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TST: Add regression test for infer_freq stateful behavior (#55794) #63224

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TST: Add regression test for infer_freq stateful behavior (#55794) #63224

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TST: Add regression test for infer_freq stateful behavior (#55794)
zhan7236 Nov 28, 2025
c314835
Address review: assert infer_freq returns None for duplicates
zhan7236 Dec 2, 2025
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16 changes: 16 additions & 0 deletions pandas/tests/tseries/frequencies/test_inference.py
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Original file line number Diff line number Diff line change
Expand Up @@ -553,3 +553,19 @@ def test_infer_freq_pyarrow():
assert frequencies.infer_freq(pd_series.values) == "30s"
assert frequencies.infer_freq(pd_index) == "30s"
assert frequencies.infer_freq(pd_series) == "30s"


def test_infer_freq_no_stateful_behavior():
# GH#55794 infer_freq should not have stateful behavior
# calling infer_freq on a full index with duplicates should not
# affect the result of calling it on a slice without duplicates
times = to_datetime(["2019-01-01", "2019-01-02", "2019-01-03", "2019-01-03"])

# Before calling infer_freq on full index
assert frequencies.infer_freq(times[:3]) == "D"

# Call on full index (which has duplicates, so returns None)
assert frequencies.infer_freq(times) is None

# After calling on full index, slice should still return "D"
assert frequencies.infer_freq(times[:3]) == "D"
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