How to use the pyproj.CRS.from_cf function in pyproj

def test_cf_rotated_latlon():
    crs = CRS.from_cf(
        dict(
            grid_mapping_name="rotated_latitude_longitude",
            grid_north_pole_latitude=32.5,
            grid_north_pole_longitude=170.0,
        )
    )
    cf_dict = crs.to_cf()
    assert cf_dict.pop("crs_wkt").startswith("GEOGCRS[")
    if LooseVersion(proj_version_str) >= LooseVersion("6.3.0"):
        with pytest.warns(UserWarning):
            assert crs.to_dict() == {
                "proj": "ob_tran",
                "o_proj": "longlat",
                "o_lat_p": 32.5,
                "o_lon_p": 170.0,
                "datum": "WGS84",

def test_cf_from_latlon__named():
    crs = CRS.from_cf(dict(spatial_ref="epsg:4326"))
    with pytest.warns(UserWarning):
        cf_dict = crs.to_cf(errcheck=True)
    assert cf_dict.pop("crs_wkt").startswith("GEOGCRS[")
    assert cf_dict == {
        "geographic_crs_name": "WGS 84",
        "grid_mapping_name": "latitude_longitude",
        "horizontal_datum_name": "WGS84",
    }

def test_mercator():
    crs = CRS.from_cf(
        {
            "grid_mapping_name": "mercator",
            "longitude_of_projection_origin": 10,
            "standard_parallel": 21.354,
            "false_easting": 0,
            "false_northing": 0,
        }
    )
    with pytest.warns(UserWarning):
        assert crs.to_dict() == {
            "datum": "WGS84",
            "lat_ts": 21.354,
            "lon_0": 10,
            "no_defs": None,
            "proj": "merc",
            "type": "crs",

def test_cf_from_latlon():
    crs = CRS.from_cf(
        dict(
            grid_mapping_name="latitude_longitude",
            semi_major_axis=6378137.0,
            inverse_flattening=298.257223,
        )
    )
    with pytest.warns(UserWarning):
        cf_dict = crs.to_cf(errcheck=True)
    assert len(cf_dict) == 4
    assert cf_dict["crs_wkt"].startswith("GEOGCRS[")
    assert cf_dict["grid_mapping_name"] == "latitude_longitude"
    assert cf_dict["semi_major_axis"] == 6378137.0
    assert cf_dict["inverse_flattening"] == 298.257223

def test_oblique_mercator():
    crs = CRS.from_cf(
        dict(
            grid_mapping_name="oblique_mercator",
            azimuth_of_central_line=0.35,
            latitude_of_projection_origin=10,
            longitude_of_projection_origin=15,
            reference_ellipsoid_name="WGS84",
            false_easting=0.0,
            false_northing=0.0,
        )
    )
    cf_dict = crs.to_cf()
    assert cf_dict.pop("crs_wkt").startswith("PROJCRS[")
    assert cf_dict == {
        "grid_mapping_name": "oblique_mercator",
        "latitude_of_projection_origin": 10,
        "longitude_of_projection_origin": 15,

def test_from_cf_transverse_mercator():
    towgs84_test = (-122.74, -34.27, -22.83, -1.884, -3.4, -3.03, -15.62)
    crs = CRS.from_cf(
        {
            "grid_mapping_name": "transverse_mercator",
            "latitude_of_projection_origin": 0,
            "longitude_of_central_meridian": 15,
            "false_easting": 2520000,
            "false_northing": 0,
            "reference_ellipsoid_name": "intl",
            "towgs84": towgs84_test,
            "unit": "m",
        }
    )
    with pytest.warns(UserWarning):
        cf_dict = crs.to_cf(errcheck=True)
    assert len(cf_dict) == 9
    assert cf_dict["crs_wkt"].startswith("BOUNDCRS[")
    assert cf_dict["grid_mapping_name"] == "transverse_mercator"

def test_cf_lambert_conformal_conic_1sp():
    crs = CRS.from_cf(
        dict(
            grid_mapping_name="lambert_conformal_conic",
            standard_parallel=25.0,
            longitude_of_central_meridian=265.0,
            latitude_of_projection_origin=25.0,
        )
    )
    with pytest.warns(UserWarning):
        cf_dict = crs.to_cf(errcheck=True)
    assert cf_dict.pop("crs_wkt").startswith("PROJCRS[")
    assert cf_dict == {
        "grid_mapping_name": "lambert_conformal_conic",
        "longitude_of_central_meridian": 265,
        "scale_factor_at_projection_origin": 1,
        "standard_parallel": 25,
        "latitude_of_projection_origin": 25,

def test_crs_fixed_angle_axis():
    crs = CRS.from_cf(
        dict(
            grid_mapping_name="geostationary",
            perspective_point_height=1,
            fixed_angle_axis="y",
        )
    )
    cf_dict = crs.to_cf()
    assert cf_dict.pop("crs_wkt").startswith("PROJCRS[")
    assert cf_dict == {
        "grid_mapping_name": "geostationary",
        "longitude_of_projection_origin": 0,
        "sweep_angle_axis": "x",
        "perspective_point_height": 1,
        "false_easting": 0,
        "false_northing": 0,
        "horizontal_datum_name": "WGS84",

def test_cf_lambert_conformal_conic_2sp():
    crs = CRS.from_cf(
        dict(
            grid_mapping_name="lambert_conformal_conic",
            standard_parallel=[25.0, 30.0],
            longitude_of_central_meridian=265.0,
            latitude_of_projection_origin=25.0,
        )
    )
    with pytest.warns(UserWarning):
        cf_dict = crs.to_cf(errcheck=True)
    assert cf_dict.pop("crs_wkt").startswith("PROJCRS[")
    assert cf_dict == {
        "grid_mapping_name": "lambert_conformal_conic",
        "longitude_of_central_meridian": 265,
        "standard_parallel": [25, 30],
        "latitude_of_projection_origin": 25,
        "false_easting": 0,

# check the variable exists
    try:
        v = nc_handle[grid_mapping_varname]
    except KeyError:
        raise KeyError("Variable '{}' does not exist in netCDF file".format(grid_mapping_varname))

    # check this indeed is a supported grid mapping variable
    try:
        if v.grid_mapping_name not in _valid_cf_type_of_grid_mapping:
            raise ValueError("Not a valid CF grid_mapping variable ({})".format(grid_mapping_varname))
    except AttributeError:
        # no :grid_mapping_name thus it cannot be a valid grid_mapping variable
        raise ValueError("Not a valid CF grid_mapping variable ({})".format(grid_mapping_varname))

    # use pyproj to load the CRS
    return pyproj.CRS.from_cf(v.attrs)