ENH: Rolling window with step size (GH-15354)

doc/source/whatsnew/v1.5.0.rst

@@ -38,6 +38,7 @@ Other enhancements
 - :meth:`to_numeric` now preserves float64 arrays when downcasting would generate values not representable in float32 (:issue:`43693`)
 - :meth:`Series.reset_index` and :meth:`DataFrame.reset_index` now support the argument ``allow_duplicates`` (:issue:`44410`)
 - :meth:`.GroupBy.min` and :meth:`.GroupBy.max` now supports `Numba <https://numba.pydata.org/>`_ execution with the ``engine`` keyword (:issue:`45428`)
+- :meth:`DataFrame.rolling` and :meth:`Series.rolling` now support a ``step`` parameter with fixed-length windows (:issue:`15354`)
 - Implemented a ``bool``-dtype :class:`Index`, passing a bool-dtype array-like to ``pd.Index`` will now retain ``bool`` dtype instead of casting to ``object`` (:issue:`45061`)
 - Implemented a complex-dtype :class:`Index`, passing a complex-dtype array-like to ``pd.Index`` will now retain complex dtype instead of casting to ``object`` (:issue:`45845`)
 - Improved error message in :class:`~pandas.core.window.Rolling` when ``window`` is a frequency and ``NaT`` is in the rolling axis (:issue:`46087`)

pandas/_libs/window/indexers.pyi

@@ -8,5 +8,6 @@ def calculate_variable_window_bounds(
     min_periods,
     center: bool,
     closed: str | None,
+    step: int | None,
     index: np.ndarray,  # const int64_t[:]
 ) -> tuple[npt.NDArray[np.int64], npt.NDArray[np.int64]]: ...

pandas/_libs/window/indexers.pyx

@@ -16,6 +16,7 @@ def calculate_variable_window_bounds(
     object min_periods,  # unused but here to match get_window_bounds signature
     bint center,
     str closed,
+    int64_t step,
     const int64_t[:] index
 ):
     """
@@ -38,6 +39,9 @@ def calculate_variable_window_bounds(
     closed : str
         string of side of the window that should be closed
 
+    step : int64
+        Spacing between windows
+
     index : ndarray[int64]
         time series index to roll over
 
@@ -52,6 +56,9 @@ def calculate_variable_window_bounds(
         int64_t start_bound, end_bound, index_growth_sign = 1
         Py_ssize_t i, j
 
+    if num_values <= 0:
+        return np.empty(0, dtype='int64'), np.empty(0, dtype='int64')
+
     # default is 'right'
     if closed is None:
         closed = 'right'
@@ -143,4 +150,4 @@ def calculate_variable_window_bounds(
             # right endpoint is open
             if not right_closed and not center:
                 end[i] -= 1
-    return start, end
+    return start[::step], end[::step]

pandas/core/generic.py

@@ -11249,6 +11249,7 @@ def rolling(
         on: str | None = None,
         axis: Axis = 0,
         closed: str | None = None,
+        step: int | None = None,
         method: str = "single",
     ):
         axis = self._get_axis_number(axis)
@@ -11263,6 +11264,7 @@ def rolling(
                 on=on,
                 axis=axis,
                 closed=closed,
+                step=step,
                 method=method,
             )
 
@@ -11275,6 +11277,7 @@ def rolling(
             on=on,
             axis=axis,
             closed=closed,
+            step=step,
             method=method,
         )
 

pandas/core/indexers/objects.py

@@ -27,6 +27,9 @@
     center passed from the top level rolling API
 closed : str, default None
     closed passed from the top level rolling API
+step : int, default None
+    step passed from the top level rolling API
+    .. versionadded:: 1.5
 win_type : str, default None
     win_type passed from the top level rolling API
 
@@ -62,6 +65,7 @@ def get_window_bounds(
         min_periods: int | None = None,
         center: bool | None = None,
         closed: str | None = None,
+        step: int | None = None,
     ) -> tuple[np.ndarray, np.ndarray]:
 
         raise NotImplementedError
@@ -77,14 +81,15 @@ def get_window_bounds(
         min_periods: int | None = None,
         center: bool | None = None,
         closed: str | None = None,
+        step: int | None = None,
     ) -> tuple[np.ndarray, np.ndarray]:
 
         if center:
             offset = (self.window_size - 1) // 2
         else:
             offset = 0
 
-        end = np.arange(1 + offset, num_values + 1 + offset, dtype="int64")
+        end = np.arange(1 + offset, num_values + 1 + offset, step, dtype="int64")
         start = end - self.window_size
         if closed in ["left", "both"]:
             start -= 1
@@ -107,8 +112,12 @@ def get_window_bounds(
         min_periods: int | None = None,
         center: bool | None = None,
         closed: str | None = None,
+        step: int | None = None,
     ) -> tuple[np.ndarray, np.ndarray]:
 
+        if step is not None:
+            raise NotImplementedError("step not implemented for variable window")
+
         # error: Argument 4 to "calculate_variable_window_bounds" has incompatible
         # type "Optional[bool]"; expected "bool"
         # error: Argument 6 to "calculate_variable_window_bounds" has incompatible
@@ -119,6 +128,7 @@ def get_window_bounds(
             min_periods,
             center,  # type: ignore[arg-type]
             closed,
+            1,
             self.index_array,  # type: ignore[arg-type]
         )
 
@@ -145,8 +155,14 @@ def get_window_bounds(
         min_periods: int | None = None,
         center: bool | None = None,
         closed: str | None = None,
+        step: int | None = None,
     ) -> tuple[np.ndarray, np.ndarray]:
 
+        if step is not None:
+            raise NotImplementedError("step not implemented for variable offset window")
+        if num_values <= 0:
+            return np.empty(0, dtype="int64"), np.empty(0, dtype="int64")
+
         # if windows is variable, default is 'right', otherwise default is 'both'
         if closed is None:
             closed = "right" if self.index is not None else "both"
@@ -215,12 +231,15 @@ def get_window_bounds(
         min_periods: int | None = None,
         center: bool | None = None,
         closed: str | None = None,
+        step: int | None = None,
     ) -> tuple[np.ndarray, np.ndarray]:
 
-        return (
-            np.zeros(num_values, dtype=np.int64),
-            np.arange(1, num_values + 1, dtype=np.int64),
-        )
+        if step is not None:
+            raise NotImplementedError("step not implemented for expanding window")
+
+        end = np.arange(1, num_values + 1, dtype=np.int64)
+        start = np.zeros(len(end), dtype=np.int64)
+        return start, end
 
 
 class FixedForwardWindowIndexer(BaseIndexer):
@@ -256,6 +275,7 @@ def get_window_bounds(
         min_periods: int | None = None,
         center: bool | None = None,
         closed: str | None = None,
+        step: int | None = None,
     ) -> tuple[np.ndarray, np.ndarray]:
 
         if center:
@@ -264,11 +284,13 @@ def get_window_bounds(
             raise ValueError(
                 "Forward-looking windows don't support setting the closed argument"
             )
+        if step is None:
+            step = 1
 
-        start = np.arange(num_values, dtype="int64")
+        start = np.arange(0, num_values, step, dtype="int64")
         end = start + self.window_size
         if self.window_size:
-            end[-self.window_size :] = num_values
+            end = np.clip(end, 0, num_values)
 
         return start, end
 
@@ -319,7 +341,11 @@ def get_window_bounds(
         min_periods: int | None = None,
         center: bool | None = None,
         closed: str | None = None,
+        step: int | None = None,
     ) -> tuple[np.ndarray, np.ndarray]:
+        if step is not None:
+            raise NotImplementedError("step not implemented for groupby window")
+
         # 1) For each group, get the indices that belong to the group
         # 2) Use the indices to calculate the start & end bounds of the window
         # 3) Append the window bounds in group order
@@ -339,7 +365,7 @@ def get_window_bounds(
                 **self.indexer_kwargs,
             )
             start, end = indexer.get_window_bounds(
-                len(indices), min_periods, center, closed
+                len(indices), min_periods, center, closed, step
             )
             start = start.astype(np.int64)
             end = end.astype(np.int64)
@@ -358,6 +384,8 @@ def get_window_bounds(
             )
             start_arrays.append(window_indices.take(ensure_platform_int(start)))
             end_arrays.append(window_indices.take(ensure_platform_int(end)))
+        if len(start_arrays) == 0:
+            return np.array([], dtype=np.int64), np.array([], dtype=np.int64)
         start = np.concatenate(start_arrays)
         end = np.concatenate(end_arrays)
         return start, end
@@ -373,6 +401,14 @@ def get_window_bounds(
         min_periods: int | None = None,
         center: bool | None = None,
         closed: str | None = None,
+        step: int | None = None,
     ) -> tuple[np.ndarray, np.ndarray]:
 
-        return np.array([0], dtype=np.int64), np.array([num_values], dtype=np.int64)
+        if step is not None:
+            raise NotImplementedError(
+                "step not implemented for exponentail moving window"
+            )
+        return (
+            np.array([0], dtype=np.int64),
+            np.array([num_values], dtype=np.int64),
+        )

pandas/core/window/ewm.py

@@ -732,6 +732,7 @@ def cov_func(x, y):
                 min_periods=min_periods,
                 center=self.center,
                 closed=self.closed,
+                step=self.step,
             )
             result = window_aggregations.ewmcov(
                 x_array,
@@ -798,6 +799,7 @@ def cov_func(x, y):
                 min_periods=min_periods,
                 center=self.center,
                 closed=self.closed,
+                step=self.step,
             )
 
             def _cov(X, Y):

pandas/core/window/numba_.py

@@ -220,8 +220,8 @@ def roll_table(
         minimum_periods: int,
         *args: Any,
     ):
-        result = np.empty(values.shape)
-        min_periods_mask = np.empty(values.shape)
+        result = np.empty((len(begin), values.shape[1]))
+        min_periods_mask = np.empty(result.shape)
         for i in numba.prange(len(result)):
             start = begin[i]
             stop = end[i]