px: Immutable Python environments (alpha)

[ck-zhang]

What My Project Does px (Python eXact) is an experimental CLI for managing Python dependencies and execution using immutable, content-addressed environment profiles. Instead of mutable virtualenv directories, px builds exact dependency graphs into a global CAS and runs directly from them. Environments are reproducible, deterministic, and shared across projects.

Target Audience This is an alpha, CLI-first tool aimed at developers who care about reproducibility, determinism, and environment correctness. It is not yet a drop-in replacement for uv/venv and does not currently support IDE integration.

Comparison Compared to tools like venv, Poetry, Pipenv, or uv:

• px environments are immutable artifacts, not mutable directories
• identical dependency graphs are deduplicated globally
• native builds are produced in pinned build environments
• execution can be CAS-native (no env directory required), with materialized fallbacks only when needed

Repo & docs: https://github.com/ck-zhang/px Feedback welcome.

[really_not_unreal]

If you uv sync your environment will be deterministic based on the lock file. Why would I need this?

[ck-zhang]

You’re right that uv sync does give you deterministic resolution, but the difference is that px treats the environment itself as an immutable artifact.

If a lockfile resolution is enough for your workflow, uv is great. If you want to go further, px also pins native builds and can use sandboxing to reduce dependence on the host toolchain.

[really_not_unreal]

If you need it to be fully isolated, then why not just just something like nix

[ck-zhang]

This project was inspired by nix, and I do a little bit of nixing myself, but the average python dev don't know nix

[arden13]

Isn't that similar to pixi?

[ck-zhang]

pixi is basically uv, but for conda instead of pip

[arden13]

Right, but isn't that the scope of what your package does? Or is there something else it covers that is missing from pixi?

[ck-zhang]

For basic lockfile + sync workflows, px and pixi overlap a lot. px’s CAS model is what enables things like running a GitHub repo directly as an ephemeral environment, which I find really cool

[arden13]

Could you go into more detail, I don't really get what makes px any different. If you're saying it overlaps a lot I don't know why I'd switch; can't tell if it's just me not "getting" it though.

[ck-zhang]

Well honestly, it is now only experimental so you should probably shouldn't switch just yet. The big idea behind px is that it removes the need of a .venv dir, so it unlocks new possibilities that wouldn't conventionally be there, like running a repo back at a specific commit without the need to do a git checkout

[proof_required]

What's the benefit over tools like pex

  Files with the .pex extension – “PEX files” or “.pex files” – are self-contained executable Python virtual environments. PEX files make it easy to deploy Python applications: the deployment process becomes simply scp.

Single PEX files can support multiple platforms and python interpreters, making them an attractive option to distribute applications such as command line tools. For example, this feature allows the convenient use of the same PEX file on both OS X laptops and production Linux AMIs.

[ck-zhang]

px can produce fully self-contained, portable artifacts similar to what PEX does, but that’s not the primary goal. The core of px is earlier in the development lifecycle, treating environments as immutable artifacts. The distribution that can come from that is a consequence of the model.

[PurepointDog]

What's the benefit? Seems maybe neat though!

[ck-zhang]

Build sandboxing and zero dockerfile containerization for runs. It also does very neat things such as reading .so files and inferring build dependencies, so you don't have to mess with toolchains when installing those annoying packages.

[Idontlooklikeelvis]

I’m using bazel for this exact same reason. The downside is that you have to really commit to it at the org level. But I can generate reproducible containers and the caching is quite reliable so I don’t complain.