How to use the databases.importer.import_from_string function in databases

def test_invalid_format():
    with pytest.raises(ImportFromStringError) as exc_info:
        import_from_string("example:")
    expected = 'Import string "example:" must be in format ":".'
    assert expected in str(exc_info.value)

def test_invalid_module():
    with pytest.raises(ImportFromStringError) as exc_info:
        import_from_string("module_does_not_exist:myattr")
    expected = 'Could not import module "module_does_not_exist".'
    assert expected in str(exc_info.value)

def test_invalid_attr():
    with pytest.raises(ImportFromStringError) as exc_info:
        import_from_string("tempfile:attr_does_not_exist")
    expected = 'Attribute "attr_does_not_exist" not found in module "tempfile".'
    assert expected in str(exc_info.value)

def test_internal_import_error():
    with pytest.raises(ImportError):
        import_from_string("tests.importer.raise_import_error:myattr")

def test_valid_import():
    instance = import_from_string("tempfile:TemporaryFile")
    from tempfile import TemporaryFile

    assert instance == TemporaryFile

def __init__(
        self, url: typing.Union[str, DatabaseURL], force_rollback: bool = False
    ):
        self.url = DatabaseURL(url)
        self.force_rollback = force_rollback
        backend_str = self.backends[self.url.dialect]
        backend_cls = import_from_string(backend_str)
        assert issubclass(backend_cls, DatabaseBackend)
        self.backend = backend_cls(self.url)
        self.is_connected = False
        self.rollback_transaction = None
        self.session_context = ContextVar("session_context")  # type: ContextVar

def __init__(
        self,
        url: typing.Union[str, "DatabaseURL"],
        *,
        force_rollback: bool = False,
        **options: typing.Any,
    ):
        self.url = DatabaseURL(url)
        self.options = options
        self.is_connected = False

        self._force_rollback = force_rollback

        backend_str = self.SUPPORTED_BACKENDS[self.url.dialect]
        backend_cls = import_from_string(backend_str)
        assert issubclass(backend_cls, DatabaseBackend)
        self._backend = backend_cls(self.url, **self.options)

        # Connections are stored as task-local state.
        self._connection_context = ContextVar("connection_context")  # type: ContextVar

        # When `force_rollback=True` is used, we use a single global
        # connection, within a transaction that always rolls back.
        self._global_connection: typing.Optional[Connection] = None
        self._global_transaction: typing.Optional[Transaction] = None