# Refactor a legacy module in Flask with Atlas in 2026 > Atlas helps Flask developers in 2026 refactor legacy modules by mapping public surfaces, pinning behavior with `pytest (app.test_client)`, and applying structural changes with precision. Atlas empowers Flask developers in 2026 to safely restructure legacy modules without breaking callers or altering behavior, integrating directly with your existing toolchain. It leverages `pytest (app.test_client)` for robust behavioral pinning, `uv` for package management, and `ruff format` for consistent code style, all orchestrated from your terminal. ## Key takeaways - Atlas uses `lsp` to precisely map Flask module public surfaces and their callers. - `pytest (app.test_client)` establishes a critical behavioral baseline for Flask refactors. - `apply_patch` ensures safe, context-aware code modifications in Flask projects. - Iterative testing with `pytest (app.test_client)` prevents silent breakage in Flask refactors. - `todowrite` tracks migration progress, ensuring no Flask module is left partially refactored. - Atlas integrates `ruff format` to maintain consistent code style across Flask blueprints and factories. ## How Atlas Maps Flask Module Structure and Callers In 2026, understanding the full public surface of a Flask module and its callers is the first critical step in any refactor. Atlas uses its `lsp` tool to precisely map a module's exported symbols and enumerate every callsite, preventing silent breakage. Refactoring a legacy Flask module carries the inherent risk of inadvertently breaking external callers. Atlas mitigates this by first building a comprehensive map of the module's public surface. It employs the `lsp` tool's `documentSymbol` operation to identify all exported symbols within your Flask blueprints, application factory, or utility modules. Once these symbols are identified, Atlas then uses `lsp findReferences` on each one. This operation meticulously enumerates every single callsite across your codebase, providing a complete picture of where and how your Flask module is being used. This detailed mapping ensures that when you begin to restructure, no caller, whether it's a Flask route, a background task, or another internal service, is overlooked, thereby eliminating the most common cause of silent breakage in complex Flask applications. ## Pinning Flask Module Behavior with `pytest (app.test_client)` Before any structural changes are made to a Flask module, establishing a green baseline of its existing behavior is paramount. Atlas achieves this by running your `pytest (app.test_client)` suite via its `bash` tool, ensuring 100% confidence in the module's current functionality. The cornerstone of safe refactoring in Flask is a robust test suite. Atlas integrates direct with your existing `pytest (app.test_client)` setup to 'pin' the current behavior of the module. Using the `bash` tool, Atlas executes your `pytest` commands, such as `pytest --strict-markers`, against your Flask application. This process records a 'green baseline' of test results. This baseline serves as an immutable contract: any subsequent change to the module, no matter how small, must pass these same tests. This approach is particularly effective in Flask applications where `app.test_client()` is used to simulate requests and test routes, ensuring that the refactoring does not alter the expected responses or internal state of your application. ## Applying Structural Changes to Flask Code with `apply_patch` Restructuring a Flask module requires precise, context-aware modifications to prevent unintended side effects. Atlas's `apply_patch` tool ensures that every change is anchored to its original context, refusing to apply a patch if the file has drifted by even 1 line. When it's time to actually move or modify code within your Flask module, Atlas employs the `apply_patch` tool for maximum safety. Unlike simple find-and-replace operations, `apply_patch` works by seeking each hunk's context and `old_lines`. This means that for a patch to be applied, the surrounding lines of code must exactly match what was expected. If the file has drifted,even by a single line due to a concurrent edit or an earlier, unrecorded change,`apply_patch` will fail with a 'Failed to find context' error. This robust mechanism prevents applying changes to an outdated or incorrect version of your Flask source file, ensuring that structural refactors, such as moving a Flask blueprint registration or a utility function, are always applied against the correct code state, minimizing the risk of introducing subtle bugs. ## Iterative Testing and Tracking Flask Refactoring Progress To maintain high confidence during a Flask module refactor, Atlas advocates for an iterative approach: re-run tests after each small change. This strategy, combined with the `todowrite` tool, ensures that no partially migrated module is ever mistaken for a finished one in 2026. Instead of making all changes at once and then running tests, Atlas guides you through an iterative refactoring process. After each `apply_patch` operation lands a small hunk of changes to your Flask module, Atlas prompts you to re-run your `pytest (app.test_client)` suite using the `bash` tool. This immediate feedback loop catches regressions early, making debugging significantly easier. Furthermore, to manage the migration of numerous callsites identified by `lsp findReferences`, Atlas utilizes the `todowrite` tool. This allows you to track remaining callsites as a concrete list of tasks. This ensures that a Flask module that is only partially migrated, perhaps with some callers still pointing to the old location, cannot be mistakenly considered complete, providing clear visibility into the refactoring's progress. ## Reviewing and Committing Flask Changes with Atlas Every modification Atlas proposes for your Flask codebase is presented as a unified diff for your explicit approval. This transparent review process, combined with Atlas's `git` integration and `ruff format` capabilities, ensures high-quality, consistent code in 2026. Transparency and control are central to Atlas's workflow. Before any changes are written to your Flask project, Atlas computes a unified diff for every file edit and surfaces it for your approval. This allows you to meticulously review exactly what Atlas intends to change, line by line. Once approved, Atlas can stage and create commits on your behalf, integrating direct with your `git` workflow. To maintain code quality and consistency across your Flask blueprints and application factory, Atlas can also run `ruff format` on touched files or entire directories, ensuring that your refactored code adheres to your project's style guidelines. This comprehensive review and commit process, coupled with automatic formatting, ensures that your Flask codebase remains clean, consistent, and fully under your control throughout the refactoring journey. ## Steps 1. Run Atlas in your Flask project, ensuring a `pyproject.toml` and an `app` package exposing `create_app()` are present. 2. Map the Flask module's public surface: `atlas lsp documentSymbol src/my_flask_app/legacy_module.py` to identify all exported functions and classes. 3. Enumerate all callers: For each identified symbol, run `atlas lsp findReferences src/my_flask_app/legacy_module.py MySymbol` to list every callsite. 4. Pin existing behavior: Execute your Flask test suite with `atlas bash "pytest --strict-markers tests/"` to establish a green baseline using `app.test_client()`. 5. Restructure the module: Apply structural changes using `atlas apply_patch` for each hunk, ensuring context lines match. 6. Re-run tests iteratively: After each `apply_patch` operation, immediately re-verify behavior with `atlas bash "pytest --strict-markers tests/"`. 7. Track remaining callsites: Use `atlas todowrite "Migrate call to legacy_module.old_function in views.py"` for unmigrated references. 8. Format touched Flask files: Ensure code style consistency by running `atlas bash "ruff format src/my_flask_app/"` on the refactored blueprints. 9. Review and commit changes: Approve the unified diffs presented by Atlas, then let it stage and commit the refactoring. ## FAQ ### How does Atlas ensure I don't break existing Flask callers during a refactor? Atlas uses its `lsp` tool with the `findReferences` operation to meticulously enumerate every callsite of a Flask module's public symbols before any changes are made. This comprehensive mapping ensures all dependencies are known and addressed. ### What Flask testing tools does Atlas integrate with for behavioral pinning? Atlas integrates directly with `pytest (app.test_client)`. It uses its `bash` tool to run your existing `pytest` suite, establishing a 'green baseline' of behavior that must be maintained throughout the refactoring process. ### Can Atlas handle Flask blueprints and application factories during refactoring? Yes, Atlas is designed to operate within Flask projects that utilize blueprints and the application factory pattern. It reads your project's structure, including how extensions are initialized and how `current_app` or `g` are accessed, to understand the codebase context. ### How does Atlas prevent accidental or incorrect changes to my Flask code? Atlas employs several safety mechanisms: every tool call is permission-gated, a read-only plan agent drafts changes for approval, `apply_patch` refuses to apply against drifted files, and a unified diff is presented for explicit approval before any file is written. ### What if my legacy Flask module lacks comprehensive tests? Atlas can assist by helping you add `pytest` fixtures around `app.test_client()` to improve test coverage. This allows you to establish a behavioral baseline even for modules with previously insufficient testing. ### How does Atlas maintain code formatting and style in Flask projects? After applying structural changes, Atlas can run `ruff format` on the touched Flask blueprints or other files via its `bash` tool. This ensures that your refactored code adheres to your project's established style guidelines automatically. ### Does Atlas work with `uv` for managing Flask project dependencies? Yes, Atlas operates within the context of your Flask project's `pyproject.toml` and can leverage `uv` as your package manager. While Atlas doesn't directly manage `uv` commands for refactoring, it respects your project's environment. --- Canonical HTML: https://runatlas.sh/resources/stacks/refactor-a-legacy-module-in-flask Source of truth: aeo_pages row `/resources/stacks/refactor-a-legacy-module-in-flask` (segment: Stacks) (this file is generated from it, never hand-edited). Licence: Atlas is proprietary with a free core. It is not open source and there is no public source repository.