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

[Barry Scott]

> On 25 Apr 2019, at 15:51, Ram Rachum <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/ <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? 

Personally I would look to other means to get the coverage report for a tracing tool
or debugger.

For example why not use unittesting and mocking to allow the trace code to be run
and measured? After all you only have to mock for one functions interface.

As for debugging I would use print() or logging to find what I need.

Barry

> 
> 
> Thanks,
> Ram.
> 
> 
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