DOC: Clarify decay argument validation in ewm when times is provided (#47026)

Author: mroeschke

doc/source/whatsnew/v1.5.0.rst

@@ -150,6 +150,7 @@ Other enhancements
 - Added ``validate`` argument to :meth:`DataFrame.join` (:issue:`46622`)
 - A :class:`errors.PerformanceWarning` is now thrown when using ``string[pyarrow]`` dtype with methods that don't dispatch to ``pyarrow.compute`` methods (:issue:`42613`)
 - Added ``numeric_only`` argument to :meth:`Resampler.sum`, :meth:`Resampler.prod`, :meth:`Resampler.min`, :meth:`Resampler.max`, :meth:`Resampler.first`, and :meth:`Resampler.last` (:issue:`46442`)
+- ``times`` argument in :class:`.ExponentialMovingWindow` now accepts ``np.timedelta64`` (:issue:`47003`)
 
 .. ---------------------------------------------------------------------------
 .. _whatsnew_150.notable_bug_fixes:

pandas/core/window/ewm.py

@@ -134,8 +134,9 @@ class ExponentialMovingWindow(BaseWindow):
     r"""
     Provide exponentially weighted (EW) calculations.
 
-    Exactly one parameter: ``com``, ``span``, ``halflife``, or ``alpha`` must be
-    provided.
+    Exactly one of ``com``, ``span``, ``halflife``, or ``alpha`` must be
+    provided if ``times`` is not provided. If ``times`` is provided,
+    ``halflife`` and one of ``com``, ``span`` or ``alpha`` may be provided.
 
     Parameters
     ----------
@@ -155,7 +156,7 @@ class ExponentialMovingWindow(BaseWindow):
         :math:`\alpha = 1 - \exp\left(-\ln(2) / halflife\right)`, for
         :math:`halflife > 0`.
 
-        If ``times`` is specified, the time unit (str or timedelta) over which an
+        If ``times`` is specified, a timedelta convertible unit over which an
         observation decays to half its value. Only applicable to ``mean()``,
         and halflife value will not apply to the other functions.
 
@@ -389,10 +390,8 @@ def __init__(
                 raise ValueError("times must be datetime64[ns] dtype.")
             if len(self.times) != len(obj):
                 raise ValueError("times must be the same length as the object.")
-            if not isinstance(self.halflife, (str, datetime.timedelta)):
-                raise ValueError(
-                    "halflife must be a string or datetime.timedelta object"
-                )
+            if not isinstance(self.halflife, (str, datetime.timedelta, np.timedelta64)):
+                raise ValueError("halflife must be a timedelta convertible object")
             if isna(self.times).any():
                 raise ValueError("Cannot convert NaT values to integer")
             self._deltas = _calculate_deltas(self.times, self.halflife)
@@ -404,7 +403,7 @@ def __init__(
                 self._com = 1.0
         else:
             if self.halflife is not None and isinstance(
-                self.halflife, (str, datetime.timedelta)
+                self.halflife, (str, datetime.timedelta, np.timedelta64)
             ):
                 raise ValueError(
                     "halflife can only be a timedelta convertible argument if "

pandas/tests/window/conftest.py

@@ -102,7 +102,7 @@ def engine_and_raw(request):
     return request.param
 
 
-@pytest.fixture(params=["1 day", timedelta(days=1)])
+@pytest.fixture(params=["1 day", timedelta(days=1), np.timedelta64(1, "D")])
 def halflife_with_times(request):
     """Halflife argument for EWM when times is specified."""
     return request.param

pandas/tests/window/test_ewm.py

@@ -90,7 +90,7 @@ def test_ewma_times_not_same_length():
 
 
 def test_ewma_halflife_not_correct_type():
-    msg = "halflife must be a string or datetime.timedelta object"
+    msg = "halflife must be a timedelta convertible object"
     with pytest.raises(ValueError, match=msg):
         Series(range(5)).ewm(halflife=1, times=np.arange(5).astype("datetime64[ns]"))