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BUG: DatetimeIndex intersection with non-anchored freq #62341

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arthurlw wants to merge 1 commit into pandas-dev:main

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BUG: DatetimeIndex intersection with non-anchored freq #62341

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Updated condition and added test

arthurlw Sep 14, 2025

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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":
Copy link Member @jbrockmendel jbrockmendel Sep 15, 2025 Choose a reason for hiding this comment The reason will be displayed to describe this comment to others. Learn more. the issue isn't just handling "C", but identifying what conditions work in the general case
		return False

		elif other.freq != self.freq:
Expand Down

15 changes: 14 additions & 1 deletion pandas/tests/indexes/datetimes/test_setops.py

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Original file line number	Diff line number	Diff line change
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年10月13日 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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