[Python-ideas] Passive tracing of function objects (was: Efficient debug logging)

C Anthony Risinger anthony at xtfx.me
Wed Feb 15 11:45:10 EST 2017

On Tue, Feb 14, 2017 at 3:55 PM, Barry <barry at barrys-emacs.org> wrote:
> The point is that the cost of creating the msg argument can be very high.
> At the point that logging decides to skip output it is to late to save the
> cost of creating the arg tuple.

Awhile back I was playing with some code for the purpose of adding debug
logs "after the fact":


I'm not trying to plug it (It's POC, broken, and Python 2) but maybe there
is some value because I was trying to solve a similar problem of fast and
easy debugging and logs.

The idea here is we target a function for "retrospection" by replacing it's
code object with a modified variant that implements call outs at different
points. Such interesting points might be "on symbol table changes" (new
vars) or "on identifier rebinding" (updating vars) or "on line changes" or
even "every single tick" to inspect TOS.

I like this approach of latching onto the function/code object directly
because it does not require a wrapper, but it can have be tricky with
decorators depending on what the decorator does with the function it
receives (we need access to the function object to replace it's __code__).

I don't much like writing logs. They are verbose in code and almost always
either much more or less than I need to deal with the problem at hand, and
it's not always clear what's important ahead-of-time. With all the
implementation details in play, you might end up wrapping code in context
managers and whatnot for the sole purpose of logging. Or you have to jump
thru hoops to get the exact info you need using many log levels or flags. I
think having a way to target functions for real time introspection, then
perfectly disable it, could allow for interesting solution to large,
cross-cutting concerns like application logging.

Is there any interest in this? Say a new function method like
fun.trace(vars=[a,b,c])? Is there already a solution or past discussion on
this topic?



C Anthony
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