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  • DataArray.resample().apply() fails to apply custom function · 2 ✖

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  • CONTRIBUTOR · 2 ✖
id html_url issue_url node_id user created_at updated_at ▲ author_association body reactions performed_via_github_app issue
1230234269 https://github.com/pydata/xarray/issues/6953#issuecomment-1230234269 https://api.github.com/repos/pydata/xarray/issues/6953 IC_kwDOAMm_X85JU-Kd st-bender 28786187 2022-08-29T12:41:47Z 2022-08-29T12:41:47Z CONTRIBUTOR

Hi @mathause

It does work if the array keeps the size:

python data.resample(index="M").apply(lambda x: x.values)

Thanks, but I am not sure I find that intuitive, why should the resampled array have the same size as the original? It seems to make only sense for DataArray.apply(), but not for a resampled one. As I indicated in my other reply, returning a scalar or equivalent should be fine, shouldn't it? At the very least the documentation is lacking, it refers to the pandas method, but clearly the behaviour is different.

As a workaround you could allow your function to consume a dummy axis. Or you could pass dim as ...

python data.resample(index="M").reduce(lambda x, axis: 1) # workaround 1 data.resample(index="M").reduce(lambda x: 1, dim=...) # workaround 2

(reduce only passes axis if dim is not None but groupby passes the group_dim per default.

That feels a bit like curing the symptoms instead of the root cause, why not set dim='...' if it is not given?

 
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   DataArray.resample().apply() fails to apply custom function 1350803561
1228267515 https://github.com/pydata/xarray/issues/6953#issuecomment-1228267515 https://api.github.com/repos/pydata/xarray/issues/6953 IC_kwDOAMm_X85JNd_7 st-bender 28786187 2022-08-26T09:22:21Z 2022-08-26T09:22:21Z CONTRIBUTOR

@mathause Thanks, that works for np.median, but .reduce() passes an axis= keyword argument, which needs to be taken care of by any custom function. I used np.median here just as an example, as it showed the problem.

@dcherian I am not sure if raising an error would be very user friendly. In my opinion, which may be biased by my personal use cases, I would expect .apply() (or .map() for that matter) of a resample object to take any function that returns a scalar: the value at the resampled point. It should be then up to xarray to iterate over any other non-resampled dimensions. I am not sure why it requires to return a DataArray in the first place. As mentioned, it works with pandas series and dataframes, e.g.:

python data.to_pandas().resample("M").apply(np.median)

 
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   DataArray.resample().apply() fails to apply custom function 1350803561

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