def coerce(cls, string):
        """Create a :class:`Requirement` object from a given package spec."""
        match = re.match(cls.spec_regex, string)
        if not match:
            raise InvalidRequirementSpecError("could not parse requirement")
        package = match.group(1)
        if all(match.group(2, 3)):
            version = "".join(match.group(2, 3))
        else:
            version = None
        return cls(package, version)
def _swdb_environment_remove(self, env_id, pkg_types, groups):
        if not self._base.history.env.get(env_id):
            self._raise_or_warn(self._ignore_installed, _("Environment id '%s' is not installed.") % env_id)
            return

        swdb_env = self._create_swdb_environment(env_id, pkg_types, groups)

        if swdb_env is not None:
            self._base.history.env.remove(swdb_env)