Message131381
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. |
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Date |
User |
Action |
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2011-03-19 02:13:57 | terry.reedy | set | recipients:
+ terry.reedy |
2011-03-19 02:13:57 | terry.reedy | set | messageid: <1300500837.93.0.136999770094.issue11604@psf.upfronthosting.co.za> |
2011-03-19 02:13:57 | terry.reedy | link | issue11604 messages |
2011-03-19 02:13:56 | terry.reedy | create | |
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