`sel` fails confusingly or silently when a dimension name matches an optional argument

Given that many Xarray methods accept dict or keyword arguments, this may effect other methods. That said, here is a minimal example with sel.

Minimal example

If I want to select based on a dimension named method, I cannot because Xarray thinks method is the method argument to sel:

>>> da1 = xr.DataArray(range(3), dims=['method'], coords={'method': range(3)})
>>> da1
<xarray.DataArray (method: 3)>
array([0, 1, 2])
Coordinates:
  * method   (method) int64 0 1 2
>>> da1.sel(method=0)
...
TypeError: ``method`` must be a string

And if the method dimension has string labels, this fails silently:

>>> da2 = xr.DataArray(range(3), dims=['method'], coords={'method': list('abc')})
>>> da2.sel(method='a')
<xarray.DataArray (method: 3)>
array([0, 1, 2])
Coordinates:
  * method   (method) <U1 'a' 'b' 'c'

Expected Output

I think raising a ValueError and providing a clarifying error message is the right call, but maybe the maintainers have a different opinion. At the very least, it seems like Xarray could note that the DataArray instance has a dimension that matches one of the function arguments and ask the user to use dict-like arguments if required.

I imagine a general error handling function could be written to check this for any function and DataArray pair.

[dcherian]

Yes add another kwarg

[dcherian]

+1. Though I prefer raising a more informative over raising a warning.