Add Select Implementation With Class Registry 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).
 
-_1829 TILs and counting..._
+_1830 TILs and counting..._
 
 See some of the other learning resources I work on:
 
@@ -1099,6 +1099,7 @@ If you've learned something here, support my efforts writing daily TILs by
 - [Override The Boolean Context Of A Class](python/override-the-boolean-context-of-a-class.md)
 - [Parse Relative Time To datetime Object](python/parse-relative-time-to-datetime-object.md)
 - [Reclassify Certain Packages As Dev Dependencies](python/reclassify-certain-packages-as-dev-dependencies.md)
+- [Select Implementation With Class Registry](python/select-implementation-with-class-registry.md)
 - [Set Up Pyright Type Checking In GitHub](python/set-up-pyright-type-checking-in-github.md)
 - [Skip Specific Pytest Test Cases](python/skip-specific-pytest-test-cases.md)
 - [Sort A List Of Dataclass Instances](python/sort-a-list-of-dataclass-instances.md)

python/select-implementation-with-class-registry.md

@@ -0,0 +1,48 @@
+# Select Implementation With Class Registry
+
+I am working on [supporting multiple storage formats (JSON and
+SQLite)](https://github.com/jbranchaud/py-vmt/pull/1) for my [`py-vmt`
+project](https://github.com/jbranchaud/py-vmt). While SQLite will be the default
+storage format, the CLI can be configured via a `config.json` file to use
+another supported `storage_format` (e.g. JSON).
+
+When the `CliContext` is initialized, I need to determine which will be used so
+that I can use the correct _repository_ implementation for reads, writes, etc. I
+decided to use a simplified _Class Registry Pattern_ to build the correct
+implementation without a messy chain of conditionals.
+
+```python
+class CliContext:
+    def __init__(self) -> None:
+        self.config = self.read_config()
+        self.repo = self._initialize_configured_repo()
+        # ...
+
+    # ...
+
+    _REPOS: dict[str, type[SessionRepository]] = {
+        "json": JsonRepository,
+        "sqlite": SqliteRepository,
+    }
+
+    def _initialize_configured_repo(self) -> SessionRepository:
+        default_format = "sqlite"
+        format = self.config.get("storage_format", default_format)
+        try:
+            return self._REPOS[format]()
+        except KeyError:
+            raise ValueError(f"Unknown storage_format: {format!r}")
+```
+
+I define a `dict` called `_REPOS` that registers each of the repository
+implementations that I support. Notice each value is a `SessionRepository`
+class. Then the `_initialize_configured_repo` function (which is called in
+`__init__`) initializes the correct repository implementation. It first tries to
+grab the `storage_format` from the config, then pulls that out of `_REPOS`, and
+then tags on `()` to initialize it. If I have a bad config, a `KeyError` will
+be raised which I will re-raise as a `ValueError`.
+
+Some class registries support decorators or have a whole mechanism registering
+and unregistering classes. I don't need anything quite that sophisticated, so I
+stuck to a hard-coded dict. If I add support for more storage formats, I can add
+their repository classes to the registry.