Multiple inheritance and a broken super() chain

[Richard Damon]

On 7/3/23 1:38 PM, Peter Slížik via Python-list wrote:
> Hello.
>
> The legacy code I'm working with uses a classic diamond inheritance. Let me
> call the classes *Top*, *Left*, *Right*, and *Bottom*.
> This is a trivial textbook example. The classes were written in the
> pre-super() era, so all of them initialized their parents and Bottom
> initialized both Left and Right in this order.
>
> The result was expected: *Top* was initialized twice:
>
> Top.__init__() Left.__init__() Top.__init__() Right.__init__()
> Bottom.__init__()
>
> Now I replaced all parent init calls with *super()*. After this, Top was
> initialized only once.
>
> Top.__init__() Right.__init__() Left.__init__() Bottom.__init__()
>
> But at this point, I freaked out. The code is complex and I don't have the
> time to examine its inner workings. And before, everything worked correctly
> even though Top was initialized twice. So I decided to break the superclass
> chain and use super() only in classes inheriting from a single parent. My
> intent was to keep the original behavior but use super() where possible to
> make the code more readable.
>
> class Top:
> def __init__(self):
> print("Top.__init__()")
>
> class Left(Top):
> def __init__(self):
> super().__init__()
> print("Left.__init__()")
>
> class Right(Top):
> def __init__(self):
> super().__init__()
> print("Right.__init__()")
>
> class Bottom(Left, Right):
> def __init__(self):
> Left.__init__(self) # Here I'm calling both parents manually
> Right.__init__(self)
> print("Bottom.__init__()")
>
> b = Bottom()
>
>
> The result has surprised me:
>
> Top.__init__() Right.__init__() Left.__init__() Top.__init__()
> Right.__init__() Bottom.__init__()
>
> Now, as I see it, from the super()'s point of view, there are two
> inheritance chains, one starting at Left and the other at Right. But
> *Right.__init__()* is called twice. What's going on here?
>
> Thanks,
> Peter

Because the MRO from Bottom is [Bottom, Left, Right, Top] so super() in 
Left is Right. It doesn't go to Top as the MRO knows that Right should 
go to Top, so Left needs to go to Right to init everything, and then 
Bottom messes things up by calling Right again.

-- 
Richard Damon