Uv sync with `--prerelease` fails unexpectedly

[dariocurr]

I’m working with the following setup:

1. library A: currently in development with version 0.2.0.dev2
2. library B: expands the functionality of library A and depends explicitly on A==0.2.0.dev2. Library B has version 0.1.0.dev1
3. application: relies on library B and specifies the dependency B==0.1.0.dev1

When running uv sync, all --prerelease options (if-necessary, explicit, if-necessary-or-explicit) fail unless the allow one. The issue arises because there’s a pre-release version in a third-party library dependency.

Since library B explicitly specifies A==0.2.0.dev2, I would expect the explicit option to handle this scenario. My intent is to avoid globally allowing pre-release versions (allow) across all dependencies and instead restrict it to cases where they are explicitly required, even if these are third-party libraries.

This behaviour works as expected with pip, where such explicit versioning doesn’t cause any issues.

uv version: 0.5.4

[charliermarsh]

I believe this is working as intended. A==0.2.0.dev2 enables pre-releases for A, but not for whatever the transitive dependency is. I think you'd want to either use allow, or add the transitive dependency as a first-party dependency with a pre-release marker.

[dariocurr]

Using allow would mean enabling pre-release versions for all dependencies, which introduces unintended side effects and it's very dangerous. For instance, if library A has a dependency like X>=1.1,<1.2, this approach could select 1.1.5.dev5 instead of the latest stable version 1.1.4 for X. These all-or-nothing options of pre-releases are not viable solutions in our scenario.

I strongly believe the if-necessary option should address this use case (as pip does)

Alternatively, uv could consider adding a new option, such as if-necessary-with-third-party, to explicitly handle situations where third-party dependencies have explicit pre-release requirements (==) specified in the dependency chain. If I explicitly specified it, I am explicitly accepting it

Such an enhancement would ensure that explicit versioning requirements are respected without globally relaxing constraints for all dependencies

[notatallshaw]

I do think the uv option is badly named, to better describe what it does it should be something like if-only-prereleases.