BUG: PeriodDtype constructor allowing invalid construction, incorrect exception class

doc/source/whatsnew/v2.1.0.rst

@@ -178,7 +178,8 @@ I/O
 
 Period
 ^^^^^^
--
+- Bug in :class:`PeriodDtype` constructor failing to raise ``TypeError`` when no argument is passed or when ``None`` is passed (:issue:`27388`)
+- Bug in :class:`PeriodDtype` constructor raising ``ValueError`` instead of ``TypeError`` when an invalid type is passed (:issue:`51790`)
 -
 
 Plotting

pandas/core/dtypes/dtypes.py

@@ -856,22 +856,15 @@ class PeriodDtype(PeriodDtypeBase, PandasExtensionDtype):
     _match = re.compile(r"(P|p)eriod\[(?P<freq>.+)\]")
     _cache_dtypes: dict[str_type, PandasExtensionDtype] = {}
 
-    def __new__(cls, freq=None):
+    def __new__(cls, freq):
         """
         Parameters
         ----------
-        freq : frequency
+        freq : PeriodDtype, BaseOffset, or string
         """
         if isinstance(freq, PeriodDtype):
             return freq
 
-        elif freq is None:
-            # empty constructor for pickle compat
-            # -10_000 corresponds to PeriodDtypeCode.UNDEFINED
-            u = PeriodDtypeBase.__new__(cls, -10_000)
-            u._freq = None
-            return u
-
         if not isinstance(freq, BaseOffset):
             freq = cls._parse_dtype_strict(freq)
 
@@ -906,7 +899,10 @@ def _parse_dtype_strict(cls, freq: str_type) -> BaseOffset:
             if freq_offset is not None:
                 return freq_offset
 
-        raise ValueError("could not construct PeriodDtype")
+        raise TypeError(
+            "PeriodDtype argument should be string or BaseOffet, "
+            f"got {type(freq).__name__}"
+        )
 
     @classmethod
     def construct_from_string(cls, string: str_type) -> PeriodDtype:

pandas/tests/dtypes/test_dtypes.py

@@ -517,11 +517,16 @@ def test_basic(self, dtype):
         assert not is_period_dtype(np.dtype("float64"))
         assert not is_period_dtype(1.0)
 
-    def test_empty(self):
-        dt = PeriodDtype()
-        msg = "object has no attribute 'freqstr'"
-        with pytest.raises(AttributeError, match=msg):
-            str(dt)
+    def test_freq_argument_required(self):
+        # GH#27388
+        msg = "missing 1 required positional argument: 'freq'"
+        with pytest.raises(TypeError, match=msg):
+            PeriodDtype()
+
+        msg = "PeriodDtype argument should be string or BaseOffet, got NoneType"
+        with pytest.raises(TypeError, match=msg):
+            # GH#51790
+            PeriodDtype(None)
 
     def test_not_string(self):
         # though PeriodDtype has object kind, it cannot be string