[Python-Dev] sys.settrace: behavior doesn't match docs (original) (raw)

Ned Batchelder ned at nedbatchelder.com
Sun May 1 00:49:11 CEST 2011

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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