issues: 819937730

What happened:

Created a new conda env for xarray as described in the contribution guide. Then I installed the h5netcdf and run all the tests (without making any changed in the xarray repo). Tests failed due to missing dependencies apparently.

What you expected to happen:

Tests should not fail if a package is not a requirement and the backends can operate independently.

Minimal Complete Verifiable Example:

bash cd xarray pip install -e . pip install h5netcdf pytest-xdist py.test -n 2 .

```python ======================================================================== FAILURES ========================================================================= ________ TestH5NetCDFAlreadyOpen.test_deepcopy ________ [gw0] darwin -- Python 3.8.8 /usr/local/anaconda3/envs/xarray/bin/python3.8

self = <xarray.tests.test_backends.TestH5NetCDFAlreadyOpen object at 0x11ce5d5e0>

def test_deepcopy(self):
    import h5netcdf

    with create_tmp_file() as tmp_file:

      with nc4.Dataset(tmp_file, mode="w") as nc:

E NameError: name 'nc4' is not defined

/Users/Victor/Documents/ProjectsDev/Qblox/xarray/xarray/tests/test_backends.py:2717: NameError ________ TestH5NetCDFAlreadyOpen.testopen_dataset_group _________ [gw1] darwin -- Python 3.8.8 /usr/local/anaconda3/envs/xarray/bin/python3.8

self = <xarray.tests.test_backends.TestH5NetCDFAlreadyOpen object at 0x11cfe6bb0>

def test_open_dataset_group(self):
    import h5netcdf

    with create_tmp_file() as tmp_file:

      with nc4.Dataset(tmp_file, mode="w") as nc:

E NameError: name 'nc4' is not defined

/Users/Victor/Documents/ProjectsDev/Qblox/xarray/xarray/tests/test_backends.py:2690: NameError _________ test_load_single_value_h5netcdf ___________ [gw0] darwin -- Python 3.8.8 /usr/local/anaconda3/envs/xarray/bin/python3.8

def _get_default_engine_netcdf():
    try:

      import netCDF4  # noqa: F401

E ModuleNotFoundError: No module named 'netCDF4'

/Users/Victor/Documents/ProjectsDev/Qblox/xarray/xarray/backends/api.py:105: ModuleNotFoundError

During handling of the above exception, another exception occurred:

def _get_default_engine_netcdf():
    try:
        import netCDF4  # noqa: F401

        engine = "netcdf4"
    except ImportError:  # pragma: no cover
        try:

          import scipy.io.netcdf  # noqa: F401

E ModuleNotFoundError: No module named 'scipy'

/Users/Victor/Documents/ProjectsDev/Qblox/xarray/xarray/backends/api.py:110: ModuleNotFoundError

During handling of the above exception, another exception occurred:

tmp_path = PosixPath('/private/var/folders/9k/k5388g_s44b_97319sslv9c40000gn/T/pytest-of-Victor/pytest-3/popen-gw0/test_load_single_value_h5netcd0')

@requires_h5netcdf
def test_load_single_value_h5netcdf(tmp_path):
    """Test that numeric single-element vector attributes are handled fine.

    At present (h5netcdf v0.8.1), the h5netcdf exposes single-valued numeric variable
    attributes as arrays of length 1, as oppesed to scalars for the NetCDF4
    backend.  This was leading to a ValueError upon loading a single value from
    a file, see #4471.  Test that loading causes no failure.
    """
    ds = xr.Dataset(
        {
            "test": xr.DataArray(
                np.array([0]), dims=("x",), attrs={"scale_factor": 1, "add_offset": 0}
            )
        }
    )

  ds.to_netcdf(tmp_path / "test.nc")

/Users/Victor/Documents/ProjectsDev/Qblox/xarray/xarray/tests/test_backends.py:4993:

---

/Users/Victor/Documents/ProjectsDev/Qblox/xarray/xarray/core/dataset.py:1689: in to_netcdf return to_netcdf( /Users/Victor/Documents/ProjectsDev/Qblox/xarray/xarray/backends/api.py:1052: in to_netcdf engine = _get_default_engine(path_or_file) /Users/Victor/Documents/ProjectsDev/Qblox/xarray/xarray/backends/api.py:145: in _get_default_engine engine = _get_default_engine_netcdf()

---

def _get_default_engine_netcdf():
    try:
        import netCDF4  # noqa: F401

        engine = "netcdf4"
    except ImportError:  # pragma: no cover
        try:
            import scipy.io.netcdf  # noqa: F401

            engine = "scipy"
        except ImportError:

          raise ValueError(
                "cannot read or write netCDF files without "
                "netCDF4-python or scipy installed"
            )

E ValueError: cannot read or write netCDF files without netCDF4-python or scipy installed

/Users/Victor/Documents/ProjectsDev/Qblox/xarray/xarray/backends/api.py:114: ValueError ==================================================================== warnings summary ===================================================================== xarray/tests/test_accessor_str.py: 18 warnings /Users/Victor/Documents/ProjectsDev/Qblox/xarray/xarray/core/duck_array_ops.py:248: FutureWarning: elementwise comparison failed; returning scalar instead, but in the future will perform elementwise comparison flag_array = (arr1 == arr2) | (isnull(arr1) & isnull(arr2))

xarray/tests/test_dataset.py::TestDataset::test_resample_loffset /Users/Victor/Documents/ProjectsDev/Qblox/xarray/xarray/tests/test_dataset.py:3904: FutureWarning: 'loffset' in .resample() and in Grouper() is deprecated.

df.resample(freq="3s", loffset="8H")

becomes:

from pandas.tseries.frequencies import to_offset df = df.resample(freq="3s").mean() df.index = df.index.to_timestamp() + to_offset("8H")

ds.bar.to_series().resample("24H", loffset="-12H").mean()

xarray/tests/test_dataarray.py::TestDataArray::test_resample /Users/Victor/Documents/ProjectsDev/Qblox/xarray/xarray/tests/test_dataarray.py:2994: FutureWarning: 'loffset' in .resample() and in Grouper() is deprecated.

df.resample(freq="3s", loffset="8H")

becomes:

from pandas.tseries.frequencies import to_offset df = df.resample(freq="3s").mean() df.index = df.index.to_timestamp() + to_offset("8H")

expected = DataArray(array.to_series().resample("24H", loffset="-12H").mean())

xarray/tests/test_dataarray.py::TestDataArray::test_polyfit[True-False] xarray/tests/test_dataarray.py::TestDataArray::test_polyfit[False-False] /Users/Victor/Documents/ProjectsDev/Qblox/xarray/xarray/core/dataset.py:6396: RuntimeWarning: overflow encountered in multiply scale = np.sqrt((lhs * lhs).sum(axis=0))

-- Docs: https://docs.pytest.org/en/stable/warnings.html ================================================================= short test summary info ================================================================= FAILED xarray/tests/test_backends.py::TestH5NetCDFAlreadyOpen::test_deepcopy - NameError: name 'nc4' is not defined FAILED xarray/tests/test_backends.py::TestH5NetCDFAlreadyOpen::test_open_dataset_group - NameError: name 'nc4' is not defined FAILED xarray/tests/test_backends.py::test_load_single_value_h5netcdf - ValueError: cannot read or write netCDF files without netCDF4-python or scipy in... ============================== 3 failed, 3474 passed, 4472 skipped, 65 xfailed, 17 xpassed, 22 warnings in 66.86s (0:01:06) =============================== ```

Anything else we need to know?:

Tests still fail when installing scipy only.

They only pass after installing both netcdf4 and scipy.

Environment:

Output of <tt>xr.show_versions()</tt>

INSTALLED VERSIONS ------------------ commit: 48378c4b11c5c2672ff91396d4284743165b4fbe python: 3.8.8 | packaged by conda-forge | (default, Feb 20 2021, 16:12:38) [Clang 11.0.1 ] python-bits: 64 OS: Darwin OS-release: 18.7.0 machine: x86_64 processor: i386 byteorder: little LC_ALL: en_US.UTF-8 LANG: en_US.UTF-8 LOCALE: en_US.UTF-8 libhdf5: 1.12.0 libnetcdf: None xarray: 0.17.1.dev3+g48378c4b pandas: 1.2.2 numpy: 1.20.1 scipy: None netCDF4: None pydap: None h5netcdf: 0.10.0 h5py: 3.1.0 Nio: None zarr: None cftime: 1.4.1 nc_time_axis: None PseudoNetCDF: None rasterio: None cfgrib: None iris: None bottleneck: None dask: None distributed: None matplotlib: None cartopy: None seaborn: None numbagg: None pint: None setuptools: 49.6.0.post20210108 pip: 21.0.1 conda: None pytest: 6.2.2 IPython: None sphinx: None