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 on_backend_added(self, sender, backend_id):
        """
        Signal callback executed when a new backend is loaded

        @param sender: not used, only here to let this function be used as a
                       callback
        @param backend_id: the id of the backend to add
        """
        # Add
        backend = self.req.get_backend(backend_id)
        if not backend:
            return
        self.add_backend(backend)
        self.refresh()
        # Select
        self.select_backend(backend_id)
        # Update it's enabled state
        self.on_backend_state_changed(None, backend.get_id())