[Python-ideas] Idea: Allow multiple levels of tracers

[Ned Batchelder]

Perhaps I misunderstand what's implied by "simple(!) monkeypatch of 
sys.settrace", but the trickiest part of Ram's proposal is that the body 
of one trace function would still trigger the remaining trace 
functions.  That to me sounds like it's going to require changes to ceval.c

--Ned.

On 4/25/19 12:26 PM, Ram Rachum wrote:
> Hmm, looks like, for this to work, you'll need the existing tracer to 
> be cooperative. Right now there are existing tracers, for example 
> coverage's tracer and Wing IDE's tracer, and I would need to modify 
> them for your idea to work, right?
>
> If I understand your idea correctly, the first tracer would 
> monkeypatch `sys.settrace` so whenever someone else adds a tracer, it 
> doesn't really do `sys.settrace` but just add a function that the real 
> tracer would be calling after it's done tracing. But this can't really 
> be done without the original tracer implementing it, right?
>
> On Thu, Apr 25, 2019 at 6:16 PM Ram Rachum <ram at rachum.com 
> <mailto:ram at rachum.com>> wrote:
>
>     Oh wow, I didn't even consider that. I think you're right, I'll do
>     more thinking about this. Thanks Anders!
>
>     On Thu, Apr 25, 2019 at 6:10 PM Anders Hovmöller
>     <boxed at killingar.net <mailto:boxed at killingar.net>> wrote:
>
>         Can't this be implemented today by a simple monkey patch of
>         sys.settrace?
>
>         On 25 Apr 2019, at 16:51, Ram Rachum <ram at rachum.com
>         <mailto:ram at rachum.com>> wrote:
>
>>         Hi,
>>
>>         Here's something I want in Python: Multiple levels of tracers
>>         working on top of each other, instead of just one.
>>
>>         I'm talking about the tracer that one can set by calling
>>         sys.settrace.
>>
>>         I've recently released PySnooper:
>>         https://github.com/cool-RR/PySnooper/
>>
>>         One of the difficulties I have, is that I can't debug or run
>>         the `coverage` tool on the core of this module. That's
>>         because the core is a trace function, and debuggers and
>>         coverage tools work by setting a trace function. When
>>         PySnooper sets its trace function using `sys.settrace`, the
>>         code that runs in that trace function runs without getting
>>         traced by the coverage tracer.
>>
>>         This means that people who develop debuggers and coverage
>>         tools can't use a debugger or a coverage tool on the core of
>>         their tool. It's quite an annoying problem.
>>
>>         My proposed solution: Multiple levels of tracing, instead of
>>         just one. When you install a tracer, you're not replacing the
>>         existing one, you're appending a tracer to the existing list
>>         of tracers.
>>
>>         If this was implemented, then when PySnooper would install
>>         its tracer, the coverage tracer would still be active and
>>         running, for every line of code including the ones in
>>         PySnooper's tracer.
>>
>>         Obviously, we'll need to figure out the API and any other
>>         kind of problems with this proposal.
>>
>>         What do you think?
>>
>>
>>         Thanks,
>>         Ram.
>>
>>
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