select-implementation-with-class-registry.md

Select Implementation With Class Registry

I am working on supporting multiple storage formats (JSON and SQLite) for my py-vmt project. 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.

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.