DEPR: `obj` argument in `GroupBy.get_group`

[rhshadrach]

From the code:

def get_group(self, name, obj=None) -> DataFrame | Series:
        """
        Construct DataFrame from group with provided name.
        Parameters
        ----------
        name : object
            The name of the group to get as a DataFrame.
        obj : DataFrame, default None
            The DataFrame to take the DataFrame out of.  If
            it is None, the object groupby was called on will
            be used.
        Returns
        -------
        same type as obj

Getting the group of an object that is unrelated to the groupby seems unnecessary. Also the implementation appears to use the indices of the requested group from self._selected_obj and return those indices (via integer location) in the passed obj. This doesn't seem meaninful/useful.

I think we should deprecate this argument.

df = pd.DataFrame({'a': [1, 1, 2], 'b': [3, 4, 5]})
gb = df.groupby('a')
df2 = pd.DataFrame({'a': [3], 'b': [6]})

print(gb.get_group(1, obj=df2))
# IndexError: indices are out-of-bounds

print(gb.get_group(3, obj=df2))
# KeyError: 3

[jbrockmendel]

+1

[lithomas1]

+1.

Were you planning to work on this, or is it fine to mark as good first issue?

[topper-123]

+1.

[rhshadrach]

@lithomas1 - I've added the tag.

[natmokval]

I would like to work on this.

[anirudh2781998]

i dont understand the issue kindly some one help me.I'm trying to make my first open source contribution