ENH: raise ValueError if invalid period freq pass to asfreq when the …

…index of df is a PeriodIndex (pandas-dev#56945)

doc/source/whatsnew/v2.2.1.rst

@@ -35,6 +35,7 @@ Bug fixes
 - Fixed bug in :func:`pandas.api.interchange.from_dataframe` which was raising for empty inputs (:issue:`56700`)
 - Fixed bug in :func:`pandas.api.interchange.from_dataframe` which wasn't converting columns names to strings (:issue:`55069`)
 - Fixed bug in :meth:`DataFrame.__getitem__` for empty :class:`DataFrame` with Copy-on-Write enabled (:issue:`57130`)
+- Fixed bug in :meth:`PeriodIndex.asfreq` which was silently converting frequencies which are not supported as period frequencies instead of raising an error (:issue:`56945`)
 
 .. ---------------------------------------------------------------------------
 .. _whatsnew_221.other:

pandas/_libs/tslibs/offsets.pyx

@@ -4271,9 +4271,7 @@ cdef class CustomBusinessDay(BusinessDay):
     @property
     def _period_dtype_code(self):
         # GH#52534
-        raise TypeError(
-            "CustomBusinessDay is not supported as period frequency"
-        )
+        raise ValueError(f"{self.base} is not supported as period frequency")
 
     _apply_array = BaseOffset._apply_array
 
@@ -4837,19 +4835,19 @@ cpdef to_offset(freq, bint is_period=False):
     if freq is None:
         return None
 
-    if isinstance(freq, BaseOffset):
-        return freq
-
     if isinstance(freq, tuple):
         raise TypeError(
             f"to_offset does not support tuples {freq}, pass as a string instead"
         )
 
+    if isinstance(freq, BaseOffset):
+        result = freq
+
     elif PyDelta_Check(freq):
-        return delta_to_tick(freq)
+        result = delta_to_tick(freq)
 
     elif isinstance(freq, str):
-        delta = None
+        result = None
         stride_sign = None
 
         try:
@@ -4950,21 +4948,27 @@ cpdef to_offset(freq, bint is_period=False):
                     offset = _get_offset(prefix)
                     offset = offset * int(np.fabs(stride) * stride_sign)
 
-                if delta is None:
-                    delta = offset
+                if result is None:
+                    result = offset
                 else:
-                    delta = delta + offset
+                    result = result + offset
         except (ValueError, TypeError) as err:
             raise ValueError(INVALID_FREQ_ERR_MSG.format(
                 f"{freq}, failed to parse with error message: {repr(err)}")
             )
     else:
-        delta = None
+        result = None
 
-    if delta is None:
+    if result is None:
         raise ValueError(INVALID_FREQ_ERR_MSG.format(freq))
 
-    return delta
+    if is_period and not hasattr(result, "_period_dtype_code"):
+        if isinstance(freq, str):
+            raise ValueError(f"{result.name} is not supported as period frequency")
+        else:
+            raise ValueError(f"{freq} is not supported as period frequency")
+
+    return result
 
 
 # ----------------------------------------------------------------------

pandas/core/arrays/period.py

@@ -733,8 +733,8 @@ def asfreq(self, freq=None, how: str = "E") -> Self:
         '2015-01'], dtype='period[M]')
         """
         how = libperiod.validate_end_alias(how)
-        if isinstance(freq, BaseOffset):
-            freq = freq_to_period_freqstr(freq.n, freq.name)
+        if isinstance(freq, BaseOffset) and hasattr(freq, "_period_dtype_code"):
+            freq = PeriodDtype(freq)._freqstr
         freq = Period._maybe_convert_freq(freq)
 
         base1 = self._dtype._dtype_code
@@ -1186,12 +1186,7 @@ def dt64arr_to_periodarr(
 
     reso = get_unit_from_dtype(data.dtype)
     freq = Period._maybe_convert_freq(freq)
-    try:
-        base = freq._period_dtype_code
-    except (AttributeError, TypeError) as err:
-        # AttributeError: _period_dtype_code might not exist
-        # TypeError: _period_dtype_code might intentionally raise
-        raise TypeError(f"{freq.name} is not supported as period frequency") from err
+    base = freq._period_dtype_code
     return c_dt64arr_to_periodarr(data.view("i8"), base, tz, reso=reso), freq
 
 

pandas/plotting/_matplotlib/timeseries.py

@@ -205,7 +205,10 @@ def _get_ax_freq(ax: Axes):
 
 
 def _get_period_alias(freq: timedelta | BaseOffset | str) -> str | None:
-    freqstr = to_offset(freq, is_period=True).rule_code
+    if isinstance(freq, BaseOffset):
+        freqstr = freq.name
+    else:
+        freqstr = to_offset(freq, is_period=True).rule_code
 
     return get_period_alias(freqstr)
 

pandas/tests/dtypes/test_dtypes.py

@@ -445,12 +445,12 @@ def test_construction(self):
 
     def test_cannot_use_custom_businessday(self):
         # GH#52534
-        msg = "CustomBusinessDay is not supported as period frequency"
+        msg = "C is not supported as period frequency"
+        msg1 = "<CustomBusinessDay> is not supported as period frequency"
         msg2 = r"PeriodDtype\[B\] is deprecated"
-        with pytest.raises(TypeError, match=msg):
-            with tm.assert_produces_warning(FutureWarning, match=msg2):
-                PeriodDtype("C")
-        with pytest.raises(TypeError, match=msg):
+        with pytest.raises(ValueError, match=msg):
+            PeriodDtype("C")
+        with pytest.raises(ValueError, match=msg1):
             with tm.assert_produces_warning(FutureWarning, match=msg2):
                 PeriodDtype(pd.offsets.CustomBusinessDay())
 

pandas/tests/indexes/datetimes/methods/test_to_period.py

@@ -221,5 +221,5 @@ def test_to_period_offsets_not_supported(self, freq):
         # GH#56243
         msg = f"{freq[1:]} is not supported as period frequency"
         ts = date_range("1/1/2012", periods=4, freq=freq)
-        with pytest.raises(TypeError, match=msg):
+        with pytest.raises(ValueError, match=msg):
             ts.to_period()

pandas/tests/indexes/period/methods/test_asfreq.py

@@ -1,3 +1,5 @@
+import re
+
 import pytest
 
 from pandas import (
@@ -7,6 +9,8 @@
 )
 import pandas._testing as tm
 
+from pandas.tseries import offsets
+
 
 class TestPeriodIndex:
     def test_asfreq(self):
@@ -136,3 +140,50 @@ def test_asfreq_with_different_n(self):
 
         excepted = Series([1, 2], index=PeriodIndex(["2020-02", "2020-04"], freq="M"))
         tm.assert_series_equal(result, excepted)
+
+    @pytest.mark.parametrize(
+        "freq",
+        [
+            "2BMS",
+            "2YS-MAR",
+            "2bh",
+        ],
+    )
+    def test_pi_asfreq_not_supported_frequency(self, freq):
+        # GH#55785
+        msg = f"{freq[1:]} is not supported as period frequency"
+
+        pi = PeriodIndex(["2020-01-01", "2021-01-01"], freq="M")
+        with pytest.raises(ValueError, match=msg):
+            pi.asfreq(freq=freq)
+
+    @pytest.mark.parametrize(
+        "freq",
+        [
+            "2BME",
+            "2YE-MAR",
+            "2QE",
+        ],
+    )
+    def test_pi_asfreq_invalid_frequency(self, freq):
+        # GH#55785
+        msg = f"Invalid frequency: {freq}"
+
+        pi = PeriodIndex(["2020-01-01", "2021-01-01"], freq="M")
+        with pytest.raises(ValueError, match=msg):
+            pi.asfreq(freq=freq)
+
+    @pytest.mark.parametrize(
+        "freq",
+        [
+            offsets.MonthBegin(2),
+            offsets.BusinessMonthEnd(2),
+        ],
+    )
+    def test_pi_asfreq_invalid_baseoffset(self, freq):
+        # GH#56945
+        msg = re.escape(f"{freq} is not supported as period frequency")
+
+        pi = PeriodIndex(["2020-01-01", "2021-01-01"], freq="M")
+        with pytest.raises(ValueError, match=msg):
+            pi.asfreq(freq=freq)

pandas/tests/resample/test_period_index.py

@@ -1032,8 +1032,8 @@ def test_resample_lowercase_frequency_deprecated(
             offsets.BusinessHour(2),
         ],
     )
-    def test_asfreq_invalid_period_freq(self, offset, frame_or_series):
-        # GH#9586
+    def test_asfreq_invalid_period_offset(self, offset, frame_or_series):
+        # GH#55785
         msg = f"Invalid offset: '{offset.base}' for converting time series "
 
         obj = frame_or_series(range(5), index=period_range("2020-01-01", periods=5))

pandas/tests/scalar/period/test_asfreq.py

@@ -820,10 +820,9 @@ def test_asfreq_MS(self):
 
         assert initial.asfreq(freq="M", how="S") == Period("2013-01", "M")
 
-        msg = INVALID_FREQ_ERR_MSG
+        msg = "MS is not supported as period frequency"
         with pytest.raises(ValueError, match=msg):
             initial.asfreq(freq="MS", how="S")
 
-        msg = "MonthBegin is not supported as period frequency"
-        with pytest.raises(TypeError, match=msg):
+        with pytest.raises(ValueError, match=msg):
             Period("2013-01", "MS")

pandas/tests/scalar/period/test_period.py

@@ -3,6 +3,7 @@
     datetime,
     timedelta,
 )
+import re
 
 import numpy as np
 import pytest
@@ -40,21 +41,22 @@ class TestPeriodDisallowedFreqs:
     )
     def test_offsets_not_supported(self, freq, freq_msg):
         # GH#55785
-        msg = f"{freq_msg} is not supported as period frequency"
-        with pytest.raises(TypeError, match=msg):
+        msg = re.escape(f"{freq} is not supported as period frequency")
+        with pytest.raises(ValueError, match=msg):
             Period(year=2014, freq=freq)
 
     def test_custom_business_day_freq_raises(self):
         # GH#52534
-        msg = "CustomBusinessDay is not supported as period frequency"
-        with pytest.raises(TypeError, match=msg):
+        msg = "C is not supported as period frequency"
+        with pytest.raises(ValueError, match=msg):
             Period("2023-04-10", freq="C")
-        with pytest.raises(TypeError, match=msg):
+        msg = f"{offsets.CustomBusinessDay().base} is not supported as period frequency"
+        with pytest.raises(ValueError, match=msg):
             Period("2023-04-10", freq=offsets.CustomBusinessDay())
 
     def test_invalid_frequency_error_message(self):
-        msg = "WeekOfMonth is not supported as period frequency"
-        with pytest.raises(TypeError, match=msg):
+        msg = "WOM-1MON is not supported as period frequency"
+        with pytest.raises(ValueError, match=msg):
             Period("2012-01-02", freq="WOM-1MON")
 
     def test_invalid_frequency_period_error_message(self):