def load(self, requirements_file=None):
        """Load or reload requirements from a requirements.txt file.

        :param requirements_file: if not given, the filename used from
            initialization will be read again.
        """
        if requirements_file is None:
            requirements_file = self.filename
            if requirements_file is None:
                raise ValueError("no filename provided")
        elif isinstance(requirements_file, text_type):
            requirements_file = Path(requirements_file)
        self.clear()
        with requirements_file.open() as f:
            self.loads(f.read())

        if isinstance(requirements_file, (text_type, Path)):
            self.filename = requirements_file
def add_backend(self, backend):
        """
        Adds a new backend to the list

        @param backend_id: the id of the backend to add
        """
        if backend:
            backend_iter = self.liststore.append([
                backend.get_id(),
                self.dialog.get_pixbuf_from_icon_name(backend.get_icon(),
                                                      16),
                backend.get_human_name(),
                self._get_markup_for_tags(backend.get_attached_tags()),
            ])
            self.backendid_to_iter[backend.get_id()] = backend_iter