
class ObjectFactory(object):

    def __init__(self, created_by_ref=None, created=None,
                 external_references=None, object_marking_refs=None,
                 list_append=True):

        self._defaults = {}
        if created_by_ref:
            self._defaults['created_by_ref'] = created_by_ref
        if created:
            self._defaults['created'] = created
            # If the user provides a default "created" time, we also want to use
            # that as the modified time.
            self._defaults['modified'] = created
        if external_references:
            self._defaults['external_references'] = external_references
        if object_marking_refs:
            self._defaults['object_marking_refs'] = object_marking_refs
        self._list_append = list_append
        self._list_properties = ['external_references', 'object_marking_refs']

    def create(self, cls, **kwargs):
        # Use self.defaults as the base, but update with any explicit args
        # provided by the user.
        properties = dict(**self._defaults)
        if kwargs:
            if self._list_append:
                # Append provided items to list properties instead of replacing them
                for list_prop in set(self._list_properties).intersection(kwargs.keys(), properties.keys()):
                    kwarg_prop = kwargs.pop(list_prop)
                    if kwarg_prop is None:
                        del properties[list_prop]
                        continue
                    if not isinstance(properties[list_prop], list):
                        properties[list_prop] = [properties[list_prop]]
                    properties[list_prop].append(kwarg_prop)

            properties.update(**kwargs)

        return cls(**properties)


class ObjectFactory(object):
    """Object Factory

    Used to easily create STIX objects with default values for certain
    properties.

    Args:
        created_by_ref: Default created_by_ref value to apply to all
            objects created by this factory.
        created: Default created value to apply to all
            objects created by this factory.
        external_references: Default `external_references` value to apply
            to all objects created by this factory.
        object_marking_refs: Default `object_marking_refs` value to apply
            to all objects created by this factory.
        list_append: When a default is set for a list property like
            `external_references` or `object_marking_refs` and a value for
            that property is passed into `create()`, if this is set to True,
            that value will be added to the list alongside the default. If
            this is set to False, the passed in value will replace the
            default. Defaults to True.
    """

    def __init__(self, created_by_ref=None, created=None,
                 external_references=None, object_marking_refs=None,
                 list_append=True):

        self._defaults = {}
        if created_by_ref:
            self._defaults['created_by_ref'] = created_by_ref
        if created:
            self._defaults['created'] = created
            # If the user provides a default "created" time, we also want to use
            # that as the modified time.
            self._defaults['modified'] = created
        if external_references:
            self._defaults['external_references'] = external_references
        if object_marking_refs:
            self._defaults['object_marking_refs'] = object_marking_refs
        self._list_append = list_append
        self._list_properties = ['external_references', 'object_marking_refs']

    def create(self, cls, **kwargs):
        # Use self.defaults as the base, but update with any explicit args
        # provided by the user.
        properties = dict(**self._defaults)
        if kwargs:
            if self._list_append:
                # Append provided items to list properties instead of replacing them
                for list_prop in set(self._list_properties).intersection(kwargs.keys(), properties.keys()):
                    kwarg_prop = kwargs.pop(list_prop)
                    if kwarg_prop is None:
                        del properties[list_prop]
                        continue
                    if not isinstance(properties[list_prop], list):
                        properties[list_prop] = [properties[list_prop]]
                    properties[list_prop].append(kwarg_prop)

            properties.update(**kwargs)

        return cls(**properties)

Add object factory docstring