issues: 354151953

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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
354151953 MDU6SXNzdWUzNTQxNTE5NTM= 2382 Add option to choose the source of global attributes in mfdataset. 1186928 closed 0     0 2018-08-27T01:45:23Z 2020-01-11T15:22:56Z 2020-01-11T15:22:56Z CONTRIBUTOR      

Code Sample

```python import numpy as np import xarray as xr

prepare fake data

time = np.arange(1000) data = time**2

write to multiple netcdf files

for i in range(10): filename = 'ds{:d}.nc'.format(i) ds = xr.Dataset({'data': (['time'], data[100i:100i+100])}, coords={'time': time[100i:100i+100]}, attrs={'history': 'Created ' + filename + '.'}) ds.to_netcdf(filename)

open as mfdataset

with xr.open_mfdataset('ds?.nc') as ds: print ds.history ```

Problem description

Currently, global attributes of multi-file datasets are taken from the first file in the list.

https://github.com/pydata/xarray/blob/b87b684b36cf5adbe4dca208aed0c69c44fc44c4/xarray/backends/api.py#L649

I think this is a problem in the context of consecutive model runs where history is appended in each subsequent run. When opening the results as mfdataset, history is taken from the first run file all subsequent history is lost.

NetCDF4 has a new keyword argument to set the master_file in a MFDataset (Unidata/netcdf4-python#835). Would it be possible to add a similar option in xarray?

Output of xr.show_versions()

INSTALLED VERSIONS ------------------ commit: None python: 2.7.15.final.0 python-bits: 64 OS: Linux OS-release: 4.14.65-1-MANJARO machine: x86_64 processor: byteorder: little LC_ALL: None LANG: fr_CH.utf8 LOCALE: None.None xarray: 0.10.8 pandas: 0.23.4 numpy: 1.15.0 scipy: 1.1.0 netCDF4: 1.4.2 h5netcdf: None h5py: None Nio: None zarr: None bottleneck: None cyordereddict: None dask: 0.18.2 distributed: None matplotlib: 2.2.3 cartopy: 0.15.1 seaborn: None setuptools: 40.0.0 pip: 18.0 conda: None pytest: None IPython: 5.8.0 sphinx: 1.7.6 
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