type_api.py 文件源码

def __init__(self, *args, **kwargs):
        """Construct a :class:`.TypeDecorator`.

        Arguments sent here are passed to the constructor
        of the class assigned to the ``impl`` class level attribute,
        assuming the ``impl`` is a callable, and the resulting
        object is assigned to the ``self.impl`` instance attribute
        (thus overriding the class attribute of the same name).

        If the class level ``impl`` is not a callable (the unusual case),
        it will be assigned to the same instance attribute 'as-is',
        ignoring those arguments passed to the constructor.

        Subclasses can override this to customize the generation
        of ``self.impl`` entirely.

        """

        if not hasattr(self.__class__, 'impl'):
            raise AssertionError("TypeDecorator implementations "
                                 "require a class-level variable "
                                 "'impl' which refers to the class of "
                                 "type being decorated")
        self.impl = to_instance(self.__class__.impl, *args, **kwargs)