MCVE Code Sample

1. Create test_accessor.py in current directory with:

```python import xarray as xr

@xr.register_dataarray_accessor('test') class TestDataArrayAccessor(object): def init(self, obj): self._obj = obj print("DataArray accessor created")

@xr.register_dataset_accessor('test') class TestDatasetAccessor(object): def init(self, obj): self._obj = obj print("Dataset accessor created") ```

1. Run the following code:

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

ds = xr.Dataset({'a': xr.DataArray(np.array([1, 2, 3])), 'b': xr.DataArray(np.array([4, 5, 6]))}) ds.test

Dataset accessor created <accessor created on first access - expected>

ds.test

<no output - expected>

ds['a'].test

DataArray accessor created <accessor created on first access - expected>

ds['a'].test

DataArray accessor created <accessor created on second access - unexpected>

var = ds['a'] var.test

DataArray accessor created <accessor created on direct instance access - expected/unexpected>

var.test

<no output - expected>

```

Expected Output

Based on the xarray accessor documentation I would have assumed that the accessor would stick around on the same DataArray object for the life of the data. My guess is that Dataset.__getitem__ is recreating the DataArray every time from the underlying Variable object which means ds['a'] is not ds['a']?

Problem Description

I'm currently working on an accessor for a new package called geoxarray to address the issues talked about in #2288. One of the cases I'm trying to handle is a NetCDF file with CR standard grid_mapping variables. This means that the easiest way to set a CRS object for all variables in a Dataset is to have the Dataset accessor use the accessor of every DataArray (to cache a _crs property). However, with the way things are working doing something like the below won't work:

python ds.geo.apply_grid_mapping() ds['Rad'].geo.crs # this is current `None` because the DataArray was recreated

Output of xr.show_versions()

INSTALLED VERSIONS ------------------ commit: None python: 3.7.3 | packaged by conda-forge | (default, Jul 1 2019, 14:38:56) [Clang 4.0.1 (tags/RELEASE_401/final)] python-bits: 64 OS: Darwin OS-release: 18.6.0 machine: x86_64 processor: i386 byteorder: little LC_ALL: None LANG: en_US.UTF-8 LOCALE: en_US.UTF-8 libhdf5: 1.10.5 libnetcdf: 4.6.2 xarray: 0.12.3 pandas: 0.25.0 numpy: 1.17.0 scipy: None netCDF4: 1.5.1.2 pydap: None h5netcdf: None h5py: 2.9.0 Nio: None zarr: None cftime: 1.0.3.4 nc_time_axis: None PseudoNetCDF: None rasterio: None cfgrib: None iris: None bottleneck: None dask: 2.2.0 distributed: 2.2.0 matplotlib: None cartopy: None seaborn: None numbagg: None setuptools: 41.0.1 pip: 19.2.2 conda: None pytest: None IPython: 7.7.0 sphinx: None