issue_comments: 1465227486

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https://github.com/pydata/xarray/issues/6870#issuecomment-1465227486 https://api.github.com/repos/pydata/xarray/issues/6870 1465227486 IC_kwDOAMm_X85XVZje 43316012 2023-03-12T15:25:47Z 2023-03-12T15:27:31Z COLLABORATOR

This is actually a bug in Dataset.reduce, it will drop the time index as well. See this example: ```python da = xr.DataArray([[1, 2, 3], [4, 5, 6]], dims=["x", "y"], coords={"x": [5, 6]}) da.reduce(lambda x, _, *__: x, dim="x") # has time coord

ds = xr.Dataset({"a": da}) ds.reduce(lambda x, _, *__: x, dim="x") # lost time coord ```

Probably the assumtion when writing this code was that the passed function actually reduces the dim.

 
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