ENH: add integer-na support via an ExtensionArray

closes pandas-dev#20700

pandas/core/algorithms.py

@@ -154,7 +154,7 @@ def _reconstruct_data(values, dtype, original):
     """
     from pandas import Index
     if is_extension_array_dtype(dtype):
-        pass
+        values = dtype.array_type._from_sequence(values)
     elif is_datetime64tz_dtype(dtype) or is_period_dtype(dtype):
         values = Index(original)._shallow_copy(values, name=None)
     elif is_bool_dtype(dtype):
@@ -705,7 +705,7 @@ def value_counts(values, sort=True, ascending=False, normalize=False,
 
     else:
 
-        if is_categorical_dtype(values) or is_sparse(values):
+        if is_extension_array_dtype(values) or is_sparse(values):
 
             # handle Categorical and sparse,
             result = Series(values)._values.value_counts(dropna=dropna)

pandas/core/arrays/base.py

@@ -82,14 +82,16 @@ class ExtensionArray(object):
     # Constructors
     # ------------------------------------------------------------------------
     @classmethod
-    def _from_sequence(cls, scalars):
+    def _from_sequence(cls, scalars, copy=False):
         """Construct a new ExtensionArray from a sequence of scalars.
 
         Parameters
         ----------
         scalars : Sequence
             Each element will be an instance of the scalar type for this
             array, ``cls.dtype.type``.
+        copy : boolean, default True
+            if True, copy the underlying data
         Returns
         -------
         ExtensionArray
@@ -567,6 +569,34 @@ def copy(self, deep=False):
         """
         raise AbstractMethodError(self)
 
+    def append(self, other):
+        """
+        Append a collection of Arrays together
+
+        Parameters
+        ----------
+        other : ExtenionArray or list/tuple of ExtenionArrays
+
+        Returns
+        -------
+        appended : ExtensionArray
+        """
+
+        to_concat = [self]
+        cls = self.__class__
+
+        if isinstance(other, (list, tuple)):
+            to_concat = to_concat + list(other)
+        else:
+            to_concat.append(other)
+
+        for obj in to_concat:
+            if not isinstance(obj, cls):
+                raise TypeError('all inputs must be of type {}'.format(
+                    cls.__name__))
+
+        return cls._concat_same_type(to_concat)
+
     # ------------------------------------------------------------------------
     # Block-related methods
     # ------------------------------------------------------------------------

pandas/core/arrays/categorical.py

@@ -2343,6 +2343,10 @@ def isin(self, values):
         return algorithms.isin(self.codes, code_values)
 
 
+# inform the Dtype about us
+CategoricalDtype.array_type = Categorical
+
+
 # The Series.cat accessor