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

[Peter J. Holzer]

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.

        hp

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

-- 
   _  | Peter J. Holzer    | Story must make more sense than reality.
|_|_) |                    |
| |   | hjp at hjp.at         |    -- Charles Stross, "Creative writing
__/   | http://www.hjp.at/ |       challenge!"
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