Issue
I can't figure out the correct way to model this problem. Here I give you a minimalistic version of my code:
# -*- coding: utf-8 -*-
from abc import ABCMeta, abstractmethod
class AreaCalculator():
__metaclass__ = ABCMeta
def __init__(self):
pass
@abstractmethod
def getArea(self):
pass
def compute(self):
self.getArea()
class PerimeterCalculator():
__metaclass__ = ABCMeta
def __init__(self):
pass
@abstractmethod
def getPerimeter(self):
pass
def compute(self):
self.getPerimeter()
class TriangleAreaCalculator(AreaCalculator):
def __init__(self):
AreaCalculator.__init__(self)
def getArea(self):
return area
class TrianglePerimeterCalculator(PerimeterCalculator):
def __init__(self):
PerimeterCalculator.__init__(self)
def getPerimeter(self):
return perimeter
a = TriangleAreaCalculator()
b = TrianglePerimeterCalculator()
Is there an elegant way to merge "TrianglePerimeterCalculator" and "TriangleAreaCalculator" classes into one, but keeping "PerimeterCalculator" and "AreaCalculator" separated?
[edit] As Kyle suggested in the comments, I can create a new class (let's call it "Triangle") that inherits from "PerimeterCalculator" and "AreaCalculator" at the same time, but what I want is to be able to tell a new instance of "Triangle" to behave as "PerimeterCalculator" or "AreaCalculator", but not both at the same time.
Solution
Here's another answer, following the editing and clarification of your question. It allows creation of a single Triangle
instance that can behave like either an AreaCalculator
or PerimeterCalculator
, as needed.
This programming pattern is called "delegation" and is used where the responsibility for implementing a particular operation is handed off to a different object—in this case an internally held instance of some other class. A common way to do this in Python is by overriding the class's default __getattr__()
method.
Since you've never responded to the comment under my other answer about exactly what it is that controls which behavior is used, I added a set_behavior()
method to allow it to be specified explicitly.
from abc import ABCMeta, abstractmethod
class AreaCalculator:
__metaclass__ = ABCMeta
def __init__(self):
pass
@abstractmethod
def getArea(self):
pass
def compute(self):
return self.getArea()
class PerimeterCalculator:
__metaclass__ = ABCMeta
def __init__(self):
pass
@abstractmethod
def getPerimeter(self):
pass
def compute(self):
return self.getPerimeter()
class TriangleAreaCalculator(AreaCalculator):
def __init__(self):
super(TriangleAreaCalculator, self).__init__()
def getArea(self):
print('TriangleAreaCalculator.getArea() called')
area = 13
return area
class TrianglePerimeterCalculator(PerimeterCalculator):
def __init__(self):
super(TrianglePerimeterCalculator, self).__init__()
def getPerimeter(self):
print('TrianglePerimeterCalculator.getPerimeter() called')
perimeter = 42
return perimeter
class Triangle:
def __init__(self):
delegate_classes = TriangleAreaCalculator, TrianglePerimeterCalculator
# Map delegate classes to instances of themselves.
self._delegates = {delegate_class: delegate_class()
for delegate_class in delegate_classes}
self.set_behavior(TriangleAreaCalculator) # Set default delegate.
def __getattr__(self, attrname):
# Called only for attributes not defined by this class (or its bases).
# Retrieve attribute from current behavior delegate class instance.
return getattr(self._behavior, attrname)
def set_behavior(self, delegate_class):
try:
self._behavior = self._delegates[delegate_class]
except KeyError:
raise TypeError("{} isn't a valid {} behavior delegate class"
.format(delegate_class, self.__class__.__name__))
if __name__ == '__main__':
triangle = Triangle()
# Uses instance's default behavior.
print('triangle.compute() -> {}'.format(triangle.compute()))
triangle.set_behavior(TrianglePerimeterCalculator) # Change behavior.
print('triangle.compute() -> {}'.format(triangle.compute()))
Output:
TriangleAreaCalculator.getArea() called
triangle.compute() -> 13
TrianglePerimeterCalculator.getPerimeter() called
triangle.compute() -> 42
Answered By - martineau
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