Implements a generic JSON-RPC server over stdin/stdout. The JSONRPCServer class handles message framing, dispatching, error handling (Fault), and method registration. This is a self-contained, reusable component for any JSON-RPC-over-stdio service, not specific to Python IDE functionality.

Provides the Jedi-based backend for all static analysis operations. The run_with_debug function wraps Jedi API calls with detailed error logging and structured failure responses, enabling robust error recovery. The backend implements a common interface (rpc_get_* methods) that can be swapped out for alternative backends, demonstrating a clean separation of concerns.

Contains test mixin classes (BackendTestCase, RPCGetCompletionsTests, etc.) that reduce duplication across backend tests. These mixins define shared test methods and assertions that can be reused when testing alternative backends, making the test suite extensible and maintainable.

The elpy-refactor.el file (tested here) implements a diff-based refactoring mode derived from diff-mode, allowing users to apply refactoring changes (renames, extractions, inlines) hunk by hunk or as a whole. This pattern is reusable for any editor that needs to present and apply code transformations with user review.

Wraps the black code formatter, handling multiple versions (20.8b0 vs 21.5b1) with different APIs. Demonstrates techniques for bridging version incompatibilities in external tools, a common pattern in tool-integration code.