Add support for strings and __future__ annotations

[dmontagu]

This pull request adds support for forward references, string annotations, and from __future__ import annotations in decorated endpoint signatures.

[tiangolo]

Nice! Good job!

This PR would introduce a small breaking change [...]

No problem with that. We bump the version to release it.

Before this is ready to merge [...]

Perfect. Let me know when you can check those things.

[dmontagu]

@tiangolo Thanks for the feedback!

I actually discussed this with Ivan Levkivskyi on the python/typing gitter, and he suggested I make a bpo post (which I did here: https://bugs.python.org/issue37923). So far it hasn't gotten much attention 😕 but I figured I'd at least ask the question.

Having dug a little more into the various related PEPs and python stdlib source, I feel pretty good about the approach in this PR, so I'm inclined to run with it, and just add more edge-case handling and tests for them. Hopefully in the future there will be first class support for this built directly into the inspect or typing packages, and we can remove any custom/redundant logic.

I'll try to get this into a mergeable state this weekend.

[dmontagu]

@tiangolo I reworked the implementation, and now it should be 100% backward compatible (no changes to the annotations in the case of optionals, etc.), and only change any of the behavior when string-valued annotations are present. (And the only behavior change will be that it works instead of raising errors.)

[codecov]

Codecov Report

Merging #451 into master will not change coverage.
The diff coverage is 100%.

@@          Coverage Diff          @@
##           master   #451   +/-   ##
=====================================
  Coverage     100%   100%           
=====================================
  Files         251    251           
  Lines        5988   6001   +13     
=====================================
+ Hits         5988   6001   +13

Impacted Files	Coverage Δ
fastapi/dependencies/utils.py	100% <100%> (ø)	⬆️
tests/test_security_oauth2.py	100% <100%> (ø)	⬆️

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[codecov]

Codecov Report

Merging #451 into master will decrease coverage by 0.16%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master     #451      +/-   ##
==========================================
- Coverage     100%   99.83%   -0.17%     
==========================================
  Files         251      251              
  Lines        5988     5899      -89     
==========================================
- Hits         5988     5889      -99     
- Misses          0       10      +10

Impacted Files	Coverage Δ
fastapi/dependencies/utils.py	100% <100%> (ø)	⬆️
tests/test_security_oauth2.py	100% <100%> (ø)	⬆️
fastapi/openapi/models.py	100% <100%> (ø)	⬆️
fastapi/utils.py	100% <100%> (ø)	⬆️
tests/test_union_inherited_body.py	65.51% <0%> (-34.49%)	⬇️
...est_additional_responses_custom_validationerror.py	100% <0%> (ø)	⬆️
docs/src/app_testing/tutorial003.py	100% <0%> (ø)	⬆️
...t_query_params_str_validations/test_tutorial001.py	100% <0%> (ø)	⬆️
tests/test_custom_swagger_ui_redirect.py	100% <0%> (ø)	⬆️
docs/src/first_steps/tutorial001.py	100% <0%> (ø)	⬆️
... and 64 more

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[tiangolo]

Awesome job! 🎉 🚀

I think it's fine to leave the tests with your modifications and test both things at the same time. I just requested comments on top of them to remind us to not "refactor" them into their non-string version.

[dmontagu]

@tiangolo

• I made the requested changes.
• I also updated the # type: ignore comments to:
  1. be compatible with the recent mypy 0.730 release (which is automatically used in the build)
  2. remove unused ignores (detected by calling mypy fastapi --disallow-untyped-defs --warn-unused-ignore)
• Do you think the docs should be updated anywhere? It seems like release notes is probably enough

[tiangolo]

Awesome! Thanks a lot!

LGTM 🎉 🚀