DEPR: Passing in a string column label for DataFrame.ewm(times=...) (pandas-dev#43265)

Author: mroeschke

doc/source/whatsnew/v1.4.0.rst

@@ -274,6 +274,7 @@ Other Deprecations
 - Deprecated dropping of nuisance columns in :class:`Rolling`, :class:`Expanding`, and :class:`EWM` aggregations (:issue:`42738`)
 - Deprecated :meth:`Index.reindex` with a non-unique index (:issue:`42568`)
 - Deprecated :meth:`.Styler.render` in favour of :meth:`.Styler.to_html` (:issue:`42140`)
+- Deprecated passing in a string column label into ``times`` in :meth:`DataFrame.ewm` (:issue:`43265`)
 
 .. ---------------------------------------------------------------------------
 

pandas/core/window/ewm.py

@@ -21,6 +21,7 @@
 
 from pandas.compat.numpy import function as nv
 from pandas.util._decorators import doc
+from pandas.util._exceptions import find_stack_level
 
 from pandas.core.dtypes.common import is_datetime64_ns_dtype
 from pandas.core.dtypes.missing import isna
@@ -315,6 +316,15 @@ def __init__(
             if not self.adjust:
                 raise NotImplementedError("times is not supported with adjust=False.")
             if isinstance(self.times, str):
+                warnings.warn(
+                    (
+                        "Specifying times as a string column label is deprecated "
+                        "and will be removed in a future version. Pass the column "
+                        "into times instead."
+                    ),
+                    FutureWarning,
+                    stacklevel=find_stack_level(),
+                )
                 self.times = self._selected_obj[self.times]
             if not is_datetime64_ns_dtype(self.times):
                 raise ValueError("times must be datetime64[ns] dtype.")

pandas/tests/window/test_ewm.py

@@ -107,7 +107,6 @@ def test_ewma_halflife_without_times(halflife_with_times):
         np.arange(10).astype("datetime64[D]").astype("datetime64[ns]"),
         date_range("2000", freq="D", periods=10),
         date_range("2000", freq="D", periods=10).tz_localize("UTC"),
-        "time_col",
     ],
 )
 @pytest.mark.parametrize("min_periods", [0, 2])
@@ -231,3 +230,14 @@ def test_float_dtype_ewma(func, expected, float_numpy_dtype):
     result = getattr(e, func)()
 
     tm.assert_frame_equal(result, expected)
+
+
+def test_times_string_col_deprecated():
+    # GH 43265
+    data = np.arange(10.0)
+    data[::2] = np.nan
+    df = DataFrame({"A": data, "time_col": date_range("2000", freq="D", periods=10)})
+    with tm.assert_produces_warning(FutureWarning, match="Specifying times"):
+        result = df.ewm(halflife="1 day", min_periods=0, times="time_col").mean()
+        expected = df.ewm(halflife=1.0, min_periods=0).mean()
+    tm.assert_frame_equal(result, expected)