[Python-bugs-list] [Bug #126254] Traceback objects not properly garbage-collected

[noreply@sourceforge.net]

Bug #126254, was updated on 2000-Dec-18 17:50
Here is a current snapshot of the bug.

Project: Python
Category: Python Interpreter Core
Status: Closed
Resolution: Invalid
Bug Group: Not a Bug
Priority: 5
Submitted by: nobody
Assigned to : gvanrossum
Summary: Traceback objects not properly garbage-collected

Details: System info:
============

  Python 2.0 (#1, Dec 18 2000, 16:47:02) 
  [GCC 2.95.2 20000220 (Debian GNU/Linux)] on linux2

  Linux phil 2.2.18 #1 Mon Dec 18 14:49:56 PST 2000 i686 unknown

Sample code:
============

    import sys
    
    class fooclass:
        def __init__(self):
            print 'CONSTRUCTED'
    
        def withtb(self, doit=0):
            try:
                raise "foo"
            except:
                if doit:
                    tb = sys.exc_info()[2]
        
        def __del__(self):
            print 'DESTROYED'
    
    if __name__ == '__main__':
        foo = fooclass()
        if len(sys.argv) > 1:
            foo.withtb(1)
        else:
            foo.withtb(0)
        del foo

How to reproduce:
=================

  Run the above python script:

   1. Without any argument: the withtb() method exception handler does
      not retrieve any traceback object. The program prints
      `CONSTRUCTED' and `DESTROYED'.

   2. With some arguments: the withtb() method exception handler
      retrieves a traceback object and stores it in the `tb' local
      variable. However `DESTROYED' never gets printed out. I think
      that the `foo' object will never be garbage collected anymore.

Workaround:
===========

   Deleting the `tb' object seems to restore things:

      if doit:
          tb = sys.exc_info()[2]
          del tb

Other:
======

   I've found this problem also in python 1.5.2 and python 1.6.

Possible cause:
===============

   I would tend to think that we're creating a circular loop which
   cannot be garbage collected:

       - `tb' holds a reference to the traceback object
       - the traceback object holds a reference to the local scope
       - the local scope holds a reference to the `tb' variable

   The only way out is to break the circular reference by hand,
   although it's annoying.

Phil - phil@commerceflow.com.


Follow-Ups:

Date: 2000-Dec-27 14:21
By: gvanrossum

Comment:
Note that the bug report was about 1.5.2 and 1.6.  This should be fixed by
the cycle collection in 2.0, shouldn't it?

-------------------------------------------------------

Date: 2000-Dec-20 18:59
By: tim_one

Comment:
"Invalid" is a just a word -- it comes with SF bug system and isn't defined
anywhere.  By convention, we pair Not-A-Bug with Invalid, for lack of
something better to do.  Not-A-Bug means it's not a bug <wink>:  you may
not like the answer, but Guido is saying it's functioning as designed and
he has no plans to change that.
-------------------------------------------------------

Date: 2000-Dec-20 18:50
By: nobody

Comment:
oops, didn't see you comment.
Forget about my question...

Phil - phil@commerceflow.com. 
-------------------------------------------------------

Date: 2000-Dec-20 18:49
By: nobody

Comment:
Could I know why this was deemed to be `Invalid' ?

Phil - phil@commerceflow.com.

-------------------------------------------------------

Date: 2000-Dec-18 20:21
By: gvanrossum

Comment:
This is not a bug.  Saving the traceback as a local variable creates a
circular reference that prevents garbage collection.

If you don't understand this answer, please write help@python.org.

-------------------------------------------------------

For detailed info, follow this link:
http://sourceforge.net/bugs/?func=detailbug&bug_id=126254&group_id=5470