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devel / comp.lang.python / Re: How does a method of a subclass become a method of the base class?

Re: How does a method of a subclass become a method of the base class?

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From: hjp-pyt...@hjp.at (Peter J. Holzer)
Newsgroups: comp.lang.python
Subject: Re: How does a method of a subclass become a method of the base class?
Date: Sun, 26 Mar 2023 20:48:25 +0200
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by: Peter J. Holzer - Sun, 26 Mar 2023 18:48 UTC

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On 2023-03-26 19:43:44 +0200, Jen Kris via Python-list wrote:
> The base class:
>
>
> class Constraint(object):
[...]
> def satisfy(self, mark):
>         global planner
>         self.choose_method(mark)
>
> The subclass:
>
> class UrnaryConstraint(Constraint):
[...]
>     def choose_method(self, mark):
>         if self.my_output.mark != mark and \
>            Strength.stronger(self.strength, self.my_output.walk_strength):
> self.satisfied = True
>         else:
>             self.satisfied = False
>
> The base class Constraint doesn’t have a "choose_method" class method,
> but it’s called as self.choose_method(mark) on the final line of
> Constraint shown above.
>
> My question is: what makes "choose_method" a method of the base
> class,

Nothing. choose_method isn't a method of the base class.

> called as self.choose_method instead of
> UrnaryConstraint.choose_method? Is it super(UrnaryConstraint,
> self).__init__(strength) or just the fact that Constraint is its base
> class?

This works only if satisfy() is called on a subclass of Constraint which
actually implements this method.

If you do something like

x = UrnaryConstraint()
x.satisfy(whatever)

Then x is a member of class UrnaryConstraint and will have a
choose_method() method which can be called.

> Also, this program also has a class BinaryConstraint that is also a
> subclass of Constraint and it also has a choose_method class method
> that is similar but not identical:
...
> When called from Constraint, it uses the one at UrnaryConstraint. How
> does it know which one to use?

By inspecting self. If you call x.satisfy() on an object of class
UrnaryConstraint, then self.choose_method will be the choose_method from
UrnaryConstraint. If you call it on an object of class BinaryConstraint,
then self.choose_method will be the choose_method from BinaryConstraint.

PS: Pretty sure there's one "r" too many in UrnaryConstraint.

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Subject	Author
Re: How does a method of a subclass become a method of the base class?	Peter J. Holzer