BUG: EWM(online) not raising NotImplementedError for unsupported

doc/source/whatsnew/v1.6.0.rst

@@ -237,6 +237,7 @@ Plotting
 
 Groupby/resample/rolling
 ^^^^^^^^^^^^^^^^^^^^^^^^
+- Bug in :class:`.ExponentialMovingWindow` with ``online`` not raising a ``NotImplementedError`` for unsupported operations (:issue:`48834`)
 - Bug in :meth:`DataFrameGroupBy.sample` raises ``ValueError`` when the object is empty (:issue:`48459`)
 -
 

pandas/core/window/ewm.py

@@ -967,10 +967,10 @@ def reset(self) -> None:
         self._mean.reset()
 
     def aggregate(self, func, *args, **kwargs):
-        return NotImplementedError
+        raise NotImplementedError("aggregate is not implemented.")
 
     def std(self, bias: bool = False, *args, **kwargs):
-        return NotImplementedError
+        raise NotImplementedError("std is not implemented.")
 
     def corr(
         self,
@@ -979,7 +979,7 @@ def corr(
         numeric_only: bool = False,
         **kwargs,
     ):
-        return NotImplementedError
+        raise NotImplementedError("corr is not implemented.")
 
     def cov(
         self,
@@ -989,10 +989,10 @@ def cov(
         numeric_only: bool = False,
         **kwargs,
     ):
-        return NotImplementedError
+        raise NotImplementedError("cov is not implemented.")
 
     def var(self, bias: bool = False, *args, **kwargs):
-        return NotImplementedError
+        raise NotImplementedError("var is not implemented.")
 
     def mean(self, *args, update=None, update_times=None, **kwargs):
         """

pandas/tests/window/test_online.py

@@ -103,3 +103,13 @@ def test_update_times_mean(
             tm.assert_equal(result, expected.tail(3))
 
             online_ewm.reset()
+
+    @pytest.mark.parametrize("method", ["aggregate", "std", "corr", "cov", "var"])
+    def test_ewm_notimplementederror_raises(self, method):
+        ser = Series(range(10))
+        kwargs = {}
+        if method == "aggregate":
+            kwargs["func"] = lambda x: x
+
+        with pytest.raises(NotImplementedError, match=".* is not implemented."):
+            getattr(ser.ewm(1).online(), method)(**kwargs)