Practice: Totalling Headcount in an Organisation Chart

Departments hold teams and people; count everyone under one department.

An organisation chart is a natural Composite, and this exercise asks you to build one and compute through it. Engineering has a head, Hal, and two teams. Platform has Eli, Fay and Gus. Product has Chen and Dee, and a Design group with Ana and Ben.

Name the leaf and the composite.
Write Person and Group with a shared headcount() method.
What does headcount() return for Product, and for Engineering?
Where would a rule such as "count contractors as half" belong?

Show a solution

from abc import ABC, abstractmethod


class Unit(ABC):
    @abstractmethod
    def headcount(self) -> int: ...


class Person(Unit):
    def __init__(self, name: str) -> None:
        self.name = name

    def headcount(self) -> int:
        return 1


class Group(Unit):
    def __init__(self, name: str, members: list[Unit]) -> None:
        self.name = name
        self._members = members

    def headcount(self) -> int:
        return sum(member.headcount() for member in self._members)


design = Group("Design", [Person("Ana"), Person("Ben")])
product = Group("Product", [Person("Chen"), Person("Dee"), design])
platform = Group("Platform", [Person("Eli"), Person("Fay"), Person("Gus")])
engineering = Group("Engineering", [Person("Hal"), platform, product])

for group in (design, product, platform, engineering):
    print(f"{group.name}: {group.headcount()}")

OutputRan with python:3.12-slim (Python 3.12.13)

Design: 2
Product: 4
Platform: 3
Engineering: 8

A person counts as one, and a group sums its members, whatever they are.

Person is the leaf and Group is the composite. Product returns 4: Chen, Dee, and the two people in Design. Engineering returns 8: Hal, the three in Platform and the four in Product. A common wrong answer for Engineering is 3, the count of its direct members, which is what a loop over _members without recursion would give.

The contractor rule belongs on the leaf: a Contractor leaf returning 0.5, or a field on Person. Group stays unchanged, because it sums whatever its members return.

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