From Monolith to Modular Architecture: Refactoring a 2,353-Line PyQt6 Desktop App Without Regressions

Description and use

Modular refactoring of a monolithic PyQt6 app reduced the main module by 35 %, increased tests by 39 %, and introduced zero regressions, preparing the codebase for Wayland support and plugins.

Problem

A 2,353-line monolithic PyQt6 desktop app (DotGhostBoard) had intertwined UI, storage, encryption, and networking logic, making future feature work fragile and hard to test.

Outcome

The refactor reduced the main UI module by 35%, expanded tests by 39%, and achieved zero user‑facing regressions while laying the groundwork for Wayland support and plugin extensibility.

Workflow

• 1. Decompose storage into database, migrations, and repository modules, exposing a facade for backward compatibility.
• 2. Implement a zero‑dependency migration engine that runs each schema change atomically.
• 3. Extract clipboard handling into a pure logic pipeline and define a ClipboardBackend protocol.
• 4. Create QtClipboardBackend and MockClipboardBackend for production and testing.
• 5. Introduce service classes (HistoryService, CollectionService, SecurityService, SyncService) to move business logic out of UI.
• 6. Harden cryptography with a two‑tier key derivation chain and envelope encryption for the Vault.
• 7. Add 86 new unit tests covering storage, clipboard, services, and cryptography.
• 8. Run the full test suite to confirm zero regressions.
• 9. Prepare the codebase for v2.0 features (Wayland, sync, plugins).

Evidence

The automated test suite expanded from 220 to 306 tests across 25 modules (+39% tests).

Source

The entire refactor passed the test suite with zero detected user-facing regressions.

Source