make construct_array_type arg optional

pandas/core/dtypes/base.py

@@ -162,18 +162,20 @@ def name(self):
         raise AbstractMethodError(self)
 
     @classmethod
-    def construct_array_type(cls, array):
+    def construct_array_type(cls, array=None):
         """Return the array type associated with this dtype
 
         Parameters
         ----------
-        string : str
+        array : array-like, optional
 
         Returns
         -------
         type
         """
-        return type(array)
+        if array is None:
+            return cls
+        raise NotImplementedError
 
     @classmethod
     def construct_from_string(cls, string):

pandas/core/dtypes/dtypes.py

@@ -318,12 +318,12 @@ def _hash_categories(categories, ordered=True):
             return np.bitwise_xor.reduce(hashed)
 
     @classmethod
-    def construct_array_type(cls, array):
+    def construct_array_type(cls, array=None):
         """Return the array type associated with this dtype
 
         Parameters
         ----------
-        array : value array
+        array : array-like, optional
 
         Returns
         -------

pandas/tests/extension/base/dtype.py

@@ -1,3 +1,4 @@
+import pytest
 import numpy as np
 import pandas as pd
 
@@ -46,3 +47,10 @@ def test_eq_with_str(self, dtype):
 
     def test_eq_with_numpy_object(self, dtype):
         assert dtype != np.dtype('object')
+
+    def test_array_type(self, data, dtype):
+        assert dtype.construct_array_type() is type(data)
+
+    def test_array_type_with_arg(self, data, dtype):
+        with pytest.raises(NotImplementedError):
+            dtype.construct_array_type('foo')

pandas/tests/extension/category/test_categorical.py

@@ -52,7 +52,9 @@ def data_for_grouping():
 
 
 class TestDtype(base.BaseDtypeTests):
-    pass
+
+    def test_array_type_with_arg(self, data, dtype):
+        assert dtype.construct_array_type('foo') is Categorical
 
 
 class TestInterface(base.BaseInterfaceTests):

pandas/tests/extension/decimal/array.py

@@ -16,12 +16,12 @@ class DecimalDtype(ExtensionDtype):
     na_value = decimal.Decimal('NaN')
 
     @classmethod
-    def construct_array_type(cls, array):
+    def construct_array_type(cls, array=None):
         """Return the array type associated with this dtype
 
         Parameters
         ----------
-        string : str
+        array : array-like, optional
 
         Returns
         -------

pandas/tests/extension/decimal/test_decimal.py

@@ -92,7 +92,9 @@ def assert_frame_equal(self, left, right, *args, **kwargs):
 
 
 class TestDtype(BaseDecimal, base.BaseDtypeTests):
-    pass
+
+    def test_array_type_with_arg(self, data, dtype):
+        assert dtype.construct_array_type('foo') is DecimalArray
 
 
 class TestInterface(BaseDecimal, base.BaseInterfaceTests):

pandas/tests/extension/json/array.py

@@ -33,12 +33,12 @@ class JSONDtype(ExtensionDtype):
         na_value = {}
 
     @classmethod
-    def construct_array_type(cls, array):
+    def construct_array_type(cls, array=None):
         """Return the array type associated with this dtype
 
         Parameters
         ----------
-        string : str
+        array : array-like, optional
 
         Returns
         -------

pandas/tests/extension/json/test_json.py

@@ -107,7 +107,9 @@ def assert_frame_equal(self, left, right, *args, **kwargs):
 
 
 class TestDtype(BaseJSON, base.BaseDtypeTests):
-    pass
+
+    def test_array_type_with_arg(self, data, dtype):
+        assert dtype.construct_array_type('foo') is JSONArray
 
 
 class TestInterface(BaseJSON, base.BaseInterfaceTests):