review comments

Author: jreback

doc/source/whatsnew/v0.24.0.txt

@@ -556,6 +556,7 @@ update the ``ExtensionDtype._metadata`` tuple to match the signature of your
 - Slicing a single row of a ``DataFrame`` with multiple ExtensionArrays of the same type now preserves the dtype, rather than coercing to object (:issue:`22784`)
 - Added :meth:`pandas.api.types.register_extension_dtype` to register an extension type with pandas (:issue:`22664`)
 - Updated the ``.type`` attribute for ``PeriodDtype``, ``DatetimeTZDtype``, and ``IntervalDtype`` to be instances of the dtype (``Period``, ``Timestamp``, and ``Interval`` respectively) (:issue:`22938`)
+- Support for reduction operations such as ``sum``, ``mean`` via opt-in base class method override (:issue:`22762`)
 
 .. _whatsnew_0240.api.incompatibilities:
 

pandas/conftest.py

@@ -132,7 +132,8 @@ def all_arithmetic_operators(request):
 
 
 _all_numeric_reductions = ['sum', 'max', 'min',
-                           'mean', 'prod', 'std', 'var', 'median']
+                           'mean', 'prod', 'std', 'var', 'median',
+                           'kurt', 'skew']
 
 
 @pytest.fixture(params=_all_numeric_reductions)

pandas/core/arrays/base.py

@@ -675,17 +675,20 @@ def _ndarray_values(self):
         return np.array(self)
 
     def _reduce(self, name, skipna=True, **kwargs):
-        """Return a scalar result of performing the op
+        """Return a scalar result of performing the reduction operation.
 
         Parameters
         ----------
         name : str
-            name of the function
+            name of the function, support values are:
+            {any, all, min, max, sum, mean, median, prod,
+             std, var, sem, kurt, skew}
         axis : int, default 0
             axis over which to apply, defined as 0 currently
         skipna : bool, default True
             if True, skip NaN values
         kwargs : dict
+            ddof is the only supported kwarg
 
         Returns
         -------
@@ -695,7 +698,8 @@ def _reduce(self, name, skipna=True, **kwargs):
         ------
         TypeError : subclass does not define reductions
         """
-        raise TypeError
+        raise TypeError("cannot perform {name} with type {dtype}".format(
+            name=name, dtype=self.dtype))
 
 
 class ExtensionOpsMixin(object):

pandas/core/arrays/integer.py

@@ -530,7 +530,7 @@ def cmp_method(self, other):
         name = '__{name}__'.format(name=op.__name__)
         return set_function_name(cmp_method, name, cls)
 
-    def _reduce(self, name, axis=0, skipna=True, **kwargs):
+    def _reduce(self, name, skipna=True, **kwargs):
         data = self._data
         mask = self._mask
 
@@ -540,17 +540,15 @@ def _reduce(self, name, axis=0, skipna=True, **kwargs):
             data[mask] = self._na_value
 
         op = getattr(nanops, 'nan' + name)
-        result = op(data, axis=axis, skipna=skipna, mask=mask)
+        result = op(data, axis=0, skipna=skipna, mask=mask)
 
-        # if we have a boolean op, provide coercion back to a bool
-        # type if possible
+        # if we have a boolean op, don't coerce
         if name in ['any', 'all']:
-            if is_integer(result) or is_float(result):
-                result = bool(int(result))
+            pass
 
         # if we have a numeric op, provide coercion back to an integer
         # type if possible
-        elif not isna(result):
+        elif notna(result):
             int_result = int(result)
             if int_result == result:
                 result = int_result

pandas/core/series.py

@@ -3408,6 +3408,9 @@ def _reduce(self, op, name, axis=0, skipna=True, numeric_only=None,
             with np.errstate(all='ignore'):
                 return op(delegate, skipna=skipna, **kwds)
 
+        # TODO(EA) dispatch to Index
+        # remove once all internals extension types are
+        # moved to ExtensionArrays
         return delegate._reduce(op=op, name=name, axis=axis, skipna=skipna,
                                 numeric_only=numeric_only,
                                 filter_type=filter_type, **kwds)

pandas/tests/extension/decimal/array.py

@@ -134,7 +134,7 @@ def _na_value(self):
     def _concat_same_type(cls, to_concat):
         return cls(np.concatenate([x._data for x in to_concat]))
 
-    def _reduce(self, name, axis=0, skipna=True, **kwargs):
+    def _reduce(self, name, skipna=True, **kwargs):
 
         if skipna:
             raise NotImplementedError("decimal does not support skipna=True")
@@ -144,7 +144,7 @@ def _reduce(self, name, axis=0, skipna=True, **kwargs):
         except AttributeError:
             raise NotImplementedError("decimal does not support "
                                       "the {} operation".format(name))
-        return op(axis=axis)
+        return op(axis=0)
 
 
 def to_decimal(values, context=None):

pandas/tests/extension/decimal/test_decimal.py

@@ -135,7 +135,7 @@ class Reduce:
 
     def check_reduce(self, s, op_name, skipna):
 
-        if skipna or op_name in ['median']:
+        if skipna or op_name in ['median', 'skew', 'kurt']:
             with pytest.raises(NotImplementedError):
                 getattr(s, op_name)(skipna=skipna)