issues: 309136602

https://travis-ci.org/pydata/xarray/builds/359093748

Example failure: ``` ___ ScipyFilePathTest.test_append_write ____ self = <xarray.tests.test_backends.ScipyFilePathTest testMethod=test_append_write> def test_append_write(self): # regression for GH1215 data = create_test_data()

  with self.roundtrip_append(data) as actual:

xarray/tests/test_backends.py:786:

../../../miniconda/envs/test_env/lib/python3.6/contextlib.py:81: in enter return next(self.gen) xarray/tests/test_backends.py:155: in roundtrip_append self.save(data[[key]], path, mode=mode, save_kwargs) xarray/tests/test_backends.py:162: in save kwargs) xarray/core/dataset.py:1131: in to_netcdf unlimited_dims=unlimited_dims) xarray/backends/api.py:657: in to_netcdf unlimited_dims=unlimited_dims) xarray/core/dataset.py:1068: in dump_to_store unlimited_dims=unlimited_dims) xarray/backends/common.py:363: in store unlimited_dims=unlimited_dims) xarray/backends/common.py:402: in set_variables self.writer.add(source, target) xarray/backends/common.py:265: in add target[...] = source xarray/backends/scipy_.py:61: in setitem data[key] = value

self = <scipy.io.netcdf.netcdf_variable object at 0x7fe3eb3ec6a0> index = Ellipsis, data = array([0. , 0.5, 1. , 1.5, 2. , 2.5, 3. , 3.5, 4. ]) def setitem(self, index, data): if self.maskandscale: missing_value = ( self._get_missing_value() or getattr(data, 'fill_value', 999999)) self._attributes.setdefault('missing_value', missing_value) self._attributes.setdefault('_FillValue', missing_value) data = ((data - self._attributes.get('add_offset', 0.0)) / self._attributes.get('scale_factor', 1.0)) data = np.ma.asarray(data).filled(missing_value) if self._typecode not in 'fd' and data.dtype.kind == 'f': data = np.round(data)

    # Expand data for record vars?
    if self.isrec:
        if isinstance(index, tuple):
            rec_index = index[0]
        else:
            rec_index = index
        if isinstance(rec_index, slice):
            recs = (rec_index.start or 0) + len(data)
        else:
            recs = rec_index + 1
        if recs > len(self.data):
            shape = (recs,) + self._shape[1:]
            # Resize in-place does not always work since
            # the array might not be single-segment
            try:
                self.data.resize(shape)
            except ValueError:
                self.__dict__['data'] = np.resize(self.data, shape).astype(self.data.dtype)

  self.data[index] = data

E ValueError: assignment destination is read-only ```