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

Author: zhan7236

pandas/tests/tseries/frequencies/test_inference.py

@@ -556,3 +556,19 @@ 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)
+    frequencies.infer_freq(times)
+
+    # After calling on full index, slice should still return "D"
+    assert frequencies.infer_freq(times[:3]) == "D"