API: Make ExtensionDtype.construct_array_type a method

This allows a single dtype to support multiple array classes.
For arrow-backed strings, we'll likely want a separate array class
for ease of implementation, clarity. But we'll have a parametrized
dtype.

```python
class StringDtype:
    def __init__(self, storage="python"):
        self.storage = storage

    def construct_array_type(self):  # regular method
        if self.storage == "python":
            return StringArray
        else:
            return ArrowStringArray
```

Closes pandas-dev#36126

Author: TomAugspurger

doc/source/whatsnew/v1.2.0.rst

@@ -57,6 +57,13 @@ Other enhancements
 -
 -
 
+.. _whatsnew_120.api_breaking.experimental:
+
+Changes to experimental APIs
+----------------------------
+
+- :meth:`pandas.api.extensions.ExtensionDtype.construct_array_type` has changed from a classmethod to a regular method to support one dtype being used for multiple arrays. To migrate, change your definition to a regular method and ensure that your method is called on instances rather than the class (:issue:`36126`).
+
 .. _whatsnew_120.api_breaking.python:
 
 Increased minimum version for Python

pandas/core/arrays/boolean.py

@@ -70,8 +70,7 @@ def kind(self) -> str:
     def numpy_dtype(self) -> np.dtype:
         return np.dtype("bool")
 
-    @classmethod
-    def construct_array_type(cls) -> Type["BooleanArray"]:
+    def construct_array_type(self) -> Type["BooleanArray"]:
         """
         Return the array type associated with this dtype.
 

pandas/core/arrays/integer.py

@@ -80,8 +80,7 @@ def itemsize(self) -> int:
         """ Return the number of bytes in this dtype """
         return self.numpy_dtype.itemsize
 
-    @classmethod
-    def construct_array_type(cls) -> Type["IntegerArray"]:
+    def construct_array_type(self) -> Type["IntegerArray"]:
         """
         Return the array type associated with this dtype.
 

pandas/core/arrays/masked.py

@@ -40,8 +40,7 @@ class BaseMaskedDtype(ExtensionDtype):
     def numpy_dtype(self) -> np.dtype:
         raise AbstractMethodError
 
-    @classmethod
-    def construct_array_type(cls) -> Type["BaseMaskedArray"]:
+    def construct_array_type(self) -> Type["BaseMaskedArray"]:
         """
         Return the array type associated with this dtype.
 

pandas/core/arrays/numpy_.py

@@ -94,8 +94,7 @@ def construct_from_string(cls, string: str) -> "PandasDtype":
             raise TypeError(msg) from err
         return cls(dtype)
 
-    @classmethod
-    def construct_array_type(cls) -> Type["PandasArray"]:
+    def construct_array_type(self) -> Type["PandasArray"]:
         """
         Return the array type associated with this dtype.
 

pandas/core/arrays/sparse/dtype.py

@@ -171,8 +171,7 @@ def name(self):
     def __repr__(self) -> str:
         return self.name
 
-    @classmethod
-    def construct_array_type(cls) -> Type["SparseArray"]:
+    def construct_array_type(self) -> Type["SparseArray"]:
         """
         Return the array type associated with this dtype.
 

pandas/core/dtypes/base.py

@@ -187,8 +187,7 @@ def names(self) -> Optional[List[str]]:
         """
         return None
 
-    @classmethod
-    def construct_array_type(cls) -> Type["ExtensionArray"]:
+    def construct_array_type(self) -> Type["ExtensionArray"]:
         """
         Return the array type associated with this dtype.
 

pandas/core/dtypes/dtypes.py

@@ -430,8 +430,7 @@ def _hash_categories(categories, ordered: Ordered = True) -> int:
         hashed = _combine_hash_arrays(iter(cat_array), num_items=len(cat_array))
         return np.bitwise_xor.reduce(hashed)
 
-    @classmethod
-    def construct_array_type(cls) -> Type["Categorical"]:
+    def construct_array_type(self) -> Type["Categorical"]:
         """
         Return the array type associated with this dtype.
 
@@ -679,8 +678,7 @@ def tz(self):
         """
         return self._tz
 
-    @classmethod
-    def construct_array_type(cls) -> Type["DatetimeArray"]:
+    def construct_array_type(self) -> Type["DatetimeArray"]:
         """
         Return the array type associated with this dtype.
 
@@ -922,8 +920,7 @@ def is_dtype(cls, dtype: object) -> bool:
                 return False
         return super().is_dtype(dtype)
 
-    @classmethod
-    def construct_array_type(cls) -> Type["PeriodArray"]:
+    def construct_array_type(self) -> Type["PeriodArray"]:
         """
         Return the array type associated with this dtype.
 
@@ -1047,8 +1044,7 @@ def subtype(self):
         """
         return self._subtype
 
-    @classmethod
-    def construct_array_type(cls) -> Type["IntervalArray"]:
+    def construct_array_type(self) -> Type["IntervalArray"]:
         """
         Return the array type associated with this dtype.
 

pandas/tests/arrays/test_array.py

@@ -277,8 +277,7 @@ def test_scalar_raises():
 class DecimalDtype2(DecimalDtype):
     name = "decimal2"
 
-    @classmethod
-    def construct_array_type(cls):
+    def construct_array_type(self):
         """
         Return the array type associated with this dtype.
 

pandas/tests/extension/arrow/arrays.py

@@ -31,8 +31,7 @@ class ArrowBoolDtype(ExtensionDtype):
     name = "arrow_bool"
     na_value = pa.NULL
 
-    @classmethod
-    def construct_array_type(cls) -> Type["ArrowBoolArray"]:
+    def construct_array_type(self) -> Type["ArrowBoolArray"]:
         """
         Return the array type associated with this dtype.
 
@@ -55,8 +54,7 @@ class ArrowStringDtype(ExtensionDtype):
     name = "arrow_string"
     na_value = pa.NULL
 
-    @classmethod
-    def construct_array_type(cls) -> Type["ArrowStringArray"]:
+    def construct_array_type(self) -> Type["ArrowStringArray"]:
         """
         Return the array type associated with this dtype.
 

pandas/tests/extension/decimal/array.py

@@ -28,8 +28,7 @@ def __init__(self, context=None):
     def __repr__(self) -> str:
         return f"DecimalDtype(context={self.context})"
 
-    @classmethod
-    def construct_array_type(cls) -> Type["DecimalArray"]:
+    def construct_array_type(self) -> Type["DecimalArray"]:
         """
         Return the array type associated with this dtype.
 

pandas/tests/extension/json/array.py

@@ -32,8 +32,7 @@ class JSONDtype(ExtensionDtype):
     name = "json"
     na_value: Mapping[str, Any] = UserDict()
 
-    @classmethod
-    def construct_array_type(cls) -> Type["JSONArray"]:
+    def construct_array_type(self) -> Type["JSONArray"]:
         """
         Return the array type associated with this dtype.
 

pandas/tests/extension/list/array.py

@@ -21,8 +21,7 @@ class ListDtype(ExtensionDtype):
     name = "list"
     na_value = np.nan
 
-    @classmethod
-    def construct_array_type(cls) -> Type["ListArray"]:
+    def construct_array_type(self) -> Type["ListArray"]:
         """
         Return the array type associated with this dtype.