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

[Ned Batchelder]

It wouldn't be difficult to have a list of trace functions, so that 
every line of "real" Python executed, would invoke all the trace 
functions.  But Ram has asked for something more: when the first trace 
function is executing, its line should themselves be traced by the 
remaining trace functions in the list.  Presumably the lines in the 
second trace function should also be traced by the function third in the 
list, and so on.  This is the thing that will be difficult to accomplish.

--Ned.

On 4/25/19 2:02 PM, Anders Hovmöller wrote:
> Well, it would trigger the top level chaining trace function, but they 
> should be able to decide when to call the sub-trace functions. Hmm... 
> Maybe :)
>
> On 25 Apr 2019, at 19:16, Ned Batchelder <ned at nedbatchelder.com 
> <mailto:ned at nedbatchelder.com>> wrote:
>
>> 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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>>>
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