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
554376164,MDU6SXNzdWU1NTQzNzYxNjQ=,3718,losing shallowness of ds.copy() on ds from xr.open_dataset,15570875,open,0,,,1,2020-01-23T20:02:55Z,2020-01-23T21:39:05Z,,NONE,,,,"#### MCVE Code Sample
<!-- In order for the maintainers to efficiently understand and prioritize issues, we ask you post a ""Minimal, Complete and Verifiable Example"" (MCVE): http://matthewrocklin.com/blog/work/2018/02/28/minimal-bug-reports -->

```
import numpy as np
import xarray as xr

xlen = 4
x = xr.DataArray(np.linspace(0.0, 1.0, xlen), dims=('x'))

varname = 'foo'
xr.Dataset({varname: xr.DataArray(np.arange(xlen, dtype='float64'), dims=('x'), coords={'x': x})}).to_netcdf('ds.nc')
    
with xr.open_dataset('ds.nc') as ds:
    ds2 = ds.copy()
    ds2[varname][0] = 11.0
    print(f'ds.equals = {ds.equals(ds2)}')

with xr.open_dataset('ds.nc') as ds:
    ds2 = ds.copy()
    print(f'ds.equals = {ds.equals(ds2)}')
    ds2[varname][0] = 11.0
    print(f'ds.equals = {ds.equals(ds2)}')
```

#### Expected Output

I expect the code to write out
```
ds.equals = True
ds.equals = True
ds.equals = True
```

However, when I run it, the last line is
```
ds.equals = False
```

#### Problem Description

The code above writes a small `xr.Dataset` to a netCDF file.
There are 2 context managers opening the netCDF file as `ds`.
Both context manager blocks start by setting `ds2` to a shallow copy of `ds`.

In the first context manager block, a value in `ds2` is modified, and `ds2` is compared to `ds`. The Datasets are still equal, confirming that the copy is shallow.

The second context manager block is the same as the first, except that `ds2` is compared to `ds` prior changing the value the value in `ds2`. When this is done, the Datasets are no longer equal, indicating that `ds2` is no longer a shallow copy of `ds`.

I don't understand how evaluating `ds.equals(ds2)`, prior to changing a value in `ds2`, could decouple `ds2` from `ds`.

I only observe this behavior when `ds` is set via `xr.open_dataset`. I don't see it when I create `ds` directly using `xr.Dataset`.

I'm rather perplexed by this.

#### Output of ``xr.show_versions()``
<details>

INSTALLED VERSIONS
------------------
commit: None
python: 3.7.6 | packaged by conda-forge | (default, Jan  7 2020, 22:33:48) 
[GCC 7.3.0]
python-bits: 64
OS: Linux
OS-release: 3.10.0-693.21.1.el7.x86_64
machine: x86_64
processor: x86_64
byteorder: little
LC_ALL: en_US.UTF-8
LANG: en_US.UTF-8
LOCALE: en_US.UTF-8
libhdf5: 1.10.5
libnetcdf: 4.7.3

xarray: 0.14.1
pandas: 0.25.3
numpy: 1.17.5
scipy: 1.4.1
netCDF4: 1.5.3
pydap: None
h5netcdf: None
h5py: None
Nio: None
zarr: None
cftime: 1.0.3.4
nc_time_axis: 1.2.0
PseudoNetCDF: None
rasterio: None
cfgrib: None
iris: None
bottleneck: 1.3.1
dask: 2.9.2
distributed: 2.9.3
matplotlib: 3.1.2
cartopy: None
seaborn: None
numbagg: None
setuptools: 45.1.0.post20200119
pip: 19.3.1
conda: None
pytest: 5.3.4
IPython: 7.11.1
sphinx: None

</details>
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