Loading data from inaccessible location gives cryptic/unhelpful error

[aidanheerdegen]

When cc.querying.getvar attempts to read data from an inaccessible location it gives a non-obvious error

ValueError: did not find a match in any of xarray's currently installed IO backends ['netcdf4', 'h5netcdf', 'scipy', 'cfgrib', 'pydap', 'rasterio', 'zarr']. Consider explicitly selecting one of the installed engines via the ``engine`` parameter, or installing additional IO dependencies, see:
http://xarray.pydata.org/en/stable/getting-started-guide/installing.html
http://xarray.pydata.org/en/stable/user-guide/io.html

It is likely they have not joined the project necessary to access the data (cj50 and qv56 are the most obvious possibilities)

Click to show full error stacktrace

---------------------------------------------------------------------------
ValueError                                Traceback (most recent call last)
/local/hh5/ed7737/tmp/ipykernel_2142946/2200008388.py in <module>
----> 1 u = cc.querying.getvar(expt,'u', session)

/g/data/hh5/public/apps/miniconda3/envs/analysis3-21.10/lib/python3.9/site-packages/cosima_cookbook/querying.py in getvar(expt, variable, session, ncfile, start_time, end_time, n, frequency, attrs, attrs_unique, **kwargs)
    358     ncfiles = list(str(f.NCFile.ncfile_path) for f in ncfiles)
    359 
--> 360     ds = xr.open_mfdataset(
    361         ncfiles,
    362         parallel=True,

/g/data/hh5/public/apps/miniconda3/envs/analysis3-21.10/lib/python3.9/site-packages/xarray/backends/api.py in open_mfdataset(paths, chunks, concat_dim, compat, preprocess, engine, data_vars, coords, combine, parallel, join, attrs_file, combine_attrs, **kwargs)
    914         # calling compute here will return the datasets/file_objs lists,
    915         # the underlying datasets will still be stored as dask arrays
--> 916         datasets, closers = dask.compute(datasets, closers)
    917 
    918     # Combine all datasets, closing them in case of a ValueError

/g/data/hh5/public/apps/miniconda3/envs/analysis3-21.10/lib/python3.9/site-packages/dask/base.py in compute(traverse, optimize_graph, scheduler, get, *args, **kwargs)
    569         postcomputes.append(x.__dask_postcompute__())
    570 
--> 571     results = schedule(dsk, keys, **kwargs)
    572     return repack([f(r, *a) for r, (f, a) in zip(results, postcomputes)])
    573 

/g/data/hh5/public/apps/miniconda3/envs/analysis3-21.10/lib/python3.9/site-packages/distributed/client.py in get(self, dsk, keys, workers, allow_other_workers, resources, sync, asynchronous, direct, retries, priority, fifo_timeout, actors, **kwargs)
   2689                     should_rejoin = False
   2690             try:
-> 2691                 results = self.gather(packed, asynchronous=asynchronous, direct=direct)
   2692             finally:
   2693                 for f in futures.values():

/g/data/hh5/public/apps/miniconda3/envs/analysis3-21.10/lib/python3.9/site-packages/distributed/client.py in gather(self, futures, errors, direct, asynchronous)
   1944             else:
   1945                 local_worker = None
-> 1946             return self.sync(
   1947                 self._gather,
   1948                 futures,

/g/data/hh5/public/apps/miniconda3/envs/analysis3-21.10/lib/python3.9/site-packages/distributed/utils.py in sync(self, func, asynchronous, callback_timeout, *args, **kwargs)
    308             return future
    309         else:
--> 310             return sync(
    311                 self.loop, func, *args, callback_timeout=callback_timeout, **kwargs
    312             )

/g/data/hh5/public/apps/miniconda3/envs/analysis3-21.10/lib/python3.9/site-packages/distributed/utils.py in sync(loop, func, callback_timeout, *args, **kwargs)
    362     if error[0]:
    363         typ, exc, tb = error[0]
--> 364         raise exc.with_traceback(tb)
    365     else:
    366         return result[0]

/g/data/hh5/public/apps/miniconda3/envs/analysis3-21.10/lib/python3.9/site-packages/distributed/utils.py in f()
    347             if callback_timeout is not None:
    348                 future = asyncio.wait_for(future, callback_timeout)
--> 349             result[0] = yield future
    350         except Exception:
    351             error[0] = sys.exc_info()

/g/data/hh5/public/apps/miniconda3/envs/analysis3-21.10/lib/python3.9/site-packages/tornado/gen.py in run(self)
    760 
    761                     try:
--> 762                         value = future.result()
    763                     except Exception:
    764                         exc_info = sys.exc_info()

/g/data/hh5/public/apps/miniconda3/envs/analysis3-21.10/lib/python3.9/site-packages/distributed/client.py in _gather(self, futures, errors, direct, local_worker)
   1809                             exc = CancelledError(key)
   1810                         else:
-> 1811                             raise exception.with_traceback(traceback)
   1812                         raise exc
   1813                     if errors == "skip":

/g/data/hh5/public/apps/miniconda3/envs/analysis3-21.10/lib/python3.9/site-packages/dask/utils.py in apply()
     35 def apply(func, args, kwargs=None):
     36     if kwargs:
---> 37         return func(*args, **kwargs)
     38     else:
     39         return func(*args)

/g/data/hh5/public/apps/miniconda3/envs/analysis3-21.10/lib/python3.9/site-packages/xarray/backends/api.py in open_dataset()
    477 
    478     if engine is None:
--> 479         engine = plugins.guess_engine(filename_or_obj)
    480 
    481     backend = plugins.get_backend(engine)

/g/data/hh5/public/apps/miniconda3/envs/analysis3-21.10/lib/python3.9/site-packages/xarray/backends/plugins.py in guess_engine()
    153         )
    154 
--> 155     raise ValueError(error_msg)
    156 
    157 

ValueError: did not find a match in any of xarray's currently installed IO backends ['netcdf4', 'h5netcdf', 'scipy', 'cfgrib', 'pydap', 'rasterio', 'zarr']. Consider explicitly selecting one of the installed engines via the ``engine`` parameter, or installing additional IO dependencies, see:
http://xarray.pydata.org/en/stable/getting-started-guide/installing.html
http://xarray.pydata.org/en/stable/user-guide/io.html

This is for documentation as much as anything (so people can search for their problem and find the solution). A nicer error message would be nice though.