issues

• [x] Closes #2037
• [ ] Tests added
• [ ] User visible changes (including notable bug fixes) are documented in whats-new.rst
• [ ] New functions/methods are listed in api.rst

What is your issue?

I was getting the following error continuously when using xarray.open_mfdataset: bash ValueError: did not find a match in any of xarray's currently installed IO backends ['netcdf4', 'h5netcdf', 'scipy', 'cfgrib', 'pydap', 'zarr']. Consider explicitly selecting one of the installed engines via the ``engine`` parameter, or installing additional IO dependencies When the file file was indeed a netcdf4: bash $ ncdump -k myfile.nc classic I was running my script with the command inside a docker container, which uses a different user and group than myself. It seems the issue was on the permisions of the file as only my user had access: bash $ ls -la total 39M -rw-r----- 1 borja borja 39M Mär 23 12:15 myfile.nc Changing permission rights with chmod a+r myfile.nc solved the thing: bash $ ls -la total 39M -rw-r--r-- 1 borja borja 39M Mär 23 12:15 myfile.nc The permission thing is perfectly fine, it was my error, however, it would be nice that the command open_mfdataset (or related) would raise PermissionError when permissions are not sufficient.

What happened: Inheritance of xarray.Dataset request the addition of __slots__: python /home/borja/miniconda3/envs/o3as/lib/python3.6/abc.py:133: FutureWarning: xarray subclass Model should explicitly define __slots__ cls = super().__new__(mcls, name, bases, namespace, **kwargs) /home/borja/Projects/o3skim/o3skim/sources.py:77: FutureWarning: Setting attribute 'dataset' on a 'Model' object. Explicitly define __slots__ to suppress this warning for legitimate custom attributes and raise an error when attempting variables assignments. self.dataset = ds however when added, produces RecursionError.

What you expected to happen: It should be possible to use Containment & Delegation on xarray classes.

Minimal Complete Verifiable Example: ```python import xarray

class MyDSet(xarray.Dataset): slots = ['dataset',] def init(self, ds): self.dataset = ds

def __getattr__(self, k):
    return getattr(self.dataset, k)

def __setattr__(self, k, v):
    setattr(self.dataset, k, v)

Returns:py

reload(mytest) Traceback (most recent call last): File "<stdin>", line 1, in <module> File "/home/borja/miniconda3/envs/o3as/lib/python3.6/importlib/init.py", line 166, in reload _bootstrap._exec(spec, module) File "<frozen importlib._bootstrap>", line 618, in _exec File "<frozen importlib._bootstrap_external>", line 678, in exec_module File "<frozen importlib._bootstrap>", line 219, in _call_with_frames_removed File "/home/borja/Projects/o3skim/o3skim/mytest.py", line 3, in <module> class BCorrect(xarray.Dataset): File "/home/borja/miniconda3/envs/o3as/lib/python3.6/typing.py", line 978, in new self = super().new(cls, name, bases, namespace, _root=True) File "/home/borja/miniconda3/envs/o3as/lib/python3.6/typing.py", line 137, in new return super().new(cls, name, bases, namespace) File "/home/borja/miniconda3/envs/o3as/lib/python3.6/abc.py", line 133, in new cls = super().new(mcls, name, bases, namespace, **kwargs) File "/home/borja/miniconda3/envs/o3as/lib/python3.6/site-packages/xarray/core/common.py", line 199, in init_subclass if not hasattr(object.new(cls), "dict"): File "/home/borja/Projects/o3skim/o3skim/mytest.py", line 9, in getattr return getattr(self.dataset, k) File "/home/borja/Projects/o3skim/o3skim/mytest.py", line 9, in getattr return getattr(self.dataset, k) File "/home/borja/Projects/o3skim/o3skim/mytest.py", line 9, in getattr return getattr(self.dataset, k) [Previous line repeated 325 more times] RecursionError: maximum recursion depth exceeded ```

Anything else we need to know?: I would recommend to add the following test or similar to ensure it is working: ```python

class X(xarray.Dataset): slots = ['dataset'] def init(self, ds): self.dataset = ds

ds = xarray.Dataset() x = X(ds) ```

Environment: