Confusing error message when a constraint is the cause of not finding a version

[notatallshaw]

Environment: Linux Python 3.10.13 uv 0.1.12

Steps to reproduce:

1. wget https://raw.githubusercontent.com/apache/airflow/constraints-2.3.4/constraints-3.10.txt
2. echo "apache-airflow==2.3.4" | uv pip compile - --constraint constraints-3.10.txt

Error:

  × No solution found when resolving dependencies:
  ╰─▶ Because there is no version of exceptiongroup==1.0.0rc8 and cattrs==22.1.0 depends on exceptiongroup==1.0.0rc8, we can conclude that
      cattrs==22.1.0 cannot be used.
      And because apache-airflow==2.3.4 depends on cattrs==22.1.0 and you require apache-airflow==2.3.4, we can conclude that the requirements
      are unsatisfiable.

      hint: exceptiongroup was requested with a pre-release marker (e.g., exceptiongroup==1.0.0rc8), but pre-releases weren't enabled (try:
      `--prerelease=allow`)

This is confusing because cattrs==22.1.0 doesn't depend only on exceptiongroup==1.0.0rc8 and further apache-airflow==2.3.4 does not depend only on cattrs==22.1.0, as you will see if you compie apache-airflow==2.3.4 or cattrs==22.1.0 or both of them together.

There is no mention that it is a user constraint which caused this failure, I beleive pip does mention when a top level user requirement or constraint caused the failure.

[zanieb]

Ah interesting, thanks for the report!

I'm not entirely sure why these constraints are not represented in the resolver (and subsequently the error report).

[potiuk]

Interesting is that while the message is confusing, the hint is correct in this case - adding --prerelease allow fixes the problem and gives the information why uv concludes that it's cattrs.

• root cause is that airflow constraints for 2.3.4 contain exceptiongroup==1.0.0rc8 (the constraints were generated when there was no 1.0.0 and >= 1.0.0rc8 is coming from pytest actually, not cattrs)

When you run the pip compile above with --prerelease allow - it will produce the right output and part of it is

exceptiongroup==1.0.0rc8
    # via cattrs

• apparently uv thinks that it's cattrs that caused exceptiongroup problem, but in this case it's the constraint file that contributed the prerelese version.

[notatallshaw]

Don't focus on the prerelease part of the error, I've reported that separately and it's already fixed: #2063

After the next release of uv I'll see if I can come up with another example.

[notatallshaw]

Okay, now there's a release with the other fix my steps to reproduce don't work. Here are a new set of steps to reproduce:

Environment: Linux Python 3.10.13 uv 0.1.14

1. wget https://raw.githubusercontent.com/apache/airflow/constraints-2.3.4/constraints-3.10.txt
2. Edit constraints-3.10.txt and replace pendulum==2.1.2 with pendulum==1.0.0
3. echo "apache-airflow==2.3.4" | uv pip compile - --constraint constraints-3.10.txt

You get the error:

$ echo "apache-airflow==2.3.4" | uv pip compile - --constraint constraints-3.10.txt
  × No solution found when resolving dependencies:
  ╰─▶ Because apache-airflow==2.3.4 depends on pendulum>=2.0 and apache-airflow==2.3.4 depends on pendulum==1.0.0, we can conclude that apache-airflow==2.3.4 cannot be used.
      And because you require apache-airflow==2.3.4, we can conclude that the requirements are unsatisfiable.

The confusing part is apache-airflow==2.3.4 depends on pendulum==1.0.0 without mentioning that pendulum==1.0.0 comes from the constraints. It would be more confusing if pendulum was a more distant transitive dependency.