issue_comments

I just realised that xarray today already implements meaningful logic when concatenating between a and c in my example above - both on an existing dimension as well as on a new one. It's just a and b that don't work. I agree that defaulting to NaN would be a good option (regardless if the dimension is new or existing).

So to recap, when concatenating [a, b, c], where b does not have the y coord: - if the dtype of the y coord in both a and c is float, numpy.float32, or numpy.float64, fill in with NaN - if it's any datetime format, fill in with NaT - if it's strings, fill in with empty string?? - in any other case, convert everything to object and fill in with None

Correct? Is there any helper function to facilitate this?

@shoyer the end goal is to replicate in concat() the same behaviour you already hav in __add__ and __mul__. So take for example a = xarray.DataArray([1, 2, 3], dims=['x'], coords={'y': 10}) b = xarray.DataArray([4, 5, 6], dims=['x']) c = xarray.DataArray([7, 8, 9], dims=['x'], coords={'y': 20}) a+b -> y is propagated to the result (unambiguous) a+c -> y is discarded (ambiguous)

I suppose I could look at how this happens in __add__ and move the logic up into broadcast() (which __add__ should already invoke)?

I updated the code and tested it in my project - it works fine. Any suggestions before I start integrating it inside xarray.broadcast()?

``` def broadcast_scalar_coords(*args, exclude=()): """Broadcast scalar coords whenever they're unambiguous, and remove them when they are ambiguous. This allows replicating the behaviour of addition or moltiplication of xarrays in concatenation. """ unambiguous = {} ambiguous = set() for arg in args: for k, v in arg.coords.items(): if k in exclude: continue # non-scalar coords are automatically ambiguous if v.shape == (): if k not in ambiguous: v = v.values.tolist() if unambiguous.get(k, v) != v: del unambiguous[k] ambiguous.add(k) else: unambiguous[k] = v else: ambiguous.add(k) unambiguous.pop(k, None)

args = tuple(arg.copy(deep=False) for arg in args)
for arg in args:
    for k, v in unambiguous.items():
        arg.coords[k] = v
    for k in ambiguous:
        if k in arg.coords and arg.coords[k].shape == ():
            del arg.coords[k]
return args

```

I wrote down this. 1. should I simply add it to xarray.broadcast()? 2. can you spot any logical flaws? 3. suggestions for cleaner code? [edit] removed obsolete code