New submission from Ned Batchelder <[email protected]>:
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.
----------
components: Library (Lib)
messages: 135089
nosy: nedbat
priority: normal
severity: normal
status: open
title: sys.settrace doesn't disable tracing if a local trace function returns
None
type: behavior
versions: Python 2.7, Python 3.2
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Python tracker <[email protected]>
<http://bugs.python.org/issue11992>
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