How would you design a plugin system in Python that allows third parties to extend your application's functionality?

A plugin system needs four things: a stable interface, a discovery mechanism, lifecycle management, and isolation. The interface is usually an abstract base class or a Protocol that plugins implement. Discovery is typically done with setuptools entry points, which let a plugin package declare a name in a group that your application reads with importlib.metadata.entry_points. This avoids hardcoding a list of plugins and lets third parties install their package alongside yours. Lifecycle matters: decide whether plugins are loaded at startup or lazily, how they register themselves, and how you handle initialization failures without crashing the whole application. Isolation is the hardest part. A plugin runs in the same process with the same privileges, so a buggy or malicious plugin can crash the process or read data it should not. For untrusted plugins, consider a subprocess or a separate service with a narrow RPC boundary. Finally, versioning: the plugin interface is a public API. Version it explicitly, provide a compatibility shim, and document what changes are breaking. Common mistakes include importing all plugins eagerly at startup, which slows startup and increases the blast radius of a failure, and not validating that a discovered entry point actually implements the interface.