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BUG: Make DTI/TDI/PI argsort match their underlying arrays #37965

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Dec 17, 2020
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5 changes: 4 additions & 1 deletion pandas/core/groupby/grouper.py
Original file line number Diff line number Diff line change
Expand Up @@ -374,7 +374,10 @@ def _set_grouper(self, obj: FrameOrSeries, sort: bool = False):
# possibly sort
if (self.sort or sort) and not ax.is_monotonic:
# use stable sort to support first, last, nth
indexer = self.indexer = ax.argsort(kind="mergesort")
# TODO: why does putting na_position="first" fix datetimelike cases?
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I am not really familiar with the history of NaTs with respect to resample. I would have assumed NaT would/should have been dropped by this point since we just exclude them from the result #13164.

I'm okay with this solution as a stop gap solution.

indexer = self.indexer = ax.array.argsort(
kind="mergesort", na_position="first"
)
ax = ax.take(indexer)
obj = obj.take(indexer, axis=self.axis)

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4 changes: 0 additions & 4 deletions pandas/core/indexes/base.py
Original file line number Diff line number Diff line change
Expand Up @@ -4776,10 +4776,6 @@ def argsort(self, *args, **kwargs) -> np.ndarray:
>>> idx[order]
Index(['a', 'b', 'c', 'd'], dtype='object')
"""
if needs_i8_conversion(self.dtype):
# TODO: these do not match the underlying EA argsort methods GH#37863
return self.asi8.argsort(*args, **kwargs)

# This works for either ndarray or EA, is overriden
# by RangeIndex, MultIIndex
return self._data.argsort(*args, **kwargs)
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8 changes: 8 additions & 0 deletions pandas/tests/indexes/datetimelike.py
Original file line number Diff line number Diff line change
Expand Up @@ -10,6 +10,14 @@


class DatetimeLike(Base):
def test_argsort_matches_array(self):
rng = self.create_index()
rng = rng.insert(1, pd.NaT)

result = rng.argsort()
expected = rng._data.argsort()
tm.assert_numpy_array_equal(result, expected)

def test_can_hold_identifiers(self):
idx = self.create_index()
key = idx[0]
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