[Python-Dev] sys.settrace: behavior doesn't match docs

[Ned Batchelder]

Indeed, the 2.0 code is very different, and got this case right.

I'm a little surprised no one is arguing that changing this code now 
could break some applications.  Maybe the fact no one noticed the docs 
were wrong proves that no one ever tried returning None from a local 
trace function.

--Ned.

On 4/30/2011 8:43 PM, Guido van Rossum wrote:
> I think you need to go back farther in time. :-) In Python 2.0 the
> call_trace function in ceval.c has a completely different signature
> (but the docs are the same). I haven't checked all history but
> somewhere between 2.0 and 2.3, SET_LINENO-less tracing was added, and
> that's where the implementation must have gone wrong. So I think we
> should fix the code.
>
> --Guido
>
> On Sat, Apr 30, 2011 at 3:49 PM, Ned Batchelder<ned at nedbatchelder.com>  wrote:
>> This week I learned something new about trace functions (how to write a C
>> trace function that survives a sys.settrace(sys.gettrace()) round-trip), and
>> while writing up what I learned, I was surprised to discover that trace
>> functions don't behave the way I thought, or the way the docs say they
>> behave.
>>
>> The docs say:
>>
>> The trace function is invoked (with event set to 'call') whenever a new
>> local scope is entered; it should return a reference to a local trace
>> function to be used that scope, or None if the scope shouldn’t be traced.
>>
>> The local trace function should return a reference to itself (or to another
>> function for further tracing in that scope), or None to turn off tracing in
>> that scope.
>>
>> It's that last part that's wrong: returning None from the trace function
>> only has an effect on the first call in a new frame.  Once the trace
>> function returns a function for a frame, returning None from subsequent
>> calls is ignored.  A "local trace function" can't turn off tracing in its
>> scope.
>>
>> To demonstrate:
>>
>> import sys
>>
>> UPTO_LINE = 1
>>
>> def t(frame, event, arg):
>>      num = frame.f_lineno
>>      print("line %d" % num)
>>      if num<  UPTO_LINE:
>>          return t
>>
>> def try_it():
>>      print("twelve")
>>      print("thirteen")
>>      print("fourteen")
>>      print("fifteen")
>>
>> UPTO_LINE = 1
>> sys.settrace(t)
>> try_it()
>>
>> UPTO_LINE = 13
>> sys.settrace(t)
>> try_it()
>>
>> Produces:
>>
>> line 11
>> twelve
>> thirteen
>> fourteen
>> fifteen
>> line 11
>> line 12
>> twelve
>> line 13
>> thirteen
>> line 14
>> fourteen
>> line 15
>> fifteen
>> line 15
>>
>> The first call to try_it() returns None immediately, preventing tracing for
>> the rest of the function.  The second call returns None at line 13, but the
>> rest of the function is traced anyway.  This behavior is the same in all
>> versions from 2.3 to 3.2, in fact, the 100 lines of code in sysmodule.c
>> responsible for Python tracing functions are completely unchanged through
>> those versions.  (A deeper mystery that I haven't looked into yet is why
>> Python 3.x intersperses all of these lines with "line 18" interjections.)
>>
>> I'm writing this email because I'm not sure whether this is a behavior bug
>> or a doc bug.  One of them is wrong, since they disagree.  The documented
>> behavior makes sense, and is what people have all along thought the trace
>> function did.  The actual behavior is a bit more complicated to explain, but
>> is what people have actually been experiencing.  FWIW, PyPy implements the
>> documented behavior.
>>
>> Should we fix the code or the docs?  I'd be glad to supply a patch for
>> either.
>>
>> --Ned.
>>
>>
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>
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