'Create an empty Message.
@raises InvalidOpenIDNamespace: if openid_namespace is not in
L{Message.allowed_openid_namespaces}'

def __init__(self, openid_namespace=None):

    self.args = {}
    self.namespaces = NamespaceMap()
    if (openid_namespace is None):
        self._openid_ns_uri = None
    else:
        implicit = (openid_namespace in OPENID1_NAMESPACES)
        self.setOpenIDNamespace(openid_namespace, implicit)
