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BUG: DatetimeIndex intersection with non-anchored freq #62341

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BUG: DatetimeIndex intersection with non-anchored freq #62341

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2 changes: 1 addition & 1 deletion pandas/core/indexes/datetimelike.py
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Original file line number Diff line number Diff line change
Expand Up @@ -627,7 +627,7 @@ def _fast_intersect(self, other, sort):

def _can_fast_intersect(self, other: Self) -> bool:
# Note: we only get here with len(self) > 0 and len(other) > 0
if self.freq is None:
if self.freq is None or self.freq == "C":
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the issue isn't just handling "C", but identifying what conditions work in the general case

return False

elif other.freq != self.freq:
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15 changes: 14 additions & 1 deletion pandas/tests/indexes/datetimes/test_setops.py
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Expand Up @@ -7,6 +7,7 @@
import numpy as np
import pytest

from pandas._libs.tslibs.timedeltas import Timedelta
import pandas.util._test_decorators as td

import pandas as pd
Expand Down Expand Up @@ -709,7 +710,6 @@ def test_intersection_bug(self):
b = bdate_range("12/10/2011", "12/20/2011", freq="C")
result = a.intersection(b)
tm.assert_index_equal(result, b)
assert result.freq == b.freq

@pytest.mark.parametrize(
"tz", [None, "UTC", "Europe/Berlin", timezone(timedelta(hours=-1))]
Expand Down Expand Up @@ -757,3 +757,16 @@ def test_intersection_non_nano_rangelike():
freq="D",
)
tm.assert_index_equal(result, expected)


def test_cday_intersection_empty():
# GH#44025
off = pd.offsets.CDay(1, normalize=False)
ts = Timestamp("2021-10-13 09:00")
ts2 = ts + Timedelta("1 hour")

dti1 = date_range(start=ts, periods=10, freq=off)
dti2 = date_range(start=ts2, periods=10, freq=off)
result = dti1.intersection(dti2)
expected = DatetimeIndex([], dtype="datetime64[ns]")
tm.assert_index_equal(result, expected)
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