接口与多态

Email today, SMS tomorrow

• The library sends due-date reminders by email. Next term it adds SMS, and later a message queue for another app.
• If the reminder code names each class, it must change every time. If it names only the behaviour "send", it never changes.

Name the behaviour

• An interface 接口 names what a thing does (send), not which class it is.
• Polymorphism 多态 is one call site running many implementations: n.send(text) works for every kind of sender.

Duck typing and Protocol

• Python is dynamically typed: any object with the right methods fits. This is duck typing 鸭子类型.
• A Protocol writes the contract down without forcing inheritance, so tools can check it.

class Notifier(Protocol):
    def send(self, text: str) -> str: ...

Dispatch follows the object

• When n.send('loan due') runs, Python looks up send on the runtime type 运行时类型: the class of the object, not the variable's annotation.
• This is dynamic dispatch 动态绑定: the call is written once, and the method is chosen for each object at call time.

Open to new types

• Adding a fourth sender class touches no existing line: the caller was never told the concrete classes.
• This is the open/closed 开闭 idea: open to new types, closed to editing.

Substitution is a promise

• Substitution 替换 (the Liskov rule): every implementation must accept at least what the interface promises and keep every guarantee. Preconditions may only widen; guarantees may only grow.
• A SilentNotify.send that raises on an empty string narrows the contract. It breaks a caller who was told that send never raises.

Replace isinstance chains

• A chain like if isinstance(n, EmailNotify): ... elif isinstance(n, SmsNotify): ... must be edited for every new type.
• Give each class its own method instead and let dispatch choose. That is what polymorphism is for.

Worked example: one loop, three senders

Three common slips

Recap