Add Define Typed Class Interface With Protocol as a Python TIL

python/define-typed-class-interface-with-protocol.md

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+# Define Typed Class Interface With Protocol
+
+In [`py-vmt`](https://github.com/jbranchaud/py-vmt) I am defining different
+storage access layers for the CLI to use. I want a consistent interface that the
+core CLI logic can depend on regardless of whether it is a JSON file or a SQLite
+database. To achieve that I can define a class of unimplemented functions that
+inherits from
+[`typing.Protocol`](https://typing.python.org/en/latest/spec/protocol.html).
+
+```python
+from typing import Protocol
+
+class SessionRepository(Protocol):
+    def active_session(self) -> Session | None: ...
+    def write_active_session(self, session) -> None: ...
+    def append_session(self, session) -> None: ...
+    def all_sessions(self) -> list[Session]: ...
+    def clear_active_session(self) -> None: ...
+```
+
+Notice that none of these have default implementations. The `...` indicates that
+class implementing this protocol will define the implementation of those
+functions.
+
+Now, my `CliContext` class, which needs some kind of `SessionRepository` to
+function can indicate as much in `__init__`.
+
+```python
+class CliContext:
+    def __init__(self, verbose: bool, repo: SessionRepository | None = None) -> None:
+        self.verbose: bool = verbose
+        self.active_session: Session | None = None
+        self.repo: SessionRepository = repo or JsonRepository()
+        self.active_session = self.repo.active_session()
+```
+
+If `JsonRepository` doesn't define all of the methods specified in the protocol,
+then a type error will occur wherever it clashes with `SessionRepository`. Now
+as I implement `SqliteRepository` I have a standard interface to build against
+that I know I can seamlessly swap in.
+
+[source](https://typing.python.org/en/latest/reference/protocols.html#simple-user-defined-protocols)