[Python-ideas] Adding a frame evaluation API to CPython

[Devin Jeanpierre]

On Mon, May 16, 2016 at 1:19 PM, Dino Viehland <dinov at microsoft.com> wrote:
> It also opens up the possibility of debugging where the frame evaluation function only performs special debugging work when it detects it is about to execute a specific code object. In that instance the bytecode could be theoretically rewritten in-place to inject a breakpoint function call at the proper point for help in debugging while not having to do a heavy-handed approach as required by sys.settrace().

To add to the debugging story: there are many debugging/profiling
tools that look at C symbol names. When you have a mixed C/Python
codebase, too often a problem straddles both sides -- the Python tools
don't tell you anything about the C end, and the C tools just tell you
that everything happened in PyEval_EvalFrameEx.

Instead of plugging in a JIT-compiled version of a code object, one
can plug in a shim, so that each code object has a function with a
unique symbol name that only calls PyEval_EvalFrameDefault. This way,
you get symbol names for all Python functions. So while it still says
that 90% CPU is used by PyEval_EvalFrameEx, it also then that breaks
down into 12% being used by PY_SYMBOL__json·load or similar, which is
a symbol corresponding to the code object for the Python function
json.load.

I'm +1 for something that makes this possible, although for the
specific API proposal here, no real opinions.

-- Devin