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People occasionally ask on python-list about the following error message when trying to create a class:
> TypeError: Error when calling the metaclass bases
> module.__init__() takes at most 2 arguments (3 given)
It is a bit cryptic. It is also accidental: if module.__init__ happened to take 3 args instead of 2, the exception would be delayed until ???
The cause is using a module as a superclass, as in
import UserDict
class MyDict(UserDict): pass
This seems to happen mostly because of duplicate (or only case-different) module/class names.
 
Suggestion: insert a specific module type check for bases in type_new() in typeobject.c. In Python:
 if basetype is module: # where 'module' is the module class
 raise TypeError('Cannot subclass module')
I see 2 possible places to put the check:
1. for all bases - after
 tmp = PyTuple_GET_ITEM(bases, i);
 tmptype = Py_TYPE(tmp);
Advantage: can add (name of) tmp to the error message.
2. Before the return call in
 if (winner != metatype) {
 if (winner->tp_new != type_new) /* Pass it to the winner */
 return winner->tp_new(winner, args, kwds);
Advantage: only add extra check when module is the winner and are about to call it as a metaclass, which raises the current error.
Any test would be CPython specific and would require checking something about the message text.
I am only suggesting a check for module because the is the only mistake I remember anyone reporting. Passing a number as a base class gives a similar message, but no one does that. And as far as I know, there is no way in general to detect whether a callable works as a metaclass except by calling it with name, bases, and dict.

> I am only suggesting a check for module because the is the only mistake > I remember anyone reporting. Passing a number as a base class gives a > > similar message, but no one does that. And as far as I know, there is > no way in general to detect whether a callable works as a metaclass > except by calling it with name, bases, and dict.
Since 3.x all the module names became lower-case and can be easily differ from class names, so I don't think this problem will present much in 3.x, right?

> raise TypeError('Cannot subclass module')
It is possible to subclass module.
Ray: right, but 2.x still has a long life ahead of it, so we still want to improve its doc.

Whoops. That should be 'Cannot subclass a module' or 'Cannot subclass module %s' % modulename. Types.ModuleType can indeed by subclassed.
I agree that this is not a pressing issue, but it will at least provide an answer to anyone searching the tracker for the current message.

I don’t know if this should be left to lint tools or addressed by CPython itself. Raymond, do you have an opinion?

Special cases aren't special enough to break the rules.
I think this issue should be closed. This is not common user error, especially in Python 3. I can even imagine that you can subclass module, if it is an instance of special ModuleType subclass. It is a feature of Python that you can subclass any objects, not only classes, if they provide the particular interface.

I cannot anymore remember seeing this. So closing.