def set_alpn_protocols(self, protocols):
        """
        Sets the ALPN protocols that will later be set on the context.

        Raises a NotImplementedError if ALPN is not supported.
        """
        if not hasattr(Security, "SSLSetALPNProtocols"):
            raise NotImplementedError(
                "SecureTransport supports ALPN only in macOS 10.12+"
            )
        self._alpn_protocols = [six.ensure_binary(p) for p in protocols]

        Sets the ALPN protocols that will later be set on the context.

        Raises a NotImplementedError if ALPN is not supported.
        
lib/urllib3/contrib/securetransport.py