ENH: Raise TypeError for offsets which are not supported as period frequency

doc/source/whatsnew/v2.2.0.rst

@@ -432,6 +432,7 @@ I/O
 Period
 ^^^^^^
 - Bug in :class:`Period` addition silently wrapping around instead of raising ``OverflowError`` (:issue:`55503`)
+- Bug in incorrectly allowing construction of :class:`Period` with certain offsets which don't have an attribute _period_dtype_code (:issue:`55785`)
 -
 
 Plotting

pandas/_libs/tslibs/offsets.pyx

@@ -4177,7 +4177,7 @@ cdef class CustomBusinessDay(BusinessDay):
  def _period_dtype_code(self):
  # GH#52534
  raise TypeError(
- "CustomBusinessDay cannot be used with Period or PeriodDtype"
+ "CustomBusinessDay is not supported as period frequency"
  )
 
  _apply_array = BaseOffset._apply_array

pandas/_libs/tslibs/period.pyx

@@ -2726,6 +2726,14 @@ class Period(_Period):
 
  if freq is not None:
  freq = cls._maybe_convert_freq(freq)
+ try:
+ period_dtype_code = freq._period_dtype_code
+ except (AttributeError, TypeError):
+ # AttributeError: _period_dtype_code might not exist
+ # TypeError: _period_dtype_code might intentionally raise
+ raise TypeError(
+ f"{(type(freq).__name__)} is not supported as period frequency"
+ )
  nanosecond = 0
 
  if ordinal is not None and value is not None:
@@ -2758,7 +2766,7 @@ class Period(_Period):
 
  elif is_period_object(value):
  other = value
- if freq is None or freq._period_dtype_code == other._dtype._dtype_code:
+ if freq is None or period_dtype_code == other._dtype._dtype_code:
  ordinal = other.ordinal
  freq = other.freq
  else:

pandas/tests/dtypes/test_dtypes.py

@@ -444,7 +444,7 @@ def test_construction(self):
 
  def test_cannot_use_custom_businessday(self):
  # GH#52534
- msg = "CustomBusinessDay cannot be used with Period or PeriodDtype"
+ msg = "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):

pandas/tests/indexes/period/test_constructors.py

@@ -553,6 +553,21 @@ def test_map_with_string_constructor(self):
  # lastly, values should compare equal
  tm.assert_index_equal(res, expected)
 
+ @pytest.mark.parametrize(
+ "freq, freq_msg",
+ [
+ (offsets.BYearBegin(), "BYearBegin"),
+ (offsets.YearBegin(2), "YearBegin"),
+ (offsets.QuarterBegin(startingMonth=12), "QuarterBegin"),
+ (offsets.BusinessMonthEnd(2), "BusinessMonthEnd"),
+ ],
+ )
+ 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):
+ Period(year=2014, freq=freq)
+
 
 class TestShallowCopy:
  def test_shallow_copy_empty(self):

pandas/tests/scalar/period/test_asfreq.py

@@ -825,7 +825,8 @@ def test_asfreq_MS(self):
  with pytest.raises(ValueError, match=msg):
  initial.asfreq(freq="MS", how="S")
 
- with pytest.raises(ValueError, match=msg):
+ msg = "MonthBegin is not supported as period frequency"
+ with pytest.raises(TypeError, match=msg):
  Period("2013-01", "MS")
 
  assert _period_code_map.get("MS") is None

pandas/tests/scalar/period/test_period.py

@@ -38,7 +38,7 @@
 class TestPeriodConstruction:
  def test_custom_business_day_freq_raises(self):
  # GH#52534
- msg = "CustomBusinessDay cannot be used with Period or PeriodDtype"
+ msg = "CustomBusinessDay is not supported as period frequency"
  with pytest.raises(TypeError, match=msg):
  Period("2023-04-10", freq="C")
  with pytest.raises(TypeError, match=msg):
@@ -1628,8 +1628,8 @@ def test_negone_ordinals():
 
 
 def test_invalid_frequency_error_message():
- msg = "Invalid frequency: <WeekOfMonth: week=0, weekday=0>"
- with pytest.raises(ValueError, match=msg):
+ msg = "WeekOfMonth is not supported as period frequency"
+ with pytest.raises(TypeError, match=msg):
  Period("2012-01-02", freq="WOM-1MON")