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 _files(self):
        if self.isexact():
            return [f for f in self._m1.files() if self(f)]
        # If m1 is not an exact matcher, we can't easily figure out the set of
        # files, because its files() are not always files. For example, if
        # m1 is "path:dir" and m2 is "rootfileins:.", we don't
        # want to remove "dir" from the set even though it would match m2,
        # because the "dir" in m1 may not be a file.
        return self._m1.files()