Exercise 3: Broker Chain — Basic Stat System

Goal

Build a basic character stat system using the Broker Chain pattern. This exercise focuses on the three-part core: Query, Broker, and Modifier.

Part A — the three core pieces

Implement:

StatQuery — a mutable object with:

character_name: str
stat: str (e.g. "attack", "defense")
value: int — starts at the base value, modifiers change it

StatBroker — an event bus with:

subscribe(handler) — registers a callable
unsubscribe(handler) — removes a callable
publish(query) — calls every registered handler with the query

Character — holds base stats and a broker reference:

base_attack = 10, base_defense = 5
get_attack() -> int — creates a StatQuery, publishes it, returns query.value
get_defense() -> int — same for defense

Verify the bare character with no modifiers:

broker = StatBroker()
hero = Character("hero", broker)
assert hero.get_attack()  == 10
assert hero.get_defense() == 5

Part B — flat bonus modifier

Add a FlatBonusModifier that adds a fixed integer to one stat of one character:

FlatBonusModifier(broker, character_name="hero", stat="attack", bonus=10)

After registering, hero.get_attack() should return 20.

Multiple FlatBonusModifier instances for the same stat should stack:

FlatBonusModifier(broker, "hero", "attack", 10)
FlatBonusModifier(broker, "hero", "attack",  5)
assert hero.get_attack() == 25

Part C — multiplier modifier and removal

Add a MultiplierModifier that multiplies a stat by a float:

MultiplierModifier(broker, character_name="hero", stat="attack", multiplier=2.0)

Add a remove() method to both modifier classes that unsubscribes them from the broker.

Verify:

sword  = FlatBonusModifier(broker, "hero", "attack", 10)   # attack = 20
berserk = MultiplierModifier(broker, "hero", "attack", 2.0) # attack = 40
assert hero.get_attack() == 40

berserk.remove()
assert hero.get_attack() == 20  # multiplier gone

sword.remove()
assert hero.get_attack() == 10  # back to base

Skeleton

See exercise3.py.

Hints

StatBroker.publish is just a loop: for h in self._handlers: h(query).
FlatBonusModifier subscribes self._handle in __init__. _handle checks query.character_name and query.stat before modifying query.value.
MultiplierModifier._handle should cast the result back to int: query.value = int(query.value * self._multiplier).
Modifier order matters: a flat bonus applied before a multiplier gives a different result than after. The order is determined by subscription order.

Exercise 2 · Solution 3 · Exercise 4