[issue16672] improve tracing performances when f_trace is NULL

[Xavier de Gaye]

Xavier de Gaye added the comment:

It is not possible to improve the performances of the trace function set with
sys.settrace without breaking backward compatibility for PyEval_SetTrace or
without introducing a new PyEval_xxx of some sort.

Yes, I suggest to revert this patch.

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Python tracker <report at bugs.python.org>
<http://bugs.python.org/issue16672>
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