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
579722569,MDExOlB1bGxSZXF1ZXN0Mzg3MDY0ODEz,3858,Backend env,2444231,closed,0,,,5,2020-03-12T06:30:28Z,2023-01-05T03:58:54Z,2023-01-05T03:58:54Z,NONE,,0,pydata/xarray/pulls/3858,"This merge request allows the user to set a `backend_env` while opening a file. This should be a dictionary, and the key/value pairs are temporarily added to `os.enviorn` while opening the file. The old environment is restored later.


 - [x] Closes #3853
    Maybe -- I'm not sure if it is clever to actually close this issue at this point. I added the `backend_env` idea.
 - [ ] Tests added

   Here, I need some help: How should I actually design the tests? The environment is only temporarily modified, so as soon as the open_dataset function ends again, the environment is restored. I would have though temporarily adding an equivalent to  `$ export lala=tada` into the environment and checking for that would be an idea; but since I restore the environment right away, there is no way I can see to actually access the new `os.environ`
 - [x] Passes `isort -rc . && black . && mypy . && flake8`
 - [x] Fully documented, including `whats-new.rst` for all changes and `api.rst` for new API

   I added a section to the relevant docstring. Not sure how much this needs to also be included in the other files.
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315381649,MDU6SXNzdWUzMTUzODE2NDk=,2066,open_mfdataset can't handle many files,2444231,closed,0,,,7,2018-04-18T08:33:15Z,2019-03-18T14:58:15Z,2019-03-18T14:58:14Z,NONE,,,,"#### Code Sample, a copy-pastable example if possible

It appears as if the `open_mfdataset` cannot handle many files (many here = 1200) 

```python
ensemble = xr.open_mfdataset(""/scratch/simulation_database/incoming/Eem125-S2/output/Eem125-S2_echam5_main_mm_26*.nc"")

OSError                                   Traceback (most recent call last)
<ipython-input-4-038705c4f255> in <module>()
----> 1 ensemble = xr.open_mfdataset(""/scratch/simulation_database/incoming/Eem125-S2/output/Eem125-S2_echam5_main_mm_26*.nc"")

~/anaconda3/lib/python3.6/site-packages/xarray/backends/api.py in open_mfdataset(paths, chunks, concat_dim, compat, preprocess, engine, lock, data_vars, coords, **kwargs)

~/anaconda3/lib/python3.6/site-packages/xarray/backends/api.py in <listcomp>(.0)

~/anaconda3/lib/python3.6/site-packages/xarray/backends/api.py in open_dataset(filename_or_obj, group, decode_cf, mask_and_scale, decode_times, autoclose, concat_characters, decode_coords, engine, chunks, lock, cache, drop_variables)

~/anaconda3/lib/python3.6/site-packages/xarray/backends/netCDF4_.py in open(cls, filename, mode, format, group, writer, clobber, diskless, persist, autoclose)

~/anaconda3/lib/python3.6/site-packages/xarray/backends/netCDF4_.py in _open_netcdf4_group(filename, mode, group, **kwargs)

netCDF4/_netCDF4.pyx in netCDF4._netCDF4.Dataset.__init__()

netCDF4/_netCDF4.pyx in netCDF4._netCDF4._ensure_nc_success()

OSError: [Errno 24] Too many open files: b'/scratch/simulation_database/incoming/Eem125-S2/output/Eem125-S2_echam5_main_mm_260001.nc'
```
#### Problem description

Often, climate simulations produce more than one output file per model component (generally 1 per saved time output, e.g. months, years, days, or something else). It would be good to access all of these as one object, rather than having to combining them by hand before with e.g. `cdo` or some other tool.

#### Expected Output

`ensemble` variable definition gives me back 1 object  😄 

#### Output of ``xr.show_versions()``

<details>
# Paste the output here xr.show_versions() here
INSTALLED VERSIONS
------------------
commit: None
python: 3.6.3.final.0
python-bits: 64
OS: Linux
OS-release: 3.13.0-144-generic
machine: x86_64
processor: x86_64
byteorder: little
LC_ALL: None
LANG: en_US.UTF-8
LOCALE: en_US.UTF-8

xarray: 0.10.0
pandas: 0.20.3
numpy: 1.13.3
scipy: 0.19.1
netCDF4: 1.3.1
h5netcdf: 0.5.0
Nio: None
bottleneck: 1.2.1
cyordereddict: None
dask: 0.15.3
matplotlib: 2.1.0
cartopy: 0.16.0
seaborn: 0.8.0
setuptools: 36.5.0.post20170921
pip: 9.0.1
conda: 4.5.1
pytest: 3.2.1
IPython: 6.1.0
sphinx: 1.6.3
</details>","{""url"": ""https://api.github.com/repos/pydata/xarray/issues/2066/reactions"", ""total_count"": 0, ""+1"": 0, ""-1"": 0, ""laugh"": 0, ""hooray"": 0, ""confused"": 0, ""heart"": 0, ""rocket"": 0, ""eyes"": 0}",,completed,13221727,issue