Class composition allows explicit relations between objects. In this example, people live in cities that belong to countries. Composition allows people to access the number of all people living in their country:
class Country(object):
def __init__(self):
self.cities=[]
def addCity(self,city):
self.cities.append(city)
class City(object):
def __init__(self, numPeople):
self.people = []
self.numPeople = numPeople
def addPerson(self, person):
self.people.append(person)
def join_country(self,country):
self.country = country
country.addCity(self)
for i in range(self.numPeople):
person(i).join_city(self)
class Person(object):
def __init__(self, ID):
self.ID=ID
def join_city(self, city):
self.city = city
city.addPerson(self)
def people_in_my_country(self):
x= sum([len(c.people) for c in self.city.country.cities])
return x
US=Country()
NYC=City(10).join_country(US)
SF=City(5).join_country(US)
print(US.cities[0].people[0].people_in_my_country())
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