Composition and inheritance

Composition stores an object inside another object. Inheritance lets a subclass specialize a base class. Prefer the simpler relationship that accurately describes your model.

Worked example

class Session:
    def __init__(self, minutes):
        self.minutes = minutes
class Plan:
    def __init__(self, sessions):
        self.sessions = sessions
    def total(self):
        return sum(item.minutes for item in self.sessions)
print(Plan([Session(20), Session(25)]).total())

45

Why this works

A plan has sessions, so composition represents that relationship directly. The total method adds their durations.

Another case

class Report:
    def label(self):
        return "report"
class DailyReport(Report):
    def label(self):
        return "daily report"
print(DailyReport().label())

daily report

Python finds DailyReport.label first. If DailyReport did not define label(), the inherited Report.label would run instead.

Practice

Would Plan inheriting from Session represent the same relationship?

Try another case

A Report holds a list of Session objects. Is that composition or inheritance?