Add Select Implementation With Class Registry as a Python TIL

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

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+# 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.