html_url,issue_url,id,node_id,user,created_at,updated_at,author_association,body,reactions,performed_via_github_app,issue
https://github.com/pydata/xarray/issues/3032#issuecomment-503747413,https://api.github.com/repos/pydata/xarray/issues/3032,503747413,MDEyOklzc3VlQ29tbWVudDUwMzc0NzQxMw==,5635139,2019-06-19T21:09:18Z,2019-06-19T21:09:18Z,MEMBER,"@shoyer thanks for the clarity

@OriolAbril would you mind if we changed this issue to ""apply_ufunc should preemptively broadcast""","{""total_count"": 0, ""+1"": 0, ""-1"": 0, ""laugh"": 0, ""hooray"": 0, ""confused"": 0, ""heart"": 0, ""rocket"": 0, ""eyes"": 0}",,457716471
https://github.com/pydata/xarray/issues/3032#issuecomment-503652824,https://api.github.com/repos/pydata/xarray/issues/3032,503652824,MDEyOklzc3VlQ29tbWVudDUwMzY1MjgyNA==,5635139,2019-06-19T17:21:34Z,2019-06-19T17:21:34Z,MEMBER,I'm trying to think whether there would be any performance cost there - i.e. are there any arrays where preemptive broadcasting would be both expensive and unnecessary?,"{""total_count"": 0, ""+1"": 0, ""-1"": 0, ""laugh"": 0, ""hooray"": 0, ""confused"": 0, ""heart"": 0, ""rocket"": 0, ""eyes"": 0}",,457716471
https://github.com/pydata/xarray/issues/3032#issuecomment-503625092,https://api.github.com/repos/pydata/xarray/issues/3032,503625092,MDEyOklzc3VlQ29tbWVudDUwMzYyNTA5Mg==,5635139,2019-06-19T16:04:58Z,2019-06-19T16:04:58Z,MEMBER,"> We should probably make apply_ufunc() explicitly broadcast arrays first.

To confirm, so that we have something like this?

```python
xr.apply_ufunc(func, a, c)
# Out
# (7, 3, 5, 6)
# (7, 3, 5, 6) 
```
","{""total_count"": 0, ""+1"": 0, ""-1"": 0, ""laugh"": 0, ""hooray"": 0, ""confused"": 0, ""heart"": 0, ""rocket"": 0, ""eyes"": 0}",,457716471
https://github.com/pydata/xarray/issues/3032#issuecomment-503337637,https://api.github.com/repos/pydata/xarray/issues/3032,503337637,MDEyOklzc3VlQ29tbWVudDUwMzMzNzYzNw==,5635139,2019-06-18T22:37:42Z,2019-06-18T22:42:14Z,MEMBER,"Because `func` receives unlabelled arrays, `apply_ufunc` aligns the axes _order_. So if they share a dimension:

```python
In [8]: import xarray as xr
   ...: import numpy as np
   ...:
   ...: a = xr.DataArray(data=np.random.normal(size=(7, 3)), dims=[""dim1"", ""dim2""])
   ...: c = xr.DataArray(data=np.random.normal(size=(7, 6)), dims=[""dim1"", ""dim4""])  # <- change here
   ...:
   ...: def func(x,y):
   ...:     print(x.shape)
   ...:     print(y.shape)
   ...:     return x
   ...:

In [9]: xr.apply_ufunc(func, a, c)
(7, 3, 1)
(7, 1, 6)
```

...otherwise `func` wouldn't know how to align.

Another option would be for your original example to put lengths of 1 in _all_ axes, rather than only 'forward filling', e.g. 

```
xr.apply_ufunc(func, a, c)
# Out
# (7, 3, 1, 1)
# (1, 1, 5, 6) # <- change here
```

I _think_ it operates without that step because functions 'in the wild' generally will handle that themselves, but that's a guess and needs someone who knows this better to weight in","{""total_count"": 0, ""+1"": 0, ""-1"": 0, ""laugh"": 0, ""hooray"": 0, ""confused"": 0, ""heart"": 0, ""rocket"": 0, ""eyes"": 0}",,457716471
https://github.com/pydata/xarray/issues/3032#issuecomment-503334445,https://api.github.com/repos/pydata/xarray/issues/3032,503334445,MDEyOklzc3VlQ29tbWVudDUwMzMzNDQ0NQ==,5635139,2019-06-18T22:24:16Z,2019-06-18T22:24:16Z,MEMBER,"Thanks for the issue & code sample @OriolAbril 

IIUC, `func` needs to do this broadcasting itself; from the `apply_ufunc` docstring:

```
func : callable
    Function to call like ``func(*args, **kwargs)`` on unlabeled arrays
    (``.data``) that returns an array or tuple of arrays. If multiple
    arguments with non-matching dimensions are supplied, this function is
    expected to vectorize (broadcast) over axes of positional arguments in
    the style of NumPy universal functions [1]_ (if this is not the case,
    set ``vectorize=True``). If this function returns multiple outputs, you
    must set ``output_core_dims`` as well.
```","{""total_count"": 0, ""+1"": 0, ""-1"": 0, ""laugh"": 0, ""hooray"": 0, ""confused"": 0, ""heart"": 0, ""rocket"": 0, ""eyes"": 0}",,457716471