[New-bugs-announce] [issue31789] Better error message when failing to overload metaclass.mro

Dan Snider report at bugs.python.org
Sat Oct 14 20:34:04 EDT 2017

New submission from Dan Snider <mr.assume.away at gmail.com>:

class meta(type):
    mros = (object,)
    def __new__(metacls, name, bases, namespace, fake_bases=()):
        print('entering __new__')
        metacls.fake_bases = fake_bases
        cls = type.__new__(metacls, name, bases, namespace)
        print('exiting __new__')
        return cls
    def mro(*args):
        print('entering mro')
        return meta.fake_bases + (object,)

class a(metaclass=meta, fake_bases=()): pass

That puts out the error message:

entering meta.__new__
entering meta.mro
Traceback (most recent call last):
  File "<pyshell#5948>", line 1, in <module>
    exec(code, u)
  File "<string>", line 14, in <module>
  File "<string>", line 6, in __new__
TypeError: super(type, obj): obj must be an instance or subtype of type

That doesn't at all explain why it doesn't work because super was never explicitly called. If the argument fake_bases isn't empty, or if mro isn't made a staticmethod/classmethod, it returns an appropriate message

TypeError: mro() returned base with unsuitable layout ('tuple')

components: Interpreter Core
messages: 304406
nosy: bup
priority: normal
severity: normal
status: open
title: Better error message when failing to overload metaclass.mro
type: enhancement
versions: Python 3.7

Python tracker <report at bugs.python.org>

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