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 test_reverse_rfc822_datetime():
    dates = [
        ("Sat, 01 Jan 2011 00:00:00 -0000", datetime(2011, 1, 1, tzinfo=pytz.utc)),
        ("Sat, 01 Jan 2011 23:59:59 -0000", datetime(2011, 1, 1, 23, 59, 59, tzinfo=pytz.utc)),
        ("Sat, 01 Jan 2011 21:59:59 -0200", datetime(2011, 1, 1, 23, 59, 59, tzinfo=pytz.utc)),
    ]

    for date_string, expected in dates:
        yield assert_equal, inputs.datetime_from_rfc822(date_string), expected