Fix crash due to runtime import of typing_extensions

[gschaffner]

hi! the issue fixed in #2507 was re-introduced when #2477 was merged.

[codecov]

Codecov Report

Merging #2537 (4bb011e) into master (4286063) will increase coverage by 1.06%.
The diff coverage is n/a.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #2537      +/-   ##
==========================================
+ Coverage   92.44%   93.50%   +1.06%     
==========================================
  Files         118      118              
  Lines       16336    16335       -1     
  Branches     3157     3156       -1     
==========================================
+ Hits        15101    15274     +173     
+ Misses       1105      955     -150     
+ Partials      130      106      -24     

Impacted Files	Coverage Δ
trio/_core/_run.py	98.92% <ø> (+0.42%)	⬆️
trio/_dtls.py	96.54% <0.00%> (+0.38%)	⬆️
trio/tests/test_subprocess.py	96.96% <0.00%> (+0.55%)	⬆️
trio/_socket.py	95.68% <0.00%> (+0.78%)	⬆️
trio/_core/tests/test_io.py	99.29% <0.00%> (+1.05%)	⬆️
trio/tests/test_socket.py	98.21% <0.00%> (+1.13%)	⬆️
trio/tests/test_threads.py	100.00% <0.00%> (+1.56%)	⬆️
trio/_highlevel_socket.py	98.19% <0.00%> (+1.80%)	⬆️
trio/tests/test_highlevel_open_tcp_stream.py	100.00% <0.00%> (+1.83%)	⬆️
... and 9 more

[pquentin]

@harahu Do you think we could add a test to avoid introducing this a third time?

[harahu]

Sorry for not picking up on this!

@pquentin My suggestion would be to, rather than adding a test, I'd make sure that dynamic analysis in CI (testing, that is) is run in an environment that is as minimal as possible. That could be achieved by separating static and dynamic analysis.

To be a bit more concrete: In my understanding, this problem was camouflaged because typing-extensions is listed in test-requirements.*. typing-extensions is a test requirement because, in this project, "test" refers both to dynamic tests, but also static analysis, like running mypy.

If we break this up into a new static-analysis-requirements.* alongside a trimmed down test-requirements.*, and we make sure pytest is run with only test-requirements.* installed, we are more likely to detect problems like this.

I'm still getting to know this project, so I don't know if my understanding is correct, so please share your thoughts.

[pquentin]

Yes that's right. We could have mypy-requirements.txt with contents like:

-r test-requirements.txt
typing-extensions=...

[harahu]

Alternatively, if flake8 is run in CI, there's a neat plugin that maybe would have caught this: https://pypi.org/project/flake8-requirements/