BUG: preserve categorical & sparse types when grouping / pivot

doc/source/whatsnew/v0.25.0.rst

@@ -322,6 +322,35 @@ of ``object`` dtype. :attr:`Series.str` will now infer the dtype data *within* t
     s
     s.str.startswith(b'a')
 
+.. _whatsnew_0250.api_breaking.groupby_categorical:
+
+Categorical dtypes are preserved during groupby
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+
+Previously, columns that were categorical, but not the groupby key(s) would be converted to ``object`` dtype during groupby operations. Pandas now will preserve these dtypes. (:issue:`18502`)
+
+.. ipython:: python
+
+   df = pd.DataFrame(
+       {'payload': [-1, -2, -1, -2],
+        'col': pd.Categorical(["foo", "bar", "bar", "qux"], ordered=True)})
+   df
+   df.dtypes
+
+*Previous Behavior*:
+
+.. code-block:: python
+
+   In [5]: df.groupby('payload').first().col.dtype
+   Out[5]: dtype('O')
+
+*New Behavior*:
+
+.. ipython:: python
+
+   df.groupby('payload').first().col.dtype
+
+
 .. _whatsnew_0250.api_breaking.incompatible_index_unions:
 
 Incompatible Index type unions
@@ -809,7 +838,7 @@ ExtensionArray
 
 - Bug in :func:`factorize` when passing an ``ExtensionArray`` with a custom ``na_sentinel`` (:issue:`25696`).
 - :meth:`Series.count` miscounts NA values in ExtensionArrays (:issue:`26835`)
-- Keyword argument ``deep`` has been removed from :method:`ExtensionArray.copy` (:issue:`27083`)
+- Keyword argument ``deep`` has been removed from :meth:`ExtensionArray.copy` (:issue:`27083`)
 
 Other
 ^^^^^

pandas/core/groupby/generic.py

@@ -158,12 +158,19 @@ def _cython_agg_blocks(self, how, alt=None, numeric_only=True,
 
                 obj = self.obj[data.items[locs]]
                 s = groupby(obj, self.grouper)
-                result = s.aggregate(lambda x: alt(x, axis=self.axis))
+                try:
+                    result = s.aggregate(lambda x: alt(x, axis=self.axis))
+                except TypeError:
+                    # we may have an exception in trying to aggregate
+                    # continue and exclude the block
+                    pass
 
             finally:
 
+                dtype = block.values.dtype
+
                 # see if we can cast the block back to the original dtype
-                result = block._try_coerce_and_cast_result(result)
+                result = block._try_coerce_and_cast_result(result, dtype=dtype)
                 newb = block.make_block(result)
 
             new_items.append(locs)

pandas/core/groupby/groupby.py

@@ -786,6 +786,8 @@ def _try_cast(self, result, obj, numeric_only=False):
             elif is_extension_array_dtype(dtype):
                 # The function can return something of any type, so check
                 # if the type is compatible with the calling EA.
+
+                # return the same type (Series) as our caller
                 try:
                     result = obj._values._from_sequence(result, dtype=dtype)
                 except Exception:
@@ -1157,7 +1159,8 @@ def mean(self, *args, **kwargs):
         """
         nv.validate_groupby_func('mean', args, kwargs, ['numeric_only'])
         try:
-            return self._cython_agg_general('mean', **kwargs)
+            return self._cython_agg_general(
+                'mean', alt=lambda x, axis: Series(x).mean(**kwargs), **kwargs)
         except GroupByError:
             raise
         except Exception:  # pragma: no cover
@@ -1179,7 +1182,11 @@ def median(self, **kwargs):
             Median of values within each group.
         """
         try:
-            return self._cython_agg_general('median', **kwargs)
+            return self._cython_agg_general(
+                'median',
+                alt=lambda x,
+                axis: Series(x).median(axis=axis, **kwargs),
+                **kwargs)
         except GroupByError:
             raise
         except Exception:  # pragma: no cover
@@ -1235,7 +1242,10 @@ def var(self, ddof=1, *args, **kwargs):
         nv.validate_groupby_func('var', args, kwargs)
         if ddof == 1:
             try:
-                return self._cython_agg_general('var', **kwargs)
+                return self._cython_agg_general(
+                    'var',
+                    alt=lambda x, axis: Series(x).var(ddof=ddof, **kwargs),
+                    **kwargs)
             except Exception:
                 f = lambda x: x.var(ddof=ddof, **kwargs)
                 with _group_selection_context(self):
@@ -1263,7 +1273,6 @@ def sem(self, ddof=1):
         Series or DataFrame
             Standard error of the mean of values within each group.
         """
-
         return self.std(ddof=ddof) / np.sqrt(self.count())
 
     @Substitution(name='groupby')
@@ -1290,7 +1299,7 @@ def _add_numeric_operations(cls):
         """
 
         def groupby_function(name, alias, npfunc,
-                             numeric_only=True, _convert=False,
+                             numeric_only=True,
                              min_count=-1):
 
             _local_template = """
@@ -1312,17 +1321,30 @@ def f(self, **kwargs):
                     kwargs['min_count'] = min_count
 
                 self._set_group_selection()
+
+                # try a cython aggregation if we can
                 try:
                     return self._cython_agg_general(
                         alias, alt=npfunc, **kwargs)
                 except AssertionError as e:
                     raise SpecificationError(str(e))
                 except Exception:
-                    result = self.aggregate(
-                        lambda x: npfunc(x, axis=self.axis))
-                    if _convert:
-                        result = result._convert(datetime=True)
-                    return result
+                    pass
+
+                # apply a non-cython aggregation
+                result = self.aggregate(
+                    lambda x: npfunc(x, axis=self.axis))
+
+                # coerce the resulting columns if we can
+                if isinstance(result, DataFrame):
+                    for col in result.columns:
+                        result[col] = self._try_cast(
+                            result[col], self.obj[col])
+                else:
+                    result = self._try_cast(
+                        result, self.obj)
+
+                return result
 
             set_function_name(f, name, cls)
 

pandas/core/groupby/ops.py

@@ -19,7 +19,7 @@
 from pandas.core.dtypes.common import (
     ensure_float64, ensure_int64, ensure_int_or_float, ensure_object,
     ensure_platform_int, is_bool_dtype, is_categorical_dtype, is_complex_dtype,
-    is_datetime64_any_dtype, is_integer_dtype, is_numeric_dtype,
+    is_datetime64_any_dtype, is_integer_dtype, is_numeric_dtype, is_sparse,
     is_timedelta64_dtype, needs_i8_conversion)
 from pandas.core.dtypes.missing import _maybe_fill, isna
 
@@ -451,9 +451,9 @@ def _cython_operation(self, kind, values, how, axis, min_count=-1,
 
         # categoricals are only 1d, so we
         # are not setup for dim transforming
-        if is_categorical_dtype(values):
+        if is_categorical_dtype(values) or is_sparse(values):
             raise NotImplementedError(
-                "categoricals are not support in cython ops ATM")
+                "{} are not support in cython ops".format(values.dtype))
         elif is_datetime64_any_dtype(values):
             if how in ['add', 'prod', 'cumsum', 'cumprod']:
                 raise NotImplementedError(

pandas/core/internals/blocks.py

@@ -594,7 +594,8 @@ def _astype(self, dtype, copy=False, errors='raise', values=None,
                         values = self.get_values(dtype=dtype)
 
                     # _astype_nansafe works fine with 1-d only
-                    values = astype_nansafe(values.ravel(), dtype, copy=True)
+                    values = astype_nansafe(
+                        values.ravel(), dtype, copy=True, **kwargs)
 
                 # TODO(extension)
                 # should we make this attribute?
@@ -1746,6 +1747,27 @@ def _slice(self, slicer):
 
         return self.values[slicer]
 
+    def _try_cast_result(self, result, dtype=None):
+        """
+        if we have an operation that operates on for example floats
+        we want to try to cast back to our EA here if possible
+
+        result could be a 2-D numpy array, e.g. the result of
+        a numeric operation; but it must be shape (1, X) because
+        we by-definition operate on the ExtensionBlocks one-by-one
+
+        result could also be an EA Array itself, in which case it
+        is already a 1-D array
+        """
+        try:
+
+            result = self._holder._from_sequence(
+                result.ravel(), dtype=dtype)
+        except Exception:
+            pass
+
+        return result
+
     def formatting_values(self):
         # Deprecating the ability to override _formatting_values.
         # Do the warning here, it's only user in pandas, since we

pandas/core/internals/construction.py

@@ -687,7 +687,10 @@ def sanitize_array(data, index, dtype=None, copy=False,
                 data = np.array(data, dtype=dtype, copy=False)
             subarr = np.array(data, dtype=object, copy=copy)
 
-    if is_object_dtype(subarr.dtype) and dtype != 'object':
+    if (not (is_extension_array_dtype(subarr.dtype) or
+             is_extension_array_dtype(dtype)) and
+            is_object_dtype(subarr.dtype) and
+            not is_object_dtype(dtype)):
         inferred = lib.infer_dtype(subarr, skipna=False)
         if inferred == 'period':
             try:

pandas/core/nanops.py

@@ -72,11 +72,12 @@ def _f(*args, **kwargs):
 
 class bottleneck_switch:
 
-    def __init__(self, **kwargs):
+    def __init__(self, name=None, **kwargs):
+        self.name = name
         self.kwargs = kwargs
 
     def __call__(self, alt):
-        bn_name = alt.__name__
+        bn_name = self.name or alt.__name__
 
         try:
             bn_func = getattr(bn, bn_name)
@@ -804,7 +805,8 @@ def nansem(values, axis=None, skipna=True, ddof=1, mask=None):
 
 
 def _nanminmax(meth, fill_value_typ):
-    @bottleneck_switch()
+
+    @bottleneck_switch(name='nan' + meth)
     def reduction(values, axis=None, skipna=True, mask=None):
 
         values, mask, dtype, dtype_max, fill_value = _get_values(
@@ -824,7 +826,6 @@ def reduction(values, axis=None, skipna=True, mask=None):
         result = _wrap_results(result, dtype, fill_value)
         return _maybe_null_out(result, axis, mask, values.shape)
 
-    reduction.__name__ = 'nan' + meth
     return reduction
 
 

pandas/tests/extension/base/groupby.py

@@ -64,6 +64,18 @@ def test_groupby_extension_apply(
         df.groupby("A").apply(groupby_apply_op)
         df.groupby("A").B.apply(groupby_apply_op)
 
+    def test_groupby_apply_identity(self, data_for_grouping):
+        df = pd.DataFrame({"A": [1, 1, 2, 2, 3, 3, 1, 4],
+                           "B": data_for_grouping})
+        result = df.groupby('A').B.apply(lambda x: x.array)
+        expected = pd.Series([df.B.iloc[[0, 1, 6]].array,
+                              df.B.iloc[[2, 3]].array,
+                              df.B.iloc[[4, 5]].array,
+                              df.B.iloc[[7]].array],
+                             index=pd.Index([1, 2, 3, 4], name='A'),
+                             name='B')
+        self.assert_series_equal(result, expected)
+
     def test_in_numeric_groupby(self, data_for_grouping):
         df = pd.DataFrame({"A": [1, 1, 2, 2, 3, 3, 1, 4],
                            "B": data_for_grouping,

pandas/tests/extension/decimal/test_decimal.py

@@ -192,7 +192,11 @@ class TestCasting(BaseDecimal, base.BaseCastingTests):
 
 
 class TestGroupby(BaseDecimal, base.BaseGroupbyTests):
-    pass
+
+    @pytest.mark.xfail(
+        reason="needs to correctly define __eq__ to handle nans, xref #27081.")
+    def test_groupby_apply_identity(self, data_for_grouping):
+        super().test_groupby_apply_identity(data_for_grouping)
 
 
 class TestSetitem(BaseDecimal, base.BaseSetitemTests):

pandas/tests/groupby/test_categorical.py

@@ -697,6 +697,27 @@ def test_preserve_categorical_dtype():
         tm.assert_frame_equal(result2, expected)
 
 
+@pytest.mark.parametrize(
+    'func, values',
+    [('first', ['second', 'first']),
+     ('last', ['fourth', 'third']),
+     ('min', ['fourth', 'first']),
+     ('max', ['second', 'third'])])
+def test_preserve_on_ordered_ops(func, values):
+    # gh-18502
+    # preserve the categoricals on ops
+    c = pd.Categorical(['first', 'second', 'third', 'fourth'], ordered=True)
+    df = pd.DataFrame(
+        {'payload': [-1, -2, -1, -2],
+         'col': c})
+    g = df.groupby('payload')
+    result = getattr(g, func)()
+    expected = pd.DataFrame(
+        {'payload': [-2, -1],
+         'col': pd.Series(values, dtype=c.dtype)}).set_index('payload')
+    tm.assert_frame_equal(result, expected)
+
+
 def test_categorical_no_compress():
     data = Series(np.random.randn(9))