API: EA._can_hold_na -> EDtype.can_hold_na

doc/source/whatsnew/v1.3.0.rst

@@ -644,6 +644,7 @@ Other API changes
 - Accessing ``_constructor_expanddim`` on a :class:`DataFrame` and ``_constructor_sliced`` on a :class:`Series` now raise an ``AttributeError``. Previously a ``NotImplementedError`` was raised (:issue:`38782`)
 - Added new ``engine`` and ``**engine_kwargs`` parameters to :meth:`DataFrame.to_sql` to support other future "SQL engines". Currently we still only use ``SQLAlchemy`` under the hood, but more engines are planned to be supported such as ``turbodbc`` (:issue:`36893`)
 - Removed redundant ``freq`` from :class:`PeriodIndex` string representation (:issue:`41653`)
+- :attr:`ExtensionArray._can_hold_na` should now be implemented on the :class:`ExtensionDtype` subclass as ``_can_hold_na`` (:issue:`40574`)
 
 Build
 =====

pandas/core/arrays/base.py

@@ -35,6 +35,7 @@
 from pandas.util._decorators import (
     Appender,
     Substitution,
+    cache_readonly,
 )
 from pandas.util._validators import (
     validate_bool_kwarg,
@@ -1273,7 +1274,9 @@ def _concat_same_type(
     # such as take(), reindex(), shift(), etc.  In addition, those results
     # will then be of the ExtensionArray subclass rather than an array
     # of objects
-    _can_hold_na = True
+    @cache_readonly
+    def _can_hold_na(self) -> bool:
+        return self.dtype._can_hold_na
 
     def _reduce(self, name: str, *, skipna: bool = True, **kwargs):
         """

pandas/core/arrays/categorical.py

@@ -353,7 +353,6 @@ class Categorical(NDArrayBackedExtensionArray, PandasObject, ObjectStringArrayMi
     # tolist is not actually deprecated, just suppressed in the __dir__
     _hidden_attrs = PandasObject._hidden_attrs | frozenset(["tolist"])
     _typ = "categorical"
-    _can_hold_na = True
 
     _dtype: CategoricalDtype
 

pandas/core/arrays/datetimelike.py

@@ -159,6 +159,10 @@ class DatetimeLikeArrayMixin(OpsMixin, NDArrayBackedExtensionArray):
     _recognized_scalars: tuple[type, ...]
     _ndarray: np.ndarray
 
+    @cache_readonly
+    def _can_hold_na(self) -> bool:
+        return True
+
     def __init__(self, data, dtype: Dtype | None = None, freq=None, copy=False):
         raise AbstractMethodError(self)
 

pandas/core/dtypes/base.py

@@ -367,6 +367,13 @@ def _get_common_dtype(self, dtypes: list[DtypeObj]) -> DtypeObj | None:
         else:
             return None
 
+    @property
+    def _can_hold_na(self) -> bool:
+        """
+        Can arrays of this dtype hold NA values?
+        """
+        return True
+
 
 def register_extension_dtype(cls: type[E]) -> type[E]:
     """

pandas/core/internals/blocks.py

@@ -175,15 +175,15 @@ def is_view(self) -> bool:
         return values.base is not None
 
     @final
-    @property
+    @cache_readonly
     def _can_hold_na(self) -> bool:
         """
         Can we store NA values in this Block?
         """
-        values = self.values
-        if isinstance(values, np.ndarray):
-            return values.dtype.kind not in ["b", "i", "u"]
-        return values._can_hold_na
+        dtype = self.dtype
+        if isinstance(dtype, np.dtype):
+            return dtype.kind not in ["b", "i", "u"]
+        return dtype._can_hold_na
 
     @final
     @cache_readonly

pandas/tests/extension/test_external_block.py

@@ -14,9 +14,11 @@
 class CustomBlock(ExtensionBlock):
 
     _holder = np.ndarray
-    # error: Cannot override final attribute "_can_hold_na"
-    # (previously declared in base class "Block")
-    _can_hold_na = False  # type: ignore[misc]
+
+    # Cannot override final attribute "_can_hold_na"
+    @property  # type: ignore[misc]
+    def _can_hold_na(self) -> bool:
+        return False
 
 
 @pytest.fixture