Exercise 4: Broker Chain — Scoped and Conditional Modifiers

Context

We continue with the stat system from Exercise 3. The base code — StatQuery, StatBroker, Character, FlatBonusModifier, MultiplierModifier — is already in place.

This exercise adds two real-world requirements: modifiers that clean themselves up automatically, and modifiers that react to live character state.

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Part A — ScopedModifier (context manager)

Make modifiers work as context managers so temporary effects are guaranteed to clean up, even if an exception is raised.

Implement a ScopedModifier base class (or mixin) that adds __enter__ and __exit__:

with FlatBonusModifier(broker, "hero", "attack", 20) as _:
    print(hero.get_attack())   # 30 (base 10 + bonus 20)

print(hero.get_attack())       # 10 — modifier removed on exit

Both FlatBonusModifier and MultiplierModifier should support the context manager protocol.

Verify that the modifier is removed even when the with block raises an exception:

try:
    with FlatBonusModifier(broker, "hero", "attack", 20):
        raise RuntimeError("something went wrong")
except RuntimeError:
    pass

assert hero.get_attack() == 10  # still cleaned up

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Part B — ConditionalModifier

Add a ConditionalModifier that only applies its effect when a runtime condition is true.

ConditionalModifier(
    broker,
    character_name="hero",
    stat="defense",
    bonus=15,
    condition=lambda: hero.health < hero.max_health * 0.25,
)

This modifier should add +15 defense, but only when the condition returns True at the moment the query fires — not at registration time.

Verify:

hero.health = 100
assert hero.get_defense() == 5    # condition false, no bonus

hero.health = 20                  # below 25% of 100
assert hero.get_defense() == 20   # condition true, bonus applied

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Part C — putting it all together

Wire a scenario with two characters sharing one broker. Use all four modifier types:

• hero has a sword (FlatBonusModifier, permanent)
• hero has a Last Stand effect (ConditionalModifier, activates at low health)
• villain gets a temporary berserk buff (MultiplierModifier, scoped)
• villain has armor (FlatBonusModifier, permanent)

Walk through a sequence of game events and assert the stats at each step:

1. Start — verify base stats plus permanent modifiers.
2. Villain’s berserk activates (enter with block) — verify villain’s boosted attack.
3. Hero’s health drops below 25% — verify hero’s Last Stand defense.
4. Villain’s berserk expires (exit with block) — verify villain’s attack is back to normal.
5. Hero’s health is restored — verify Last Stand is inactive again.

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Skeleton

See exercise4.py.

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Hints

• ScopedModifier.__enter__ should return self. __exit__ should call self.remove() and return False (don’t suppress exceptions).
• ConditionalModifier._handle should call self._condition() at handle time, not at registration time. The lambda captures a reference to hero, so it always reads the current value of hero.health.
• For Part C, the with block for the villain’s berserk should contain all the assertions that depend on the buff being active.

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Exercise 3 · Solution 4