[issue27157] Unhelpful error message when one calls a subclass of type with a custom metaclass

[Emanuel Barry]

Emanuel Barry added the comment:

In Objects/typeobject.c#L2290, the code checks that the (meta?)class is exactly `type` and that there's one argument, then returns `Py_TYPE(x)`. Subclasses of `type` allowing a single argument is a side-effect of not overriding __new__, not a documented feature.

Changing the call from `PyType_CheckExact` to `PyType_Check` makes it work, but I'm assuming there may be something I didn't think of. Or maybe there isn't, but either way, I don't consider that this is worth fixing -- if you want to call your subclass with only one argument, override __new__ and do the logic in there. And if you want the type of an object, use type directly.

Also, there may be performance concerns here. `type` is heavily optimized in many places; I thought that `PyType_Check` called Python code, but after checking the macro definitions and testing a bit, it turns out I'm wrong. But if there *is* a negative performance impact, this probably can't go in -- this check runs everytime that type() is called, no matter how many arguments, and including in class creation; that's also probably why `PyType_CheckExact` was chosen to begin with.

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<http://bugs.python.org/issue27157>
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