Add Define Typed Class Interface With Protocol as a Python TIL

README.md

@@ -10,7 +10,7 @@ working across different projects via [VisualMode](https://www.visualmode.dev/).
 
 For a steady stream of TILs, [sign up for my newsletter](https://visualmode.kit.com/newsletter).
 
-_1804 TILs and counting..._
+_1805 TILs and counting..._
 
 See some of the other learning resources I work on:
 
@@ -1069,6 +1069,7 @@ If you've learned something here, support my efforts writing daily TILs by
 - [Create A Range Of Descending Values](python/create-a-range-of-descending-values.md)
 - [Deduplicate A List Into A Tuple](python/deduplicate-a-list-into-a-tuple.md)
 - [Define Sequence Of Tests With Parametrize Decorator](python/define-sequence-of-tests-with-parametrize-decorator.md)
+- [Define Typed Class Interface With Protocol](python/define-typed-class-interface-with-protocol.md)
 - [Dunder Methods](python/dunder-methods.md)
 - [Easy Key-Value Aggregates With defaultdict](python/easy-key-value-aggregates-with-defaultdict.md)
 - [Get Absolute Seconds From `timedelta` Object](python/get-absolute-seconds-from-timedelta-object.md)

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

@@ -0,0 +1,42 @@
+# 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)