id,node_id,number,title,user,state,locked,assignee,milestone,comments,created_at,updated_at,closed_at,author_association,active_lock_reason,draft,pull_request,body,reactions,performed_via_github_app,state_reason,repo,type
833518574,MDU6SXNzdWU4MzM1MTg1NzQ=,5043,open_mfdataset failed to open tarfile filestream when it locates in the context of dask.distributed Client,12339722,open,0,,,1,2021-03-17T08:17:47Z,2021-06-20T21:06:23Z,,NONE,,,,"Recently, i use `open_mfdataset` to open a local tar.gz file of multiple netcdf files, 
it failed to open it and raise a `distributed.scheduler.KilledWorker: Error` and 
`TypeError: cannot serialize 'ExFileObject' object` .

My code is like following, 

```python
import tarfile
from dask.distributed import Client

client = Client()

tar = tarfile.open(my_multiple_netcdf_tar_gz_file)
flist = [tar.extractfile(member) for member in tar.getmembers()]

ds = xr.open_mfdataset(flist)

# This line will raise Exception
print(ds.MyNcVar.values)

# ....
# blah blah my other client calcualation codes
# ....

client.close()
```

In above code, the elements of variable `flist` will be type of `ExFileObject`, 
which can't be serialized to distributed.Client cluster and 
therefore will result in the failure of `open_mfdataset` .

The reason is `xr.open_mfdataset` auto convert `chunks=None`  to `{}` , 
which will force the method  `xr.open_dataset`  to use dask.

We can see in this line of `open_mfdataset` , 

https://github.com/pydata/xarray/blob/37fe5441c8a2fb981f2c50b8379d7d4f8492ae19/xarray/backends/api.py#L897

```python
    # Notes this line will force chunks=None into chunks={} and result in the involvement of dask
    open_kwargs = dict(engine=engine, chunks=chunks or {}, **kwargs)

    if parallel:
        import dask

        # wrap the open_dataset, getattr, and preprocess with delayed
        open_ = dask.delayed(open_dataset)
        getattr_ = dask.delayed(getattr)
        if preprocess is not None:
            preprocess = dask.delayed(preprocess)
    else:
        open_ = open_dataset
        getattr_ = getattr

    datasets = [open_(p, **open_kwargs) for p in paths]
    closers = [getattr_(ds, ""_close"") for ds in datasets]
```

Even if i set the chunks=None, it will be a error cause the `chunks` always not be None
 when it is passed into `open_dataset` .

I think maybe we can keep the chunks value and if anyone want change it, 
he or she can set it to `{}` or any other values as they want ?

```python
    open_kwargs = dict(engine=engine, chunks=chunks, **kwargs)
```

Or may you have a better solution for my problem ?

Also, Thank You for your great jobs on this excellent package.
","{""url"": ""https://api.github.com/repos/pydata/xarray/issues/5043/reactions"", ""total_count"": 0, ""+1"": 0, ""-1"": 0, ""laugh"": 0, ""hooray"": 0, ""confused"": 0, ""heart"": 0, ""rocket"": 0, ""eyes"": 0}",,,13221727,issue