def __init__(self, app, project):
self.project = project<EOL>self.app = app<EOL>self.sources = set()<EOL>self.smart_sources = []<EOL>self.index = None<EOL>self.source_roots = OrderedSet()<EOL>self._created_symbols = DefaultOrderedDict(OrderedSet)<EOL>self.__package_root = None<EOL>self.__toplevel_comments = OrderedSet()<EOL>self.formatter = self._make_formatter()<EOL>
Constructor for `Extension`.

        This should never get called directly.

        Args:
            project: The `project.Project` instance which documentation
                is being generated.