homepage

Right now, when a debugger is active, the number of local variables can affect the tracing speed quite a lot.
For instance, having tracing setup in a program such as the one below takes 4.64 seconds to run, yet, changing all the variables to have the same name -- i.e.: change all assignments to `a = 1` (such that there's only a single variable in the namespace), it takes 1.47 seconds (in my machine)... the higher the number of variables, the slower the tracing becomes.
```
import time
t = time.time()
def call():
 a = 1
 b = 1
 c = 1
 d = 1
 e = 1
 f = 1
def noop(frame, event, arg):
 return noop
import sys
sys.settrace(noop)
for i in range(1_000_000):
 call()
print('%.2fs' % (time.time() - t,))
```
This happens because `PyFrame_FastToLocalsWithError` and `PyFrame_LocalsToFast` are called inside the `call_trampoline` (https://github.com/python/cpython/blob/master/Python/sysmodule.c#L946).
So, I'd like to simply remove those calls.
Debuggers can call `PyFrame_LocalsToFast` when needed -- otherwise mutating non-current frames doesn't work anyways. As a note, pydevd already has such a call: https://github.com/fabioz/PyDev.Debugger/blob/0d4d210f01a1c0a8647178b2e665b53ab113509d/_pydevd_bundle/pydevd_save_locals.py#L57 and PyPy also has a counterpart.
As for `PyFrame_FastToLocalsWithError`, I don't really see any reason to call it at all.
i.e.: something as the code below prints the `a` variable from the `main()` frame regardless of that and I checked all pydevd tests and nothing seems to be affected (it seems that accessing f_locals already does this: https://github.com/python/cpython/blob/cb9879b948a19c9434316f8ab6aba9c4601a8173/Objects/frameobject.c#L35, so, I don't see much reason to call it at all).
```
def call():
 import sys
 frame = sys._getframe()
 print(frame.f_back.f_locals)
 
def main():
 a = 1
 call()
 
if __name__ == '__main__':
 main()
```
So, the idea is removing those lines (I expect that I'll be able to provide a pull request for this).