Misleading error when opening file that does not exist

[naught101]

What happened:

In [1]: import xarray as xr

In [2]: xr.__version__
Out[2]: '0.18.2'

In [3]: xr.open_dataset('/not-a-real-file')
---------------------------------------------------------------------------
ValueError                                Traceback (most recent call last)
<ipython-input-3-4cc5243e5a90> in <module>
----> 1 xr.open_dataset('/not-a-real-file')

~/miniconda3/envs/ana38/lib/python3.8/site-packages/xarray/backends/api.py in open_dataset(filename_or_obj, engine, chunks, cache, decode_cf, mask_and_scale, decode_times, decode_timedelta, use_cftime, concat_characters, decode_coords, drop_variables, backend_kwargs, *args, **kwargs)
    478 
    479     if engine is None:
--> 480         engine = plugins.guess_engine(filename_or_obj)
    481 
    482     backend = plugins.get_backend(engine)

~/miniconda3/envs/ana38/lib/python3.8/site-packages/xarray/backends/plugins.py in guess_engine(store_spec)
    109     installed = [k for k in engines if k != "store"]
    110     if installed:
--> 111         raise ValueError(
    112             "did not find a match in any of xarray's currently installed IO "
    113             f"backends {installed}. Consider explicitly selecting one of the "

ValueError: did not find a match in any of xarray's currently installed IO backends ['netcdf4', 'scipy']. Consider explicitly selecting one of the installed backends via the ``engine`` parameter to xarray.open_dataset(), or installing additional IO dependencies:
http://xarray.pydata.org/en/stable/getting-started-guide/installing.html
http://xarray.pydata.org/en/stable/user-guide/io.html

What you expected to happen:

Should produce a "FileNotFound" error first.

Engine hunting on a non-existent file is pointless, and the error message is pretty wordy, so my skim-reading originally misinterpreted it to think that for some reason my netcdf4 library wasn't installed, which lead me on to a 4-hour environment rebuild, with a sudden realisation that I'm an idiot at the end of it...

Possible solution:

assert(os.path.isfile(path)) before anything else.

[d70-t]

I think there's more to think of then the suggested solution. For example when opening remote datasets (e.g. OPeNDAP resources), the supplied path will be a string which does not refer to a local path. The decision if a supplied "path" is valid might thus require to find an appropriate IO backend and then ask the backend if the supplied "path" is a valid one.

[keewis]

See also #5329 for more discussion and #5351 for a proposed fix. I'm closing this as a duplicate, but feel free to continue in that issue / PR.