Don't generate trivial __new__

[serhiy-storchaka]

Cython generates the __new__ method even for classes without data. Recent change in CPython broke multiple inheritance from Cython classes. That change was reverted for backward compatibility, but if Cython would not generate trivial __new__ for empty classes, it would help to push that change back.

[scoder]

@serhiy-storchaka, I'm not sure what to make of this. Cython does not generate a __new__ Python method directly. It only generates a tp_new slot function. CPython then generates the __new__ method wrapper for it.

If Cython left tp_new empty, then the type couldn't be instantiated. So a tp_new, even an empty one that calls the base type's tp_new, is always needed.

We could try to inherit the tp_new function of the base type instead of creating an empty one that calls it, but I don't know if that changes anything. Is that what you were referring to?

[scoder]

Also, for a type without base types, we could then use PyType_GenericNew for tp_new, but that doesn't seem to be special-cased anywhere. There doesn't seem to be a benefit to that, so is this only about a tp_new that calls the base class tp_new, i.e. subtypes that do not add anything to tp_new ?

[scoder]

#2900 and #2909 should resolve this.