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devel / comp.lang.python / Re: Multiple inheritance and a broken super() chain

Re: Multiple inheritance and a broken super() chain

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From: learn2pr...@gmail.com (Alan Gauld)
Newsgroups: comp.lang.python
Subject: Re: Multiple inheritance and a broken super() chain
Date: Tue, 4 Jul 2023 23:33:51 +0100
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by: Alan Gauld - Tue, 4 Jul 2023 22:33 UTC

On 03/07/2023 19:39, Chris Angelico via Python-list wrote:
> On Tue, 4 Jul 2023 at 03:39, Peter Slížik via Python-list
>> The legacy code I'm working with uses a classic diamond inheritance.

> What happens when Top is initialized twice? This seems like a problem
> waiting to happen, and when you moved to using super(), you more than
> likely simplified things and fixed things.

Slightly off topic but I wonder how many real world problems
people have experienced having the top of a diamond initialized
twice? The reason I ask is that I ran a maintenance team for
about 5 years (early 1990s) working on various OOP projects using MI;
in Lisp Flavors, C++(*) and a homebrew variant of C that supported MI.
In that time I don't recall ever having problems with top objects
being initialized twice (apart from redundant execution of code
of course).

In most cases the top object was so abstract that its init()/constructor
was only doing generic type stuff or opening database sessions/networks
etc which got lost and tidied up by garbage collectors.

So I'm curious about how big this "big problem with MI" is in
practice. I'm sure there are scenarios where it has bitten folks
but it never (or very rarely) occurred in our projects. (Of
course, being maintenance programmers, the problems may have
been ironed out before the code ever reached us! But that
wasn't usually the case...)

(*) C++ is the odd one out because it doesn't have GC, but then
neither does it have an Object superclass so very often MI in C++
does not involve creating diamonds! And especially if the MI
style is mixin based.

--
Alan G
Author of the Learn to Program web site
http://www.alan-g.me.uk/
http://www.amazon.com/author/alan_gauld
Follow my photo-blog on Flickr at:
http://www.flickr.com/photos/alangauldphotos

Re: Multiple inheritance and a broken super() chain

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From: greg.ew...@canterbury.ac.nz (Greg Ewing)
Newsgroups: comp.lang.python
Subject: Re: Multiple inheritance and a broken super() chain
Date: Wed, 5 Jul 2023 12:26:35 +1200
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by: Greg Ewing - Wed, 5 Jul 2023 00:26 UTC

Re: Multiple inheritance and a broken super() chain

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Subject: Re: Multiple inheritance and a broken super() chain
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by: Chris Angelico - Wed, 5 Jul 2023 00:51 UTC

On Wed, 5 Jul 2023 at 10:31, Greg Ewing via Python-list
<python-list@python.org> wrote:
>
> On 5/07/23 10:33 am, Alan Gauld wrote:
> > (*) C++ is the odd one out because it doesn't have GC, but then
> > neither does it have an Object superclass so very often MI in C++
> > does not involve creating diamonds! And especially if the MI
> > style is mixin based.
>
> Even if all your mixins have empty constructors, in C++ there
> is still a diamond problem if they have any data members, because
> you end up with multiple copies of them.
>
> But C++ has the concept of virtual base classes, which avoids the
> diamond problem, albeit at the expense of making you explicitly
> call all the base class constructors in your ancestry.

Yeah, non-virtual MI in C++ is basically composition with a shorthand
for calling non-conflicting methods or accessing non-conflicting data
members. Point of random interest: Pike actually allows that sort of
"composition MI" but will give you back an array of all parents when
you seek a superclass's method, giving a very elegant syntax for MI.

inherit Thing1;
inherit Thing2;

void method() {
::method(); //this is actually calling an array of two methods
}

Python's way of doing it requires that every class choose to cooperate
in the MI and then be aware that they are all operating on the same
object. Pike's and C++'s can sometimes be used as composition in
disguise, but in general, MI does require proper cooperation.

ChrisA

Subject	Author
Re: Multiple inheritance and a broken super() chain	Alan Gauld
Re: Multiple inheritance and a broken super() chain	Greg Ewing
Re: Multiple inheritance and a broken super() chain	Chris Angelico