From 8504ac8163fc2113cb3734a0c20b8b7ffd66024c Mon Sep 17 00:00:00 2001
From: Natalia Mokeeva <91160475+natmokval@users.noreply.github.com>
Date: Tue, 16 Jan 2024 21:29:53 +0100
Subject: [PATCH] Backport PR #56906: DEPR: freq ''2BQ-SEP" for to_period
 should raise an error

---
 pandas/core/arrays/datetimes.py                 | 11 +++++------
 .../indexes/datetimes/methods/test_to_period.py | 17 -----------------
 .../tests/indexes/period/test_constructors.py   |  9 +++++++++
 3 files changed, 14 insertions(+), 23 deletions(-)

diff --git a/pandas/core/arrays/datetimes.py b/pandas/core/arrays/datetimes.py
index 6b7ddc4a72957..02656b655a0c6 100644
--- a/pandas/core/arrays/datetimes.py
+++ b/pandas/core/arrays/datetimes.py
@@ -39,10 +39,7 @@
     tz_convert_from_utc,
     tzconversion,
 )
-from pandas._libs.tslibs.dtypes import (
-    abbrev_to_npy_unit,
-    freq_to_period_freqstr,
-)
+from pandas._libs.tslibs.dtypes import abbrev_to_npy_unit
 from pandas.errors import PerformanceWarning
 from pandas.util._exceptions import find_stack_level
 from pandas.util._validators import validate_inclusive
@@ -1232,8 +1229,10 @@ def to_period(self, freq=None) -> PeriodArray:
 
         if freq is None:
             freq = self.freqstr or self.inferred_freq
-            if isinstance(self.freq, BaseOffset):
-                freq = freq_to_period_freqstr(self.freq.n, self.freq.name)
+            if isinstance(self.freq, BaseOffset) and hasattr(
+                self.freq, "_period_dtype_code"
+            ):
+                freq = PeriodDtype(self.freq)._freqstr
 
             if freq is None:
                 raise ValueError(
diff --git a/pandas/tests/indexes/datetimes/methods/test_to_period.py b/pandas/tests/indexes/datetimes/methods/test_to_period.py
index 42a3f3b0f7b42..00c0216a9b3b5 100644
--- a/pandas/tests/indexes/datetimes/methods/test_to_period.py
+++ b/pandas/tests/indexes/datetimes/methods/test_to_period.py
@@ -111,23 +111,6 @@ def test_to_period_frequency_M_Q_Y_A_deprecated(self, freq, freq_depr):
         with tm.assert_produces_warning(FutureWarning, match=msg):
             assert prng.freq == freq_depr
 
-    @pytest.mark.parametrize(
-        "freq, freq_depr",
-        [
-            ("2BQE-SEP", "2BQ-SEP"),
-            ("2BYE-MAR", "2BY-MAR"),
-        ],
-    )
-    def test_to_period_frequency_BQ_BY_deprecated(self, freq, freq_depr):
-        # GH#9586
-        msg = f"'{freq_depr[1:]}' is deprecated and will be removed "
-        f"in a future version, please use '{freq[1:]}' instead."
-
-        rng = date_range("01-Jan-2012", periods=8, freq=freq)
-        prng = rng.to_period()
-        with tm.assert_produces_warning(FutureWarning, match=msg):
-            prng.freq == freq_depr
-
     def test_to_period_infer(self):
         # https://github.com/pandas-dev/pandas/issues/33358
         rng = date_range(
diff --git a/pandas/tests/indexes/period/test_constructors.py b/pandas/tests/indexes/period/test_constructors.py
index 387dc47c48d20..0cd4ddef5a3c9 100644
--- a/pandas/tests/indexes/period/test_constructors.py
+++ b/pandas/tests/indexes/period/test_constructors.py
@@ -48,6 +48,15 @@ def test_period_index_frequency_invalid_freq(self, freq_depr):
         with pytest.raises(ValueError, match=msg):
             PeriodIndex(["2020-01", "2020-05"], freq=freq_depr)
 
+    @pytest.mark.parametrize("freq", ["2BQE-SEP", "2BYE-MAR", "2BME"])
+    def test_period_index_from_datetime_index_invalid_freq(self, freq):
+        # GH#56899
+        msg = f"Invalid frequency: {freq[1:]}"
+
+        rng = date_range("01-Jan-2012", periods=8, freq=freq)
+        with pytest.raises(ValueError, match=msg):
+            rng.to_period()
+
 
 class TestPeriodIndex:
     def test_from_ordinals(self):