warn if dim is passed to rolling operations.

doc/whats-new.rst

@@ -217,6 +217,9 @@ Bug fixes
   By `Deepak Cherian <https://github.com/dcherian>`_.
 - Fix error in concatenating unlabeled dimensions (:pull:`3362`).
   By `Deepak Cherian <https://github.com/dcherian/>`_.
+- Warn if the ``dim`` kwarg is passed to rolling operations. This is redundant since a dimension is
+  specified when the :py:class:`DatasetRolling` or :py:class:`DataArrayRolling` object is created.
+  (:pull:`3362`). By `Deepak Cherian <https://github.com/dcherian/>`_.
 
 Documentation
 ~~~~~~~~~~~~~

xarray/core/rolling.py

@@ -1,4 +1,5 @@
 import functools
+import warnings
 from typing import Callable
 
 import numpy as np
@@ -351,6 +352,14 @@ def _bottleneck_reduce(self, func, **kwargs):
     def _numpy_or_bottleneck_reduce(
         self, array_agg_func, bottleneck_move_func, **kwargs
     ):
+        if "dim" in kwargs:
+            warnings.warn(
+                f"Reductions will be applied along the rolling dimension '{self.dim}'. Passing the 'dim' kwarg to reduction operations has no effect and will raise an error in xarray 0.16.0.",
+                DeprecationWarning,
+                stacklevel=3,
+            )
+            del kwargs["dim"]
+
         if bottleneck_move_func is not None and not isinstance(
             self.obj.data, dask_array_type
         ):

xarray/tests/test_dataarray.py

@@ -4188,6 +4188,9 @@ def test_rolling_wrapped_bottleneck(da, name, center, min_periods):
     )
     assert_array_equal(actual.values, expected)
 
+    with pytest.warns(DeprecationWarning, match="Reductions will be applied"):
+        getattr(rolling_obj, name)(dim="time")
+
     # Test center
     rolling_obj = da.rolling(time=7, center=center)
     actual = getattr(rolling_obj, name)()["time"]
@@ -4203,6 +4206,9 @@ def test_rolling_wrapped_dask(da_dask, name, center, min_periods, window):
     # dask version
     rolling_obj = da_dask.rolling(time=window, min_periods=min_periods, center=center)
     actual = getattr(rolling_obj, name)().load()
+    if name != "count":
+        with pytest.warns(DeprecationWarning, match="Reductions will be applied"):
+            getattr(rolling_obj, name)(dim="time")
     # numpy version
     rolling_obj = da_dask.load().rolling(
         time=window, min_periods=min_periods, center=center