On Wed, Aug 19, 2020 at 11:32:10PM +0200, Wojtek Porczyk wrote:

Hi Philipp, (Cc: Daniel, because IIUC you reviewed !16 which got this merged),

I'm sorry I didn't notice this earlier, but the commit f4be03b3 dated 2020-04-20 [0] is wrong. The super().__init__(*args, **kwargs) in Callback.__init__ was there on purpose, because of how Python's inheritance in new-style classes works.

Let me explain this a bit, because it is not obvious.

Suppose you had diamond inheritance like this:

class A(object): pass class B(A): pass class C(A): pass class D(B,C): pass

And those classes needed a common function with varying arguments:

class A(object): def spam(self, a): print(f'A: {a}') class B(A): def spam(self, b): print(f'B: {b}') class C(A): def spam(self, c): print(f'C: {c}') class D(B,C): def spam(self, d): print(f'D: {d}')

The way to call all parent's functions exactly once (as per MRO) and accept all arguments and also forbid unknown arguments is to accept **kwargs everywhere and pass them to super().spam():

class A: def spam(self, a): print(f'A: {a}') class B(A): def spam(self, b, **kwargs): print(f'B: {b}') super().spam(**kwargs) class C(A): def spam(self, c, **kwargs): print(f'C: {c}') super().spam(**kwargs) class D(B, C): def spam(self, d, **kwargs): print(f'D: {d}') super().spam(**kwargs)

Let's run this:

>>> B().spam(a=1, b=2) B: 2 A: 1 >>> D().spam(a=1, b=2, c=3, d=4) D: 4 B: 2 C: 3 A: 1

You may notice that super() in B.spam refers to two different classes, either A or C, depending on inheritance order in yet undefinded classes (as of B's definition).

That's why the conclusion that super() in Callback.__init__ refers to object is wrong. In this example, spam=__init__, A=object, B=Callback and C and D are not yet written, but theoretically possible classes that could be written by someone else. Why would they be needed, I don't know, but if someone written them, s/he would be out of options to invent new arguments to C.__init__.

Note that super().__init__(*args, **kwargs) when super() refers to object isn't harmful, and just ensures that args and kwargs are empty (i.e. no unknown arguments were passed). In fact, this is exactly why object.__init__() takes no arguments since Python 2.6 [1][2], as you correctly point out in the commit message.

I don't think this breaks anything (I very much doubt anyone would need to write code that would trigger this), nevertheless, as the commit is both pointless and wrong, and as the original author of libvirtaio I'd like to ask for this commit to be reverted. If this breaks some static analysis tool, could you just suppress it for this particular line?

Could you open a merge request providing the revert along with your description of why the change was wrong and I'll review & approve it. Regards, Daniel -- |: https://berrange.com -o- https://www.flickr.com/photos/dberrange :| |: https://libvirt.org -o- https://fstop138.berrange.com :| |: https://entangle-photo.org -o- https://www.instagram.com/dberrange :|