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Update resolver internals docs (#11098)

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Since the resolver internals docs were written, we added a lot more
features to the resolver, which should be documented.

As usual, these docs are not targeted at regular users, but should give
interested readers an insight into the internals of uv and help advanced
users with especially hard resolver problems.
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104
docs/reference/resolver-internals.md
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@ -25,6 +25,8 @@ there are no general, fast algorithms. In practice, this is misleading for a num
- When resolution fails, more information is needed than a message that there is no solution (as is
seen in SAT solvers). Instead, the resolver should produce an understandable error trace that
states which packages are involved in away to allows a user to remove the conflict.
- The most important heuristic for performance and user experience is determining the order in which
decisions are made through prioritization.
uv uses [pubgrub-rs](https://github.com/pubgrub-rs/pubgrub), the Rust implementation of
[PubGrub](https://nex3.medium.com/pubgrub-2fb6470504f), an incremental version solver. PubGrub in uv
@ -32,10 +34,10 @@ works in the following steps:
- Start with a partial solution that declares which packages versions have been selected and which
are undecided. Initially, only a virtual root package is decided.
- The highest priority package is selected from the undecided packages. Package with URLs (including
file, git, etc.) have the highest priority, then those with more exact specifiers (such as `==`),
then those with less strict specifiers. Inside each category, packages are ordered by when they
were first seen (i.e. order in a file), making the resolution deterministic.
- The highest priority package is selected from the undecided packages. Roughly, packages with URLs
(including file, git, etc.) have the highest priority, then those with more exact specifiers (such
as `==`), then those with less strict specifiers. Inside each category, packages are ordered by
when they were first seen (i.e. order in a file), making the resolution deterministic.
- A version is picked for the selected package. The version must works with all specifiers from the
requirements in the partial solution and must not be previously marked as incompatible. The
resolver prefers versions from a lockfile (`uv.lock` or `-o requirements.txt`) and those installed
@ -115,16 +117,6 @@ between forks is written to the lockfile. When performing a new resolution, the
lockfile are used to ensure the resolution is stable. When requirements change, new forks may be
added to the saved forks.
## Requires-python
To ensure that a resolution with `requires-python = ">=3.9"` can actually be installed for the
included Python versions, uv requires that all dependencies have the same minimum Python version.
Package versions that declare a higher minimum Python version, e.g., `requires-python = ">=3.10"`,
are rejected, because a resolution with that version can't be installed on Python 3.9. For
simplicity and forward compatibility, only lower bounds in `requires-python` are respected. For
example, if a package declares `requires-python = ">=3.8,<4"`, the `<4` marker is not propagated to
the entire resolution.
## Wheel tags
While uv's resolution is universal with respect to environment markers, this doesn't extend to wheel
@ -136,3 +128,87 @@ system and architecture. Most projects have a universally compatible source dist
used when attempted to install a package that has no compatible wheel, but some packages, such as
`torch`, don't publish a source distribution. In this case an installation on, e.g., Python 3.13, an
uncommon operating system, or architecture, will fail and complain that there is no matching wheel.
## Marker and wheel tag filtering
In every fork, we know what markers are possible. In non-universal resolution, we know their exact
values. In universal mode, we know at least a constraint for the python requirement, e.g.,
`requires-python = ">=3.12"` means that `importlib_metadata; python_version < "3.10"` can be
discarded because it can never be installed. If additionally `tool.uv.environments` is set, we can
filter out requirements with markers disjoint with those environments. Inside each fork, we can
additionally filter by the fork markers.
There is some redundancy in the marker expressions, where the value of one marker field implies the
value of another field. Internally, we normalize `python_version` and `python_full_version` as well
as known values of `platform_system` and `sys_platform` to a shared canonical representation, so
they can match against each other.
When we selected a version with a local tag (e.g.,`1.2.3+localtag`) and the wheels don't cover
support for Windows, Linux and macOS, and there is a base version without tag (e.g.,`1.2.3`) with
support for a missing platform, we fork trying to extend the platform support by using both the
version with local tag and without local tag depending on the platform. This helps with packages
that use the local tag for different hardware accelerators such as torch. While there is no 1:1
mapping between wheel tags and markers, we can do a mapping for well-known platforms, including
Windows, Linux and macOS.
## Requires-python
To ensure that a resolution with `requires-python = ">=3.9"` can actually be installed for the
included Python versions, uv requires that all dependencies have the same minimum Python version.
Package versions that declare a higher minimum Python version, e.g., `requires-python = ">=3.10"`,
are rejected, because a resolution with that version can't be installed on Python 3.9. For
simplicity and forward compatibility, only lower bounds in `requires-python` are respected. For
example, if a package declares `requires-python = ">=3.8,<4"`, the `<4` marker is not propagated to
the entire resolution.
This default is a problem for packages that use the version-dependent C API of CPython, such as
numpy. Each numpy release support 4 Python minor versions, e.g., numpy 2.0.0 has wheels for CPython
3.9 through 3.12 and declares `requires-python = ">=3.9"`, while numpy 2.1.0 has wheels for CPython
3.10 through 3.13 and declares `requires-python = ">=3.10"`. The means that when we resolve a
`numpy>=2,<3` requirement in a project with `requires-python = ">=3.9"`, we resolve numpy 2.0.0 and
the lockfile doesn't install on Python 3.13 or newer. To alleviate this, whenever we reject a
version due to a too high Python requirement, we fork on that Python version. This behavior is
controlled by `--fork-strategy`. In the example case, upon encountering numpy 2.1.0 we fork into
Python versions `>=3.9,<3.10` and `>=3.10` and resolve two different numpy versions:
```
numpy==2.0.0; python_version >= "3.9" and python_version < "3.10"
numpy==2.1.0; python_version >= "3.10"
```
## Prioritization
Prioritization is important for both performance and for better resolutions.
If we try many versions we have to later discard, resolution is slow, both because we have to read
metadata we didn't need and because we have to track a lot of (conflict) information for this
discarded subtree.
There are expectations about which solution uv should choose, even if the version constraints allow
multiple solutions. Generally, a desirable solution prioritizes use the highest versions for direct
dependencies over those for indirect dependencies, it avoids backtracking to very old versions and
can be installed on a target machine.
Internally, uv represent each package with a given package name as a number of virtual packages, for
example, one package for each activated extra, for dependency groups, or for having a marker. While
PubGrub needs to choose a version for each virtual package, uv's prioritization works on the package
name level.
Whenever we encounter a requirement on a package, we match it to a priority. The root package and
URL requirements have the highest priority, then singleton requirements with the `==` operator, as
their version can be directly determined, then highly conflicting packages (next paragraph), and
finally all other packages. Inside each category, packages are sorted by when they were first
encountered, creating a breadth first search that prioritizes direct dependencies including
workspace dependencies over transitive dependencies.
A common problem is that we have a package A with a higher priority than package B, and B is only
compatible with older versions of A. We decide the latest version for package A. Each time we decide
a version for B, it is immediately discarded due to the conflict with A. We have to try all possible
versions of B, until we have either exhausted the possible range (slow), pick a very old version
that doesn't depend on A, but most likely isn't compatible with the project either (bad) or fail to
build a very old version (bad). Once we see such conflict happen five time, we set A and B to
special highly-conflicting priority levels, and set them so that B is decided before A. We then
manually backtrack to a state before deciding A, in the next iteration now deciding B instead of A.
See [#8157](https://github.com/astral-sh/uv/issues/8157) and
[#9843](https://github.com/astral-sh/uv/pull/9843) for a more detailed description with real world
examples.