Some things we can learn from pytypes

[gvanrossum]

I checked the profiling hook in pytypes, and they do two things we should also do:

• save and restore the previously active profiler
• in a thread, when the profiling callback is run after profiling has been stopped, reset the hook (note: this should only be done after it's been stopped -- not after it's been paused)

[gvanrossum]

Moreover if a previous profiler is already running they pass events to that too!

[gvanrossum]

Lots of other stuff too, e.g. their utilities for finding the actual function. (OTOH I don't see any downsampling or other speed hacks in their code.)

[Stewori]

Yes, the downsampling idea is something we can learn from pyannotate. I will surely add this.