The following code causes a SystemError:
class BadMetaclass(type):
    def __prepare__(*args):
        pass

class Foo(metaclass=BadMetaclass):
    pass


This is because builtin___build_class__() assumes that __prepare__() returned a
mapping, and passes it to PyEval_EvalCodeEx(), which passes it to
_PyEval_EvalCodeWithName(), which passes it to _PyFrame_New_NoTrack(), which
raises the SystemError.


This issue seems related to #17421.

I'm not a metaclass expert. Eric, Nick, could you please take a look at the patch?

There's no __prepare__ method in 2.7, so that version can't be affected by this.

For 3.6 and 3.7, I can't reproduce the reported SystemError. Instead, I get:

```
>>> class BadMetaclass(type):
...     def __prepare__(*args):
...         pass
... 
>>> class Foo(metaclass=BadMetaclass):
...     pass
... 
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "<stdin>", line 1, in Foo
TypeError: 'NoneType' object is not subscriptable

```

The error remains the same if I add `@classmethod` to the __prepare__ definition.


That's still thoroughly cryptic and worth changing, but indicates the test should be checking the error message details, not just the error type.

Nick, maybe you tried to reproduce in release?
In debug (where I got the SystemError), you have in the beginning of _PyFrame_New_NoTrack():
#ifdef Py_DEBUG
    if (code == NULL || globals == NULL || !PyDict_Check(globals) ||
        (locals != NULL && !PyMapping_Check(locals))) {
        PyErr_BadInternalCall();
        return NULL;
    }

Aye, that's exactly what I did.

New changeset 5837d0418f47933b2e3c139bdee8a79c248a943c by Nick Coghlan (Oren Milman) in branch 'master':
bpo-31588: Validate return value of __prepare__() methods (GH-3764)
https://github.com/python/cpython/commit/5837d0418f47933b2e3c139bdee8a79c248a943c

CI is still running for the backport, so I'll merge that tomorrow.

Thanks for the patch!

New changeset 084f80b82c564c8a3cef26fc6e56da188a379be2 by Nick Coghlan (Miss Islington (bot)) in branch '3.6':
[3.6] bpo-31588: Validate return value of __prepare__() methods (GH-3790)
https://github.com/python/cpython/commit/084f80b82c564c8a3cef26fc6e56da188a379be2