Prevent wild assignment in DEBUG mode only

[asvetlov]

For sake of performance

Related to #2834

[codecov-io]

Codecov Report

Merging #2860 into master will increase coverage by <.01%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master    #2860      +/-   ##
==========================================
+ Coverage   97.97%   97.97%   +<.01%     
==========================================
  Files          39       39              
  Lines        7444     7447       +3     
  Branches     1307     1309       +2     
==========================================
+ Hits         7293     7296       +3     
  Misses         48       48              
  Partials      103      103

Impacted Files	Coverage Δ
aiohttp/web_app.py	99.08% <100%> (ø)	⬆️
aiohttp/web_request.py	99.69% <100%> (ø)	⬆️
aiohttp/helpers.py	96.83% <100%> (ø)	⬆️

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update b432ab3...9cf70d4. Read the comment docs.

[spcharc]

I like it.
but would this check impact performance?
of course disabling it would be better, I just wonder the extent

[asvetlov]

Yes, technically it is a Python function call on every internal attribute assignment.
Very cheap if the check is implemented by CPython C/Cython but quite expensive if __setattr__ is a pure Python function.

[lock]

This thread has been automatically locked since there has not been any recent activity after it was closed. Please open a [new issue] for related bugs.
If you feel like there's important points made in this discussion, please include those exceprts into that [new issue].
[new issue]: https://github.com/aio-libs/aiohttp/issues/new