Class ForwardRef

Lib/annotationlib.py:53–324

Wrapper that holds a forward reference. Constructor arguments: * arg: a string representing the code to be evaluated. * module: the module where the forward reference was created. Must be a string, not a module object. * owner: The owning object (module, class, or function).

class ForwardRef:
    """Wrapper that holds a forward reference.

    Constructor arguments:
    * arg: a string representing the code to be evaluated.
    * module: the module where the forward reference was created.
      Must be a string, not a module object.
    * owner: The owning object (module, class, or function).
    * is_argument: Does nothing, retained for compatibility.
    * is_class: True if the forward reference was created in class scope.

    """

    __slots__ = _SLOTS

    def __init__(
        self,
        arg,
        *,
        module=None,
        owner=None,
        is_argument=True,
        is_class=False,
    ):
        if not isinstance(arg, str):
            raise TypeError(f"Forward reference must be a string -- got {arg!r}")

        self.__arg__ = arg
        self.__forward_is_argument__ = is_argument
        self.__forward_is_class__ = is_class
        self.__forward_module__ = module
        self.__owner__ = owner
        # These are always set to None here but may be non-None if a ForwardRef
        # is created through __class__ assignment on a _Stringifier object.
        self.__globals__ = None
        # This may be either a cell object (for a ForwardRef referring to a single name)
        # or a dict mapping cell names to cell objects (for a ForwardRef containing references
        # to multiple names).
        self.__cell__ = None
        self.__extra_names__ = None
        # These are initially None but serve as a cache and may be set to a non-None
        # value later.
        self.__code__ = None
        self.__ast_node__ = None

    def __init_subclass__(cls, /, *args, **kwds):
        raise TypeError("Cannot subclass ForwardRef")

    def evaluate(
        self,
        *,
        globals=None,
        locals=None,
        type_params=None,
        owner=None,
        format=Format.VALUE,
    ):
        """Evaluate the forward reference and return the value.