requests: allow immutable headers

[Akuli]

Fixes a problem reported by @rpdelaney in #7773 (comment)

I spent a while looking at requests source code, and I'm quite convinced that the headers can't be mutated. Each request is prepared before auth etc headers are applied, and the headers are copied when preparing.

[JelleZijlstra]

Thanks! Is it worth adding some test cases here? We've gone through quite a few iterations with these requests mappings.

[Akuli]

Most of the iterations were before mypy_primer worked for third-party stubs. Ideally we would just add @rpdelaney's project to mypy_primer, if it happens to be open-source.

[Akuli]

Actually it is open-source.

[Akuli]

closed accidentally

[github-actions]

According to mypy_primer, this change has no effect on the checked open source code. 🤖🎉

[AlexWaygood]

Most of the iterations were before mypy_primer worked for third-party stubs. Ideally we would just add @rpdelaney's project to mypy_primer, if it happens to be open-source.

+1 to adding more projects to mypy-primer if they use code that isn't otherwise covered. But I feel like "adding more projects to mypy-primer" and "adding specific regression tests" aren't necessarily mutually exclusive.

The huge value of mypy-primer is that it can detect problems that we don't even know are problems yet. But the disadvantage is we'd instantly lose coverage if a certain project decided to move from using requests to HTTPie (or whatever). We have more control over the regression tests in the test_cases directory.

[rpdelaney]

downforeveryone is Apache 2.0 licensed. I don't know how mypy_primer works, but can't you check out a package at a particular commit? Or just copy-paste in the code you need to check the regression.

[AlexWaygood]

downforeveryone is Apache 2.0 licensed. I don't know how mypy_primer works, but can't you check out a package at a particular commit? Or just copy-paste in the code you need to check the regression.

I think we probably all agree that adding downforeverone to mypy-primer would be both Possible And Good — @Akuli's filed hauntsaninja/mypy_primer#33 to track that :)

What we're debating is whether to also add regression tests to typeshed itself via our fairly new test_cases directory. We currently have a smattering of test cases for the stdlib stubs, but no test cases as yet for the third-party stubs typeshed maintains.