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

[Richard Damon]

On 3/26/23 1:43 PM, Jen Kris via Python-list wrote:
> The base class:
>
>
> class Constraint(object):
>
> def __init__(self, strength):
>          super(Constraint, self).__init__()
>          self.strength = strength
>
> def satisfy(self, mark):
>          global planner
>          self.choose_method(mark)
>
> The subclass:
>
> class UrnaryConstraint(Constraint):
>
> def __init__(self, v, strength):
>          super(UrnaryConstraint, self).__init__(strength)
>          self.my_output = v
>          self.satisfied = False
>          self.add_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, 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?
>
> 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:
>
> def choose_method(self, mark):
>      if self.v1.mark == mark:
>              if self.v2.mark != mark and Strength.stronger(self.strength, self.v2.walk_strength):
>                  self.direction = Direction.FORWARD
>              else:
>                  self.direction = Direction.BACKWARD
>
> When called from Constraint, it uses the one at UrnaryConstraint.  How does it know which one to use?
>
> Thanks,
>
> Jen

Perhaps the key point to remember is that when looking up the methods on 
an object, those methods are part of the object as a whole, not 
particually "attached" to a given class. When creating the subclass 
typed object, first the most base class part is built, and all the 
methods of that class are put into the object, then the next level, and 
so on, and if a duplicate method is found, it just overwrites the 
connection. Then when the object is used, we see if there is a method by 
that name to use, so methods in the base can find methods in subclasses 
to use.

Perhaps a more modern approach would be to use the concept of an 
"abstract base" which allows the base to indicate that a derived class 
needs to define certain abstract methods, (If you need that sort of 
support, not defining a method might just mean the subclass doesn't 
support some optional behavior defined by the base)

-- 
Richard Damon