Resolve field aliases before calling field_transformer

[veeceey]

Summary

Fixes #1479

• Moves default alias resolution (e.g., stripping leading underscores from _private to private) to before the field_transformer callback, so transformers receive fully populated Attribute objects with usable alias values instead of None.
• Adds a second alias resolution pass after the transformer to handle any new fields the transformer may have added without setting an alias.
• Updates Attribute.evolve() to automatically update the alias when the name changes, if the alias was auto-generated (matches the default for the old name). Explicit aliases are preserved.

Before (broken)

def transformer(cls, fields):
    for f in fields:
        print(f.alias)  # None for auto-generated aliases
    return fields

After (fixed)

def transformer(cls, fields):
    for f in fields:
        print(f.alias)  # 'private' for _private, 'my_alias' for explicit
    return fields

Test plan

• Added test_hook_alias_available - verifies transformer sees resolved aliases
• Added test_hook_evolve_name_updates_auto_alias - verifies evolve(name=...) updates auto-generated alias
• Added test_hook_evolve_name_keeps_explicit_alias - verifies evolve(name=...) preserves explicit alias
• Updated 2 existing test expectations where error messages now show the resolved alias instead of None
• Full test suite passes: 1367 passed, 6 skipped, 2 xfailed

[veeceey]

Friendly ping - any chance someone could take a look at this when they get a chance? Happy to make any changes if needed.

[AdrianSosic]

Hi @veeceey 👋🏼 Thank you very much for taking care of this! As the one who opened the issue, I can confirm that it indeed solves the problem for me 👌🏼 Regarding the integration into attrs, I'd hope that @hynek could have a final look and give his approval?

[veeceey]

Thanks for confirming @AdrianSosic, really glad it solves the problem on your end! Hopefully @hynek can find a moment to take a look when he gets a chance.

[hynek]

Sorry for the delays, but I got caught up in a PR slop wave last week and while your PR looks legit on a brief look, given the current legal status I cannot merge it as long as there's a credit for Claude in the history.

This is not meant belligerently in any way; I appreciate that your PR conforms to our current contribution rules and one of the reasons I didn't reply earlier is because I was drafting #1518. I'm happy to take your feedback over there too, FWIW, but until we have more legal clarity, we can't permit the copyright status of attrs to become unclear by having a paper trail of legally questionable contributions.

I'm genuinely sorry for the inconvenience, and I would've preferred to spend my time differently too, but these are crazy times.

---

There's also the technical issue of whether this change is too breaking, but I don't want to waste your time/split the discussion until we have mergeable PR.

[veeceey]

Hey @hynek, good catch - I've cleaned up the git history and removed the co-author attribution. All commits are now solely under my name. Let me know if there's anything else needed!

[hynek]

amazing, thank you for that @veeceey!

Before I zoom in on the implementation, and given we had some breakage related to field transformers in the past: hey @ashb is this likely to break Airflow? It looks additive/harmless enough but I'm not sure what kind of assumptions y'all are making.

Also cc @sscherfke.

[ashb]

@hynek first thought is that it should be fine as i don't think we use field transformers much if at all.

I'll run this through our tests today

[ashb]

All good on the Airflow side, he have exactly one field_transformer which doesn't make any assumptions about private fields or not.

[veeceey]

Thanks for running the tests @ashb, glad to hear it's all clear on the Airflow side!

@hynek just a heads up - I noticed the commit messages still had co-author lines from earlier, and the PR description had a leftover attribution line too. I've now cleaned both up completely - the git history and PR description are fully clean. Ready for your review whenever you get a chance!