How to use the everett.ext.yamlfile.ConfigYamlEnv function in everett

def test_missing_file(self):
        cie = ConfigYamlEnv(["/a/b/c/bogus/filename"])
        assert cie.get("foo") == NO_VALUE

def test_multiple_files(self, tmpdir):
        """Test multiple files--uses first found"""
        yaml_filename = tmpdir / "config.yaml"
        yaml_filename.write(YAML_FLAT)

        yaml_filename_2 = tmpdir / "config2.yaml"
        yaml_filename_2.write(YAML_FLAT_2)

        cie = ConfigYamlEnv([str(yaml_filename), str(yaml_filename_2)])
        # Only the first found file is loaded, so foo_original does not exist
        assert cie.get("foo_original") == NO_VALUE

        cie = ConfigYamlEnv([str(yaml_filename_2)])
        # ... but it is there if only the original is loaded (safety check)
        assert cie.get("foo_original") == "original"

def test_hierarchical(self, tmpdir):
        """Test hierarchical specification works"""
        yaml_filename = tmpdir / "config.yaml"
        yaml_filename.write(YAML_HIERARCHY)

        cie = ConfigYamlEnv([str(yaml_filename)])
        assert cie.get("foo") == "bar"
        assert cie.get("foo", namespace="nsbaz") == "bat"
        assert cie.get("foo", namespace=["nsbaz"]) == "bat"
        assert cie.get("foo", namespace=["nsbaz", "nsbaz2"]) == "bat2"

def test_non_string_values(self, tmpdir):
        yaml_filename = tmpdir / "config.yaml"
        yaml_filename.write(YAML_NON_STRING_VALUES)

        with pytest.raises(ConfigurationError):
            ConfigYamlEnv([str(yaml_filename)])

def test_flat(self, tmpdir):
        """Test flat specification works"""
        yaml_filename = tmpdir / "config.yaml"
        yaml_filename.write(YAML_FLAT)

        cie = ConfigYamlEnv([str(yaml_filename)])
        assert cie.get("foo") == "bar"
        assert cie.get("foo", namespace="nsbaz") == "bat"
        assert cie.get("foo", namespace=["nsbaz"]) == "bat"
        assert cie.get("foo", namespace=["nsbaz", "nsbaz2"]) == "bat2"

def test_flat_caps(self, tmpdir):
        """Test case-insensitive"""
        yaml_filename = tmpdir / "config.yaml"
        yaml_filename.write(YAML_FLAT)

        cie = ConfigYamlEnv([str(yaml_filename)])
        assert cie.get("foo") == "bar"
        assert cie.get("FOO") == "bar"
        assert cie.get("Foo") == "bar"
        assert cie.get("foo", namespace="nsbaz") == "bat"
        assert cie.get("foo", namespace="NsBaz") == "bat"
        assert cie.get("FOO", namespace="NSBAZ") == "bat"