Python collections 模块，html() 实例源码

def _load(self, body, resource, nested_in=None):
        """Load the composite field.

        :param body: parent JSON body.
        :param resource: parent resource.
        :param nested_in: parent resource name (for error reporting only).
        :returns: a new object with sub-fields attached to it.
        """
        nested_in = (nested_in or []) + self._path
        value = super(CompositeField, self)._load(body, resource)
        if value is None:
            return None

        # We need a new instance, as this method is called a singleton instance
        # that is attached to a class (not instance) of a resource or another
        # CompositeField. We don't want to end up modifying this instance.
        instance = copy.copy(self)
        for attr, field in self._subfields.items():
            # Hide the Field object behind the real value
            setattr(instance, attr, field._load(value, resource, nested_in))

        return instance

    # Satisfy the mapping interface, see
    # https://docs.python.org/2/library/collections.html#collections.Mapping.