classification
Title: destructors of local variables are not traced
Type: behavior Stage: patch review
Components: Interpreter Core, Library (Lib) Versions: Python 3.8, Python 3.7, Python 3.6
process
Status: open Resolution:
Dependencies: Superseder:
Assigned To: Nosy List: serhiy.storchaka, xdegaye
Priority: normal Keywords: patch

Created on 2018-05-08 18:29 by xdegaye, last changed 2018-05-08 19:00 by serhiy.storchaka.

Pull Requests
URL Status Linked Edit
PR 6730 open xdegaye, 2018-05-08 18:45
Messages (2)
msg316290 - (view) Author: Xavier de Gaye (xdegaye) * (Python triager) Date: 2018-05-08 18:29
In the following code, the destructors of objects referenced by the 'a' and 'b' local variables are not traced by the 'step' command of pdb.

 1 class C:
 2     def __init__(self, name):
 3         self.name = name
 4
 5     def __del__(self):
 6         print('"%s" destructor' % self.name)
 7
 8 def main():
 9     a = C('a')
10     b = C('b')
11     import pdb; pdb.set_trace()
12     a = 1
13
14 main()
msg316292 - (view) Author: Xavier de Gaye (xdegaye) * (Python triager) Date: 2018-05-08 18:49
In both cases the destructor cannot be traced because it is invoked from functions called from [1] call_trace() where tracing is disabled:

  Case local variable 'a':
    On line 12, just before executing this line with a pdb step command, the object referenced by 'a' lives in frame->f_locals. The step command causes the ceval loop to execute the bytecodes corresponding to the statement on line 12 and to eventualy call [2] call_trampoline() with a 'return' trace event and to call PyFrame_FastToLocalsWithError() here. This last call causes the last reference to the previous 'a' object in frame->f_locals to be decremented and the object to be deallocated but without its destructor being traced as tracing has been disabled in call_trace().

  Case local variable 'b':
    Upon exiting the frame of the 'main' function, pdb keeps a reference to frame->f_locals through its own attribute curframe_locals.  Next, after returning to the 'main' caller, this reference is decremented since pdb.curframe_locals references now another object and the dictionary referenced by the previous frame f_locals (the frame of 'main') is deallocated, but this happens within pdb where tracing is disabled and the destructor of the object that had been referenced by 'b' is therefore not traced.

PR 6730 proposes a fix for both cases.

[1] https://github.com/python/cpython/blob/master/Python/ceval.c#L4247
[2] https://github.com/python/cpython/blob/master/Python/sysmodule.c#L462
History
Date User Action Args
2018-05-08 19:00:03serhiy.storchakasetnosy: + serhiy.storchaka
2018-05-08 18:49:55xdegayesetmessages: + msg316292
2018-05-08 18:45:41xdegayesetkeywords: + patch
stage: patch review
pull_requests: + pull_request6422
2018-05-08 18:29:15xdegayecreate