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 _regex(kind, pat, globsuffix):
    """Convert a (normalized) pattern of any kind into a regular expression.
    globsuffix is appended to the regexp of globs."""
    if not pat and kind in ("glob", "relpath"):
        return ""
    if kind == "re":
        return pat
    if kind in ("path", "relpath"):
        if pat == ".":
            return ""
        return util.re.escape(pat) + "(?:/|$)"
    if kind == "rootfilesin":
        if pat == ".":
            escaped = ""
        else:
            # Pattern is a directory name.
            escaped = util.re.escape(pat) + "/"
        # Anything after the pattern must be a non-directory.
        return escaped + "[^/]+$"
    if kind == "relglob":
        return "(?:|.*/)" + _globre(pat) + globsuffix
    if kind == "relre":
        if pat.startswith("^"):
            return pat
        return ".*" + pat
    return _globre(pat) + globsuffix