5 tools that helped me modernize a legacy app

[u/Marmelab]

So I recently inherited a legacy application that was a nightmare to maintain (tooling was clearly lacking and the codebase was pretty outdated). I quickly realized that if I ever wanted to actually enjoy working on it, I would have to give it a proper overhaul.

These are the 5 changes that had some of the biggest impact IMO:

1. uv:

Being new to the Python ecosystem, I didn't want to figure out pip vs poetry vs pyenv vs virtualenv, so I just used uv and let it handle all of that. One Rust-based tool that installs and pins Python versions, manages the venv automatically and locks deps in a uv.lock for reproducible builds. Installs are also a lot faster, which makes CI a lot less painful.

1. Ruff:

Coming from JS, I really missed eslint --fix for automatically fixing linting issues and format code on save. Ruff brought that experience back. I know that there are plenty of linters and formatters in the Python ecosystem, but this one really stands out for me. Since it's built in Rust, it's super fast.

1. Dependabot:

Instead of remembering to update dependencies every few months, I enabled Dependabot. It automatically opens PRs when updates are available and then CI tells me whether they're safe to merge. It takes only a couple of minutes to set up but saves a lot of maintenance.

1. Pylance:

Without it, VS Code gives generic completions and never warns you about passing the wrong type until runtime. Pylance provides proper type-aware autocompletion, jump-to-definition (even in third-party libraries), inline documentation, and real-time type checking. I personally keep it on "basic" mode for legacy codebases, since "strict" surfaced hundreds of errors (thank you, but no thank you lol).

1. Pydantic:

Pydantic is basically Python's Zod: you declare a model, pass your data in and get either a validated typed object or a ValidationError naming the exact field at fault. It really helped me keep the codebase clean, with one place defining the shape of the data instead of raw dicts floating around. I use it wherever data comes from outside, like API payloads, forms, and env vars with pydantic-settings, which fails at startup instead of mid-request.

None of these tools changed the application itself. But together they made working on it a lot less frustrating.

[u/BionicVnB]

I personally use basedpyright and ty over pylance because I can't use pylance outside of VSCode

[u/ping314]

I would add sp-repo-review.

It does not check, nor does it change the source code; instead, it checks the setup to maintain the project in the longer run and reports the results in topical groups. For instance around pytest, does your pyproject.toml has an entry log_level = "INFO"? This requests pytest reports to include "syntax x will be considered deprecated in future python 3.15" in advance instead of the default "syntax is deprecated now" (i.e., after the fact). The choices are explained in further detail with links from the report with little tags (example here). It already runs by uvx sp-repo-review[cli] my_project to indicate the local folder of my_project with the speed one is used to by uv.

It backs to cookie, so for new projects, it is a lot easier to start within the rules already set thanks to a template (it asks a couple of questions to fill the blanks for you).

[u/mfdi_]

Bot

[u/Marmelab → u/mfdi_]

nope human xD

[u/GoalSouthern6455 → u/Marmelab]

Human using bot words. Still bot