ENH: Add online operations for EWM.mean

doc/source/user_guide/window.rst

@@ -37,14 +37,14 @@ pandas supports 4 types of windowing operations:
 #. Expanding window: Accumulating window over the values.
 #. Exponentially Weighted window: Accumulating and exponentially weighted window over the values.
 
-=============================   =================  ===========================   ===========================  ========================  ===================================
-Concept                         Method             Returned Object               Supports time-based windows  Supports chained groupby  Supports table method
-=============================   =================  ===========================   ===========================  ========================  ===================================
-Rolling window                  ``rolling``        ``Rolling``                   Yes                          Yes                       Yes (as of version 1.3)
-Weighted window                 ``rolling``        ``Window``                    No                           No                        No
-Expanding window                ``expanding``      ``Expanding``                 No                           Yes                       Yes (as of version 1.3)
-Exponentially Weighted window   ``ewm``            ``ExponentialMovingWindow``   No                           Yes (as of version 1.2)   No
-=============================   =================  ===========================   ===========================  ========================  ===================================
+=============================   =================  ===========================   ===========================  ========================  ===================================  ===========================
+Concept                         Method             Returned Object               Supports time-based windows  Supports chained groupby  Supports table method                Supports online operations
+=============================   =================  ===========================   ===========================  ========================  ===================================  ===========================
+Rolling window                  ``rolling``        ``Rolling``                   Yes                          Yes                       Yes (as of version 1.3)              No
+Weighted window                 ``rolling``        ``Window``                    No                           No                        No                                   No
+Expanding window                ``expanding``      ``Expanding``                 No                           Yes                       Yes (as of version 1.3)              No
+Exponentially Weighted window   ``ewm``            ``ExponentialMovingWindow``   No                           Yes (as of version 1.2)   No                                   Yes (as of version 1.3)
+=============================   =================  ===========================   ===========================  ========================  ===================================  ===========================
 
 As noted above, some operations support specifying a window based on a time offset:
 
@@ -98,6 +98,26 @@ be calculated with :meth:`~Rolling.apply` by specifying a separate column of wei
    df = pd.DataFrame([[1, 2, 0.6], [2, 3, 0.4], [3, 4, 0.2], [4, 5, 0.7]])
    df.rolling(2, method="table", min_periods=0).apply(weighted_mean, raw=True, engine="numba")  # noqa:E501
 
+.. versionadded:: 1.3
+
+Some windowing operations also support an ``online`` method after constructing a windowing object
+which returns a new object that supports passing in new :class:`DataFrame` or :class:`Series` objects
+to continue the windowing calculation with the new values (i.e. online calculations).
+
+The methods on this new windowing objects must call the aggregation method first to "prime" the initial
+state of the online calculation. Then, new :class:`DataFrame` or :class:`Series` objects can be passed in
+the ``update`` argument to continue the windowing calculation.
+
+.. ipython:: python
+
+   df = pd.DataFrame([[1, 2, 0.6], [2, 3, 0.4], [3, 4, 0.2], [4, 5, 0.7]])
+   df.ewm(0.5).mean()
+
+.. ipython:: python
+
+   online_ewm = df.head(2).ewm(0.5).online()
+   online_ewm.mean()
+   online_ewm.mean(update=df.tail(1))
 
 All windowing operations support a ``min_periods`` argument that dictates the minimum amount of
 non-``np.nan`` values a window must have; otherwise, the resulting value is ``np.nan``.

doc/source/whatsnew/v1.3.0.rst

@@ -237,7 +237,8 @@ For example:
 Other enhancements
 ^^^^^^^^^^^^^^^^^^
 
-- :meth:`DataFrame.rolling`, :meth:`Series.rolling`, :meth:`DataFrame.expanding`, and :meth:`Series.expanding` now support a ``method`` argument with a ``'table'`` option that performs the windowing operation over an entire DataFrame. See :ref:`Window Overview <window.overview>` for performance and functional benefits (:issue:`15095`, :issue:`38995`)
+- :meth:`DataFrame.rolling`, :meth:`Series.rolling`, :meth:`DataFrame.expanding`, and :meth:`Series.expanding` now support a ``method`` argument with a ``'table'`` option that performs the windowing operation over an entire :class:`DataFrame`. See :ref:`Window Overview <window.overview>` for performance and functional benefits (:issue:`15095`, :issue:`38995`)
+- :class:`.ExponentialMovingWindow` now support a ``online`` method that can perform ``mean`` calculations in an online fashion. See :ref:`Window Overview <window.overview>` (:issue:`41673`)
 - Added :meth:`MultiIndex.dtypes` (:issue:`37062`)
 - Added ``end`` and ``end_day`` options for the ``origin`` argument in :meth:`DataFrame.resample` (:issue:`37804`)
 - Improved error message when ``usecols`` and ``names`` do not match for :func:`read_csv` and ``engine="c"`` (:issue:`29042`)

pandas/core/generic.py

@@ -10892,7 +10892,7 @@ def ewm(
         span: float | None = None,
         halflife: float | TimedeltaConvertibleTypes | None = None,
         alpha: float | None = None,
-        min_periods: int = 0,
+        min_periods: int | None = 0,
         adjust: bool_t = True,
         ignore_na: bool_t = False,
         axis: Axis = 0,

pandas/core/window/ewm.py

@@ -41,6 +41,10 @@
     GroupbyIndexer,
 )
 from pandas.core.window.numba_ import generate_numba_ewma_func
+from pandas.core.window.online import (
+    EWMMeanState,
+    generate_online_numba_ewma_func,
+)
 from pandas.core.window.rolling import (
     BaseWindow,
     BaseWindowGroupby,
@@ -263,7 +267,7 @@ def __init__(
         span: float | None = None,
         halflife: float | TimedeltaConvertibleTypes | None = None,
         alpha: float | None = None,
-        min_periods: int = 0,
+        min_periods: int | None = 0,
         adjust: bool = True,
         ignore_na: bool = False,
         axis: Axis = 0,
@@ -273,7 +277,7 @@ def __init__(
     ):
         super().__init__(
             obj=obj,
-            min_periods=max(int(min_periods), 1),
+            min_periods=1 if min_periods is None else max(int(min_periods), 1),
             on=None,
             center=False,
             closed=None,
@@ -338,6 +342,48 @@ def _get_window_indexer(self) -> BaseIndexer:
         """
         return ExponentialMovingWindowIndexer()
 
+    def online(self, engine="numba", engine_kwargs=None):
+        """
+        Return an ``OnlineExponentialMovingWindow`` object to calculate
+        exponentially moving window aggregations in an online method.
+
+        .. versionadded:: 1.3.0
+
+        Parameters
+        ----------
+        engine: str, default ``'numba'``
+            Execution engine to calculate online aggregations.
+            Applies to all supported aggregation methods.
+
+        engine_kwargs : dict, default None
+            Applies to all supported aggregation methods.
+
+            * For ``'numba'`` engine, the engine can accept ``nopython``, ``nogil``
+              and ``parallel`` dictionary keys. The values must either be ``True`` or
+              ``False``. The default ``engine_kwargs`` for the ``'numba'`` engine is
+              ``{{'nopython': True, 'nogil': False, 'parallel': False}}`` and will be
+              applied to the function
+
+        Returns
+        -------
+        OnlineExponentialMovingWindow
+        """
+        return OnlineExponentialMovingWindow(
+            obj=self.obj,
+            com=self.com,
+            span=self.span,
+            halflife=self.halflife,
+            alpha=self.alpha,
+            min_periods=self.min_periods,
+            adjust=self.adjust,
+            ignore_na=self.ignore_na,
+            axis=self.axis,
+            times=self.times,
+            engine=engine,
+            engine_kwargs=engine_kwargs,
+            selection=self._selection,
+        )
+
     @doc(
         _shared_docs["aggregate"],
         see_also=dedent(
@@ -655,3 +701,167 @@ def _get_window_indexer(self) -> GroupbyIndexer:
             window_indexer=ExponentialMovingWindowIndexer,
         )
         return window_indexer
+
+
+class OnlineExponentialMovingWindow(ExponentialMovingWindow):
+    def __init__(
+        self,
+        obj: FrameOrSeries,
+        com: float | None = None,
+        span: float | None = None,
+        halflife: float | TimedeltaConvertibleTypes | None = None,
+        alpha: float | None = None,
+        min_periods: int | None = 0,
+        adjust: bool = True,
+        ignore_na: bool = False,
+        axis: Axis = 0,
+        times: str | np.ndarray | FrameOrSeries | None = None,
+        engine: str = "numba",
+        engine_kwargs: dict[str, bool] | None = None,
+        *,
+        selection=None,
+    ):
+        if times is not None:
+            raise NotImplementedError(
+                "times is not implemented with online operations."
+            )
+        super().__init__(
+            obj=obj,
+            com=com,
+            span=span,
+            halflife=halflife,
+            alpha=alpha,
+            min_periods=min_periods,
+            adjust=adjust,
+            ignore_na=ignore_na,
+            axis=axis,
+            times=times,
+            selection=selection,
+        )
+        self._mean = EWMMeanState(
+            self._com, self.adjust, self.ignore_na, self.axis, obj.shape
+        )
+        if maybe_use_numba(engine):
+            self.engine = engine
+            self.engine_kwargs = engine_kwargs
+        else:
+            raise ValueError("'numba' is the only supported engine")
+
+    def reset(self):
+        """
+        Reset the state captured by `update` calls.
+        """
+        self._mean.reset()
+
+    def aggregate(self, func, *args, **kwargs):
+        return NotImplementedError
+
+    def std(self, bias: bool = False, *args, **kwargs):
+        return NotImplementedError
+
+    def corr(
+        self,
+        other: FrameOrSeriesUnion | None = None,
+        pairwise: bool | None = None,
+        **kwargs,
+    ):
+        return NotImplementedError
+
+    def cov(
+        self,
+        other: FrameOrSeriesUnion | None = None,
+        pairwise: bool | None = None,
+        bias: bool = False,
+        **kwargs,
+    ):
+        return NotImplementedError
+
+    def var(self, bias: bool = False, *args, **kwargs):
+        return NotImplementedError
+
+    def mean(self, *args, update=None, update_times=None, **kwargs):
+        """
+        Calculate an online exponentially weighted mean.
+
+        Parameters
+        ----------
+        update: DataFrame or Series, default None
+            New values to continue calculating the
+            exponentially weighted mean from the last values and weights.
+            Values should be float64 dtype.
+
+            ``update`` needs to be ``None`` the first time the
+            exponentially weighted mean is calculated.
+
+        update_times: Series or 1-D np.ndarray, default None
+            New times to continue calculating the
+            exponentially weighted mean from the last values and weights.
+            If ``None``, values are assumed to be evenly spaced
+            in time.
+            This feature is currently unsupported.
+
+        Returns
+        -------
+        DataFrame or Series
+
+        Examples
+        --------
+        >>> df = pd.DataFrame({"a": range(5), "b": range(5, 10)})
+        >>> online_ewm = df.head(2).ewm(0.5).online()
+        >>> online_ewm.mean()
+              a     b
+        0  0.00  5.00
+        1  0.75  5.75
+        >>> online_ewm.mean(update=df.tail(3))
+                  a         b
+        1  1.615385  6.615385
+        2  2.550000  7.550000
+        3  3.520661  8.520661
+        >>> online_ewm.reset()
+        >>> online_ewm.mean()
+              a     b
+        0  0.00  5.00
+        1  0.75  5.75
+        """
+        result_kwargs = {}
+        is_frame = True if self._selected_obj.ndim == 2 else False
+        if update_times is not None:
+            raise NotImplementedError("update_times is not implemented.")
+        else:
+            update_deltas = np.ones(
+                max(self._selected_obj.shape[self.axis - 1] - 1, 0), dtype=np.float64
+            )
+        if update is not None:
+            if self._mean.last_ewm is None:
+                raise ValueError(
+                    "Must call mean with update=None first before passing update"
+                )
+            result_from = 1
+            result_kwargs["index"] = update.index
+            if is_frame:
+                last_value = self._mean.last_ewm[np.newaxis, :]
+                result_kwargs["columns"] = update.columns
+            else:
+                last_value = self._mean.last_ewm
+                result_kwargs["name"] = update.name
+            np_array = np.concatenate((last_value, update.to_numpy()))
+        else:
+            result_from = 0
+            result_kwargs["index"] = self._selected_obj.index
+            if is_frame:
+                result_kwargs["columns"] = self._selected_obj.columns
+            else:
+                result_kwargs["name"] = self._selected_obj.name
+            np_array = self._selected_obj.astype(np.float64).to_numpy()
+        ewma_func = generate_online_numba_ewma_func(self.engine_kwargs)
+        result = self._mean.run_ewm(
+            np_array if is_frame else np_array[:, np.newaxis],
+            update_deltas,
+            self.min_periods,
+            ewma_func,
+        )
+        if not is_frame:
+            result = result.squeeze()
+        result = result[result_from:]
+        result = self._selected_obj._constructor(result, **result_kwargs)
+        return result