<SYSTEM_TASK:>
Solve the following problem using Python, implementing the functions described below, one line at a time
<END_TASK>
<USER_TASK:>
Description:
 def toArgs(self):
 """Return all namespaced arguments, failing if any non-namespaced arguments exist."""

        # FIXME - undocumented exception
        post_args = self.toPostArgs()
        kvargs = {}
        for k, v in post_args.items():
            if not k.startswith('openid.'):
                raise ValueError(
                    'This message can only be encoded as a POST, because it '
                    'contains arguments that are not prefixed with "openid."')
            else:
                kvargs[k[7:]] = v

        return kvargs