#!/opt/local/bin/python

# https://twitter.com/mengwong/status/1271434036976603136

from abc import ABC, abstractmethod

class Person:
    def __init__(self, children=[]):
        self.children = children

class TaxCredit(ABC):
    @abstractmethod
    def rate(self, person):
        pass

class TaxCreditB:
    def rate(self, person):
        return 100
    
class TaxCreditA(TaxCreditB):
    def rate(self, person):
        return super().rate(person) / 2 if len(person.children) == 2 else super().rate(person)

alice = Person()
bob   = Person()
carol = Person(children = [alice, bob])
    
taxcB = TaxCreditB()
print ("tax credit B: alice:", taxcB.rate(alice))
print ("tax credit B: bob:",   taxcB.rate(bob))
print ("tax credit B: carol:", taxcB.rate(carol))

taxcA = TaxCreditA()
print ("tax credit A: alice:", taxcA.rate(alice))
print ("tax credit A: bob:",   taxcA.rate(bob))
print ("tax credit A: carol:", taxcA.rate(carol))

# 20200612-21:47:23 mengwong@venice4:~/tmp/python/taxc% python taxc.py
# tax credit B: alice: 100
# tax credit B: bob: 100
# tax credit B: carol: 100
# tax credit A: alice: 100
# tax credit A: bob: 100
# tax credit A: carol: 50.0