def __init__(self, package, version):
        """Construct a new requirement.

        :param package: the package name.
        :param version: a semver compatible version specification.
        """
        self.package = package
        self.version = version
        if self.version and not re.match(r"[<=>~]", version[:2]):
            self.version = "=={}".format(self.version)
def switch(self, dest, url, rev_options):
        # type: (str, HiddenText, RevOptions) -> None
        self.run_command(
            make_command('config', 'remote.origin.url', url),
            cwd=dest,
        )
        cmd_args = make_command('checkout', '-q', rev_options.to_args())
        self.run_command(cmd_args, cwd=dest)

        self.update_submodules(dest)