How is __slots__ implemented in Python?

Question

• How is __slots__ implemented in Python?
• Is this exposed in the C interface?
• How do I get __slots__ behaviour when defining a Python class in C via PyTypeObject?

Mystical

Can't you just assign to the __slots__ attribute from the C interface just as you would in a .py file? – Brosy

@David Heffernan: Only on non-legacy systems ;). – Mystical

Answer 1

When creating Python classes, they by default have a __dict__ and you can set any attribute on them. The point of slots is to not create a __dict__ to save space.

In the C interface it's the other way around, an extension class has by default no __dict__, and you would instead explicitly have to add one and add getattr/setattr support to handle it (although luckily there are methods for this already, PyObject_GenericGetAttr and PyObject_GenericSetAttr, so you don't have to implement them, just use them. (Funnily there is not PyObject_GenericDelAttr, though, I'm not sure what that is about. (I should probably stop nesting parenthesis like this (or not)))).

Slots therefore aren't needed nor make sense for Extension types. By default you just let your getattr/setatttr methods access only those attributes that the class has.

As for how it's implemented, the code is in typeobject.c, and it's pretty much just a question of "If the object has a __slots__ attribute, don't create a __dict__. Quite unexciting. :-)

Placencia

PyObject_GenericDelAttr = PyObject_GenericSetAttr with a NULL value argument. – Sophey

Huh, the C API docs are really slack on documenting the fact that the tp_setattr and tp_setattro methods are supposed to handle deletion as well. We should probably do something about that... – Sophey

For more detail on how slots is implemented, the Python near-equivalent at code.activestate.com/recipes/532903-how-slots-are-implemented/ is worth looking at. – Vaquero