issue_comments: 276537615

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https://github.com/pydata/xarray/issues/60#issuecomment-276537615 https://api.github.com/repos/pydata/xarray/issues/60 276537615 MDEyOklzc3VlQ29tbWVudDI3NjUzNzYxNQ== 8982598 2017-02-01T00:26:24Z 2017-02-01T00:26:24Z CONTRIBUTOR

Ah yes both ways are working now, thanks. Just had a little play around with timings, and this seems like a reasonably quick way to achieve correct NaN behaviour:

```python def xr_idxmax(obj, dim): sig = ([(dim,), (dim,)], [()]) kwargs = {'axis': -1}

allna = obj.isnull().all(dim)

return apply_ufunc(gufunc_idxmax, obj.fillna(-np.inf), obj[dim],
                   signature=sig, kwargs=kwargs,
                   dask_array='allowed').where(~allna).fillna(np.nan)

```

i.e. originally replace all NaN values with -Inf, use the usual argmax, and remask the all-NaN values afterwards.

 
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