Support multiple constraints and constraint files

[tiran]

Fromager 0.82 and earlier only supports a single constraints file with unique entries. packaging 26.1 added a new feature to check if a SpecifierSet is satisfiable. This feature finally allows us to support multiple constraints and constraints files safely.

>>> from packaging.specifiers import SpecifierSet
>>> SpecifierSet("<1").is_unsatisfiable()
False
>>> SpecifierSet("<1,>2").is_unsatisfiable()
True

1. extend fromager.constraints.Constraints.add_constraint to combine constraints and verify that the combined constraints are satisfiable.
2. update the global -c / --constraints-file option to support multiple constraints files

[LalatenduMohanty]

@smoparth I would like to see a proposal on this before you implement the actual feature. I will help us agree on the benefit it will bring to downstream usage of fromager.

[LalatenduMohanty]

For example how fromager support of multiple constraint will look, is it going to be something like this fromager -c global.txt -c collection.txt -c variant.txt bootstrap

Lookslike SpecifierSet doesn't just look for string mismatches; it understands the numerical logic of PEP 440 versions. It detects:

• Disjoint ranges: e.g., >5.0 and <3.0.
• Exact match conflicts: e.g., ==1.0 and ==2.0.
• Exclusion conflicts: e.g., ==1.2 and !=1.2.

[tiran]

fromager -c global.txt -c collection.txt -c variant.txt

Yes, it is as simple as adding multiple=True to the click parameter and change the parameter from str to tuple[str].

[smoparth]

Makes sense, happy to write a proposal. I'll cover:

The downstream benefit I see -- multiple constraint sources over manual merging.
Key design decisions that need agreement: error type change (KeyError → ValueError), packaging>=26.1 pin, and whether to pass multiple --constraint files to pip vs a single merged file but I thought these were more of implementation decisions than architecture.

I don't see a proposal template in the repo, not a need of the hour but we can have one to standardize the proposals. I'll follow existing proposal formats and draft one.

[tiran]

IMHO we don't need a design proposal for this feature. It's a simple improvement and we discuss the details in this ticket.

• pinning packaging >= 26.1 is okay. We can always require newer version as long as the newer versions are compatible with minimum Python version
• raising a different exception is also fine. You could even create a custom exception ConflictingSpecifier

  >>> packaging.specifiers.InvalidSpecifier.__mro__
  (<class 'packaging.specifiers.InvalidSpecifier'>, <class 'ValueError'>, <class 'Exception'>, <class 'BaseException'>, <class 'object'>)
  

• Let's write a single constraints file and pass that file to uv.

[LalatenduMohanty]

yes, we can discuss the details here in this issue. It will also help the implementation and review smother.

[smoparth]

PR #1125 already covers constraint merging, validation, and the new InvalidConstraintError exception.

PR #1128 addresses remaining: the global -c/--constraints-file option now accepts multiple values.

• Passing multiple constraints
  

• All constraints are loaded in order and merged (leveraging the combining logic from #1125)
  

• A single merged constraints file is written and passed to uv (as discussed in this thread)
  

• Backward compatible — single -c and no -c both work unchanged