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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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1c545b9
Updated condition and added test
arthurlw Sep 14, 2025
8265c7d
Updated condition logic
arthurlw Oct 18, 2025
526bcc6
add comment back
arthurlw Oct 18, 2025
7cee449
add removed test condition
arthurlw Oct 18, 2025
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7 changes: 7 additions & 0 deletions pandas/core/indexes/datetimelike.py
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Original file line number Diff line number Diff line change
Expand Up @@ -637,6 +637,13 @@ def _can_fast_intersect(self, other: Self) -> bool:
# Because freq is not None, we must then be monotonic decreasing
return False

# for non-anchored frequencies, need to check that the two
# indexes actually share a common point
# GH#44025
diff = other[0] - self[0]
if diff != Timedelta(0) and diff.total_seconds() % 86400 != 0:
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@jbrockmendel jbrockmendel Oct 18, 2025

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clearer just to write diff != Timedelta(days=1)?

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@jbrockmendel jbrockmendel Oct 18, 2025

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of never mind, didn't see the %

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@jbrockmendel jbrockmendel Oct 27, 2025

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i dont think this condition ensures they share a point in common. e.g. what if both freqs are Week and they are offset by one day?

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@arthurlw arthurlw Nov 4, 2025

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Assuming the week freqs are created with CDay(7), then self.freq.n is equal to 7 and _can_fast_intersect returns false on the final check:

return self.freq.n == 1

If the freqs are "W" then it seems they snap to Sunday regardless of starting offset.

For specific "W" freqs (e.g. "W-MON", "W-TUE") it returns false on this check:

elif other.freq != self.freq:
    return False

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@jbrockmendel jbrockmendel Nov 4, 2025

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Assuming the week freqs are created with CDay(7)

Why would you assume that. I was thinking pd.offsets.Week(1)

return False

# this along with matching freqs ensure that we "line up",
# so intersection will preserve freq
# Note we are assuming away Ticks, as those go through _range_intersect
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14 changes: 14 additions & 0 deletions 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 @@ -757,3 +758,16 @@ 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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