BUG: support corr and cov functions for custom BaseIndexer rolling windows

doc/source/whatsnew/v1.1.0.rst

@@ -175,8 +175,7 @@ Other API changes
 - Added :meth:`DataFrame.value_counts` (:issue:`5377`)
 - :meth:`Groupby.groups` now returns an abbreviated representation when called on large dataframes (:issue:`1135`)
 - ``loc`` lookups with an object-dtype :class:`Index` and an integer key will now raise ``KeyError`` instead of ``TypeError`` when key is missing (:issue:`31905`)
-- Using a :func:`pandas.api.indexers.BaseIndexer` with ``cov``, ``corr`` will now raise a ``NotImplementedError`` (:issue:`32865`)
-- Using a :func:`pandas.api.indexers.BaseIndexer` with ``count``, ``min``, ``max``, ``median``, ``skew`` will now return correct results for any monotonic :func:`pandas.api.indexers.BaseIndexer` descendant (:issue:`32865`)
+- Using a :func:`pandas.api.indexers.BaseIndexer` with ``count``, ``min``, ``max``, ``median``, ``skew``,  ``cov``, ``corr`` will now return correct results for any monotonic :func:`pandas.api.indexers.BaseIndexer` descendant (:issue:`32865`)
 - Added a :func:`pandas.api.indexers.FixedForwardWindowIndexer` class to support forward-looking windows during ``rolling`` operations.
 -
 

pandas/core/window/common.py

@@ -324,25 +324,3 @@ def func(arg, window, min_periods=None):
         return cfunc(arg, window, min_periods)
 
     return func
-
-
-def validate_baseindexer_support(func_name: Optional[str]) -> None:
-    # GH 32865: These functions work correctly with a BaseIndexer subclass
-    BASEINDEXER_WHITELIST = {
-        "count",
-        "min",
-        "max",
-        "mean",
-        "sum",
-        "median",
-        "std",
-        "var",
-        "skew",
-        "kurt",
-        "quantile",
-    }
-    if isinstance(func_name, str) and func_name not in BASEINDEXER_WHITELIST:
-        raise NotImplementedError(
-            f"{func_name} is not supported with using a BaseIndexer "
-            f"subclasses. You can use .apply() with {func_name}."
-        )

pandas/core/window/rolling.py

@@ -48,7 +48,6 @@
     calculate_center_offset,
     calculate_min_periods,
     get_weighted_roll_func,
-    validate_baseindexer_support,
     zsqrt,
 )
 from pandas.core.window.indexers import (
@@ -393,12 +392,11 @@ def _get_cython_func_type(self, func: str) -> Callable:
             return self._get_roll_func(f"{func}_variable")
         return partial(self._get_roll_func(f"{func}_fixed"), win=self._get_window())
 
-    def _get_window_indexer(self, window: int, func_name: Optional[str]) -> BaseIndexer:
+    def _get_window_indexer(self, window: int) -> BaseIndexer:
         """
         Return an indexer class that will compute the window start and end bounds
         """
         if isinstance(self.window, BaseIndexer):
-            validate_baseindexer_support(func_name)
             return self.window
         if self.is_freq_type:
             return VariableWindowIndexer(index_array=self._on.asi8, window_size=window)
@@ -444,7 +442,7 @@ def _apply(
 
         blocks, obj = self._create_blocks()
         block_list = list(blocks)
-        window_indexer = self._get_window_indexer(window, name)
+        window_indexer = self._get_window_indexer(window)
 
         results = []
         exclude: List[Scalar] = []
@@ -1632,20 +1630,23 @@ def quantile(self, quantile, interpolation="linear", **kwargs):
     """
 
     def cov(self, other=None, pairwise=None, ddof=1, **kwargs):
-        if isinstance(self.window, BaseIndexer):
-            validate_baseindexer_support("cov")
-
         if other is None:
             other = self._selected_obj
             # only default unset
             pairwise = True if pairwise is None else pairwise
         other = self._shallow_copy(other)
 
-        # GH 16058: offset window
-        if self.is_freq_type:
-            window = self.win_freq
+        # GH 32865. We leverage rolling.mean, so we pass
+        # to the rolling constructors the data used when constructing self:
+        # window width, frequency data, or a BaseIndexer subclass
+        if isinstance(self.window, BaseIndexer):
+            window = self.window
         else:
-            window = self._get_window(other)
+            # GH 16058: offset window
+            if self.is_freq_type:
+                window = self.win_freq
+            else:
+                window = self._get_window(other)
 
         def _get_cov(X, Y):
             # GH #12373 : rolling functions error on float32 data
@@ -1778,15 +1779,19 @@ def _get_cov(X, Y):
     )
 
     def corr(self, other=None, pairwise=None, **kwargs):
-        if isinstance(self.window, BaseIndexer):
-            validate_baseindexer_support("corr")
-
         if other is None:
             other = self._selected_obj
             # only default unset
             pairwise = True if pairwise is None else pairwise
         other = self._shallow_copy(other)
-        window = self._get_window(other) if not self.is_freq_type else self.win_freq
+
+        # GH 32865. We leverage rolling.cov and rolling.std here, so we pass
+        # to the rolling constructors the data used when constructing self:
+        # window width, frequency data, or a BaseIndexer subclass
+        if isinstance(self.window, BaseIndexer):
+            window = self.window
+        else:
+            window = self._get_window(other) if not self.is_freq_type else self.win_freq
 
         def _get_corr(a, b):
             a = a.rolling(

pandas/tests/window/test_base_indexer.py

@@ -82,19 +82,6 @@ def get_window_bounds(self, num_values, min_periods, center, closed):
         df.rolling(indexer, win_type="boxcar")
 
 
-@pytest.mark.parametrize("func", ["cov", "corr"])
-def test_notimplemented_functions(func):
-    # GH 32865
-    class CustomIndexer(BaseIndexer):
-        def get_window_bounds(self, num_values, min_periods, center, closed):
-            return np.array([0, 1]), np.array([1, 2])
-
-    df = DataFrame({"values": range(2)})
-    indexer = CustomIndexer()
-    with pytest.raises(NotImplementedError, match=f"{func} is not supported"):
-        getattr(df.rolling(indexer), func)()
-
-
 @pytest.mark.parametrize("constructor", [Series, DataFrame])
 @pytest.mark.parametrize(
     "func,np_func,expected,np_kwargs",
@@ -210,3 +197,40 @@ def test_rolling_forward_skewness(constructor):
         ]
     )
     tm.assert_equal(result, expected)
+
+
+@pytest.mark.parametrize(
+    "func,expected",
+    [
+        ("cov", [2.0, 2.0, 2.0, 97.0, 2.0, -93.0, 2.0, 2.0, np.nan, np.nan],),
+        (
+            "corr",
+            [
+                1.0,
+                1.0,
+                1.0,
+                0.8704775290207161,
+                0.018229084250926637,
+                -0.861357304646493,
+                1.0,
+                1.0,
+                np.nan,
+                np.nan,
+            ],
+        ),
+    ],
+)
+def test_rolling_forward_cov_corr(func, expected):
+    values1 = np.arange(10).reshape(-1, 1)
+    values2 = values1 * 2
+    values1[5, 0] = 100
+    values = np.concatenate([values1, values2], axis=1)
+
+    indexer = FixedForwardWindowIndexer(window_size=3)
+    rolling = DataFrame(values).rolling(window=indexer, min_periods=3)
+    # We are interested in checking only pairwise covariance / correlation
+    result = getattr(rolling, func)().loc[(slice(None), 1), 0]
+    result = result.reset_index(drop=True)
+    expected = Series(expected)
+    expected.name = result.name
+    tm.assert_equal(result, expected)