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_warnings.c was initially created to help with startup performance. It turned out to be a tricky bit of code to get right to continue to support the Python version of the module.
But now that we live in a world where we have startup benchmarks instead of hunches and we freeze code like importlib, maybe it's time to re-evaluate whether warnings.py is such a bad thing to have as part of the startup process? I would be curious to know what the performance impact is if we made _warnings the frozen version of warnings.py instead of the C code and measured the startup performance.

I should also mention the other motivating factor was providing C access to the warnings system, but once again importlib blazed that trail already by providing a C API which simply uses the Python code.

Here are the benchmark results of freezing warnings.py as _frozen_warnings (very quick hack which doesn't use _frozen_warnings is attached). Basically -S slows down by about 3% and normal startup has no measurable impact. Not sure if that's worth it to simplify the warnings code or not.
INFO:root:Automatically selected timer: perf_counter
INFO:root:Skipping benchmark bzr_startup; not compatible with Python 3.6
INFO:root:Skipping benchmark hg_startup; not compatible with Python 3.6
Running normal_startup...
INFO:root:Running `../cpython/default/python.exe -c ` 1000 times
INFO:root:Running `../cpython/pristine/python.exe -c ` 1000 times
Running startup_nosite...
INFO:root:Running `../cpython/default/python.exe -S -c ` 2000 times
INFO:root:Running `../cpython/pristine/python.exe -S -c ` 2000 times
Report on Darwin Bretts-MacBook-Pro.local 14.5.0 Darwin Kernel Version 14.5.0: Wed Jul 29 02:26:53 PDT 2015; root:xnu-2782409~1/RELEASE_X86_64 x86_64 i386
Total CPU cores: 4
### startup_nosite ###
Min: 0.311247 -> 0.317280: 1.02x slower
Avg: 0.316682 -> 0.325945: 1.03x slower
Significant (t=-17.89)
Stddev: 0.00326 -> 0.00403: 1.2360x larger
The following not significant results are hidden, use -v to show them:
normal_startup.

After thinking about it and with _warnings.c not exactly changing very much I think I'm going to call YAGNI on ripping out _warnings.c.