method).

In my two examples above under "Reproducible Example" and "Expected Behavior," the key difference is only the data type of the series; pandas correctly aligns the series with object data type but does not align the series with a categorical data type.

In [14]: series.align(grouper)
Out[14]: 
(0    0
 1    1
 2    2
 3    3
 4    4
 5    5
 6    6
 dtype: int64,
 0      A
 1      B
 2      B
 3      A
 4    NaN
 5    NaN
 6    NaN
 dtype: category
 Categories (2, object): ['A', 'B'])

this is not what one would expect here. yes you can do this but i would expect an error to accidently align on non-sensical things.

@stanwest i see you opened a PR, will look.

Elaborating on the display of the aligned objects, if we assign

aligned_series, aligned_grouper = series.align(grouper)

then pandas currently will group the series by the given categories (dropping the missing categories) with either of the following:

# Group by a Series with categorical data type and matching index.
aligned_series.groupby(aligned_grouper).groups
# Returns {'A': [0, 3], 'B': [1, 2]}

# Group by a Categorical with matching length.
aligned_series.groupby(aligned_grouper.values).groups
# Returns {'A': [0, 3], 'B': [1, 2]}

As I understand the concepts and the documentation, series.groupby(grouper) should also produce the same result by performing the alignment internally. However, the check for categorical groupers derails the grouping operation before alignment can happen. I believe that the PR narrows that check appropriately and makes it consistent with length checks for other groupers. Thanks, @jreback, for looking over the PR and for the dialog.