import ast

class FieldsMissingMeta(type):
    def __getattribute__(self, name):
        if armed and name == '_fields':
            # PyObject_GetOptionalAttr() returns 0 now, *fields is NULL.
            # The returned sentinel 0 is not handled.
            raise AttributeError
        return type.__getattribute__(self, name)

class FieldsMissing(ast.Del, metaclass=FieldsMissingMeta):
    pass

armed = False  # don't raise during construction
f = FieldsMissing()
armed = True  # raise in repr()
repr(f)  # problem is in ast_repr_max_depth()