Fork when wheels only support a specific range

[cbrnr]

I don't know if this is intentional, but the following project cannot be installed on Python 3.13 if I want to avoid building source distributions:

1. uv init example
2. cd example
3. Use this pyproject.toml:

   [project]
   name = "example"
   version = "0.1.0"
   description = "Add your description here"
   readme = "README.md"
   requires-python = ">=3.9"
   dependencies = [
       "scipy >= 1.13.1",
   ]

4. uv sync --python=3.12 works (same for 3.9, 3.10, and 3.11)
5. uv sync --python=3.13 --no-build-package=scipy fails, even though it could (should?) use scipy==1.14.1, which is available as a binary wheel.

This surprised me because I expected that by specifying scipy >= 1.13.1, uv would automatically choose a newer version with a binary wheel for Python 3.13. Indeed, there are binary wheels for version 1.13.1 available for Python 3.9 through 3.12, but only version ≥ 1.14.0 has a binary wheel for Python 3.13 (though it no longer supports 3.9). However, it seems that uv is trying to maintain consistent package versions across all supported Python versions.

[konstin]

This is known problem with scientific packages unfortunately. Does splitting the scipy version on the python version as documented at the bottom of https://github.com/astral-sh/uv/blob/main/docs/reference/build_failures.md (this doc will ship with the next release) help?

[cbrnr]

Unfortunately not:

[project]
name = "example"
version = "0.1.0"
description = "Add your description here"
readme = "README.md"
requires-python = ">=3.9"
dependencies = [
    "scipy >= 1.13.1; python_version <= '3.12'",
    "scipy >= 1.14.0; python_version >= '3.13'",
]

It tries to build NumPy 2.0.2 from source on Python 3.13, no idea why it doesn't just use >= 2.1:

❯ uv sync --python=3.13 --no-build-package=scipy --no-build-package=numpy
Using CPython 3.13.0
Removed virtual environment at: .venv
Creating virtual environment at: .venv
Resolved 4 packages in 0.97ms
error: Distribution `numpy==2.0.2 @ registry+https://pypi.org/simple` can't be installed because it is marked as `--no-build` but has no binary distribution

Maybe because it also uses NumPy 2.0.2 on Python <= 3.12?

Besides, I think splitting like this feels a bit off, since the ranges actually overlap (unlike in the example).

[konstin]

You need to also split numpy, probably as a constraint since it's not a direct dependency:

[project]
name = "example"
version = "0.1.0"
description = "Add your description here"
readme = "README.md"
requires-python = ">=3.9"
dependencies = [
  "scipy >=1.13.1,<1.14.0; python_version < '3.13'",
  "scipy >=1.14.0; python_version >= '3.13'",
]

[tool.uv]
constraint-dependencies = [
  # Supports 3.8 to 3.10
  "numpy <=2.1.0,>=1.22.4; python_version < '3.13'",
  # Support 3.10 to 3.13
  "numpy >=2.1.0; python_version >= '3.13'",
]

Overlapping version ranges are fine in uv's model, but you can also make them exclusive as in the example above.

Fwiw it would be nice if we could make this split automatically for non-abi3 wheels.

[cbrnr]

Uhm, can you explain the reasoning behind this resolution? Why does uv not pick scipy 1.14.1 for Python 3.13? I'm explicitly specifying scipy >= 1.13.1, so 1.14.1 is perfectly valid?

[hoechenberger]

This feels very un-intuitive to me, too 🤯

[konstin]

There's three parts of the uv resolver interacting here: requires-python, forking and wheel compatibility. uv will only select packages with a requires-python lowest bound at least as high as your project. For example, your project has requires-python = ">=3.9", while scipy 1.14.1 has Requires-Python: >=3.10 (https://inspector.pypi.io/project/scipy/1.14.1/packages/64/68/3bc0cfaf64ff507d82b1e5d5b64521df4c8bf7e22bc0b897827cbee9872c/scipy-1.14.1-cp310-cp310-macosx_10_13_x86_64.whl/scipy-1.14.1.dist-info/METADATA), so we can't select this version and must choose an older version. (If we would select that version, you would get an error trying to uv sync on Python 3.9)

uv also has the forking mechanism (https://docs.astral.sh/uv/reference/resolver-internals/#forking): Whenever we see that a dependency has different requirements for different markers in the same package, we fork and solve once for the one markers and once for the other markers (assuming the markers are disjoint, if they aren't we split some more until we have disjoint environments, there are some implementation details to it). Forking can tighten the requires-python range: If we know we're in an environment from a python_version >= "3.12" marker, we can pick a more recent version - such as scipy 1.14.1 - since the python_version >= "3.9" and python_version < "3.12" range is already handled by the other fork, which can select an older version.

uv is conservative with forking, we only fork on above condition. In a single fork, we have to enforce that there is only a single version per package per resolution (since we can only install a single version). uv also tries to reuse versions between forks when possible, to reduce the divergence between them (less different versions, less things that can go wrong/need to be tested). The latter is the reason that you often have to add an upper bound on the version for getting the right forking results.

Pure python projects don't have an upper bound on their python compatibility: Usually a package published now will work on future python versions. Even for most compiled projects with per-python version shared libraries, you can use them on a newer version by building the source dist. For these and reasons related to how our universal dependency resolution works, we're discarding upper bounds on requires-python (long explanation in https://github.com/astral-sh/uv/issues/4022).

For numpy, scipy and a number of related projects this assumption isn't true, they really only work for a specific Python range for each version, and you're also not supposed to build them from source (often failing to compile on newer Python versions). Here's the part where uv falls short and you need the manual intervention of specifying python_version markers: uv can't detect that we're trying a package that has compatibility and wheels only for a specific range, and we need to fork to satisfy the full Python version range from the lowest version your project requires to the latest version the dependency supports.

[cbrnr]

Thanks for this detailed explanation! I do like how thoroughly uv has been designed, but given that the scientific ecosystem now pushes it beyond its boundaries, I feel like it makes it very hard to use uv in this context. In the simple example with just a single scipy dependency I have to manually specify constraints for numpy, but in a more realistic project there will probably be dozens of such constraints that I don't want to handle manually.

Fwiw it would be nice if we could make this split automatically for non-abi3 wheels.

I'm not sure if I understand what you're saying, but do you think that uv should do this automatically so that my original example "just works"?

[cbrnr]

Also, pip resolves these requirements differently: it installs scipy==1.14.1 on >=3.10 and scipy==1.13.1 on ==3.9 (together with suitable binary wheels for numpy). This is the expected behavior at least for me, but probably also for the Python community.

[hoechenberger]

This is the expected behavior at least for me, but probably also for the Python community.

I would like to second this ☝️

[konstin]

For numpy at least, Requires-Python doesn't tell us about the upper bounds (https://inspector.pypi.io/project/numpy/2.1.2/packages/1c/a2/40a76d357f168e9f9f06d6cc2c8e22dd5fb2bfbe63fe2c433057258c145a/numpy-2.1.2-cp310-cp310-macosx_10_9_x86_64.whl/numpy-2.1.2.dist-info/METADATA#line.999), so we would need to find a strategy of forking dependent on the wheel on PyPI, e.g.:

Pick a numpy version, analyse the range of https://pypi.org/project/numpy/#files, and if we see consistent range of compiled cp3x tags while there exists a newer version that covers a wider range, fork and so we can pick a newer numpy on newer Python versions.