review comments

doc/source/whatsnew/v0.24.0.txt

@@ -27,7 +27,7 @@ ExtensionType Changes
 ^^^^^^^^^^^^^^^^^^^^^
 
 - ``ExtensionArray`` has gained the abstract methods ``.dropna()`` and ``.append()``, and attribute ``array_type`` (:issue:`21185`)
-- ``ExtensionDtype`` has gained the ability to instantiate from string dtypes, e.g. ``decimal`` would instaniate a registered ``DecimalDtype`` (:issue:`21185`)
+- ``ExtensionDtype`` has gained the ability to instantiate from string dtypes, e.g. ``decimal`` would instantiate a registered ``DecimalDtype`` (:issue:`21185`)
 - The ``ExtensionArray`` constructor, ``_from_sequence`` now take the keyword arg ``copy=False`` (:issue:`21185`)
 
 .. _whatsnew_0240.api.other:

pandas/core/arrays/base.py

@@ -38,7 +38,6 @@ class ExtensionArray(object):
     * copy
     * append
     * _concat_same_type
-    * array_type
 
     An additional method is available to satisfy pandas' internal,
     private block API.

pandas/core/dtypes/base.py

@@ -109,6 +109,12 @@ class ExtensionDtype(_DtypeOpsMixin):
     * name
     * construct_from_string
 
+
+    Optionally one can assign an array_type for construction with the name
+    of this dtype via the Registry
+
+    * array_type
+
     The `na_value` class attribute can be used to set the default NA value
     for this type. :attr:`numpy.nan` is used by default.
 
@@ -118,6 +124,8 @@ class ExtensionDtype(_DtypeOpsMixin):
     provided for registering virtual subclasses.
     """
 
+    array_type = None
+
     def __str__(self):
         return self.name
 

pandas/core/dtypes/dtypes.py

@@ -2,7 +2,6 @@
 
 import re
 import numpy as np
-from collections import OrderedDict
 from pandas import compat
 from pandas.core.dtypes.generic import ABCIndexClass, ABCCategoricalIndex
 
@@ -18,22 +17,19 @@ class Registry(object):
 
     These are tried in order for inference.
     """
-    dtypes = OrderedDict()
+    dtypes = []
 
     @classmethod
-    def register(self, dtype, constructor=None):
+    def register(self, dtype):
         """
         Parameters
         ----------
-        dtype : PandasExtension Dtype
+        dtype : ExtensionDtype
         """
         if not issubclass(dtype, (PandasExtensionDtype, ExtensionDtype)):
             raise ValueError("can only register pandas extension dtypes")
 
-        if constructor is None:
-            constructor = dtype.construct_from_string
-
-        self.dtypes[dtype] = constructor
+        self.dtypes.append(dtype)
 
     def find(self, dtype):
         """
@@ -54,9 +50,9 @@ def find(self, dtype):
 
             return None
 
-        for dtype_type, constructor in self.dtypes.items():
+        for dtype_type in self.dtypes:
             try:
-                return constructor(dtype)
+                return dtype_type.construct_from_string(dtype)
             except TypeError:
                 pass
 
@@ -610,29 +606,23 @@ def _parse_dtype_strict(cls, freq):
     @classmethod
     def construct_from_string(cls, string):
         """
-        attempt to construct this type from a string, raise a TypeError
-        if its not possible
+        Strict construction from a string, raise a TypeError if not
+        possible
         """
         from pandas.tseries.offsets import DateOffset
-        if isinstance(string, (compat.string_types, DateOffset)):
+
+        if (isinstance(string, compat.string_types) and
+            (string.startswith('period[') or
+             string.startswith('Period[')) or
+                isinstance(string, DateOffset)):
+            # do not parse string like U as period[U]
             # avoid tuple to be regarded as freq
             try:
                 return cls(freq=string)
             except ValueError:
                 pass
         raise TypeError("could not construct PeriodDtype")
 
-    @classmethod
-    def construct_from_string_strict(cls, string):
-        """
-        Strict construction from a string, raise a TypeError if not
-        possible
-        """
-        if string.startswith('period[') or string.startswith('Period['):
-            # do not parse string like U as period[U]
-            return PeriodDtype.construct_from_string(string)
-        raise TypeError("could not construct PeriodDtype")
-
     def __unicode__(self):
         return "period[{freq}]".format(freq=self.freq.freqstr)
 
@@ -747,21 +737,13 @@ def construct_from_string(cls, string):
         attempt to construct this type from a string, raise a TypeError
         if its not possible
         """
-        if isinstance(string, compat.string_types):
+        if (isinstance(string, compat.string_types) and
+            (string.startswith('interval') or
+             string.startswith('Interval'))):
             return cls(string)
         msg = "a string needs to be passed, got type {typ}"
         raise TypeError(msg.format(typ=type(string)))
 
-    @classmethod
-    def construct_from_string_strict(cls, string):
-        """
-        Strict construction from a string, raise a TypeError if not
-        possible
-        """
-        if string.startswith('interval') or string.startswith('Interval'):
-            return IntervalDtype.construct_from_string(string)
-        raise TypeError("cannot construct IntervalDtype")
-
     def __unicode__(self):
         if self.subtype is None:
             return "interval"
@@ -806,6 +788,6 @@ def is_dtype(cls, dtype):
 
 # register the dtypes in search order
 registry.register(DatetimeTZDtype)
-registry.register(PeriodDtype, PeriodDtype.construct_from_string_strict)
-registry.register(IntervalDtype, IntervalDtype.construct_from_string_strict)
+registry.register(PeriodDtype)
+registry.register(IntervalDtype)
 registry.register(CategoricalDtype)

pandas/core/indexes/interval.py

@@ -800,7 +800,7 @@ def astype(self, dtype, copy=True):
     @cache_readonly
     def dtype(self):
         """Return the dtype object of the underlying data"""
-        return IntervalDtype.construct_from_string(str(self.left.dtype))
+        return IntervalDtype(str(self.left.dtype))
 
     @property
     def inferred_type(self):