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

Anders Hovmöller boxed at killingar.net
Thu Apr 25 11:10:01 EDT 2019


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> 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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