Bugs item #1553819, was opened at 2006-09-06 23:26
Message generated for change (Comment added) made by peterdonis
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Category: Python Interpreter Core
Group: Python 2.4
Status: Open
Resolution: None
Priority: 5
Submitted By: Peter Donis (peterdonis)
Assigned to: Nobody/Anonymous (nobody)
Summary: Class instance apparently not destructed when expected
Initial Comment:
When an instance variable of a class with the same
name as a class variable in a base class is assigned
a value (making the class variable of the base class
invisible), the class instance does not appear to be
destructed when it should.
Here is the simplest test script I've been able to
come up with that reproduces the error, along with
its output when run from a shell prompt. I've
included the dir() commands to make sure that the
variable referencing the class instance is in fact
deleted in both cases. As you can see, the instance
of the base class gets destructed as expected, but
the instance of the derived class does not.
--- Test script ---
#!/usr/bin/env python
# Test script to see when objects are freed
class Test(object):
testfield = None
def __init__(self):
print "Initializing test object."
def __del__(self):
print "Freeing test object."
class Test2(Test):
def __init__(self):
# This masks Test.testfield
self.testfield = self.meth
Test.__init__(self)
def meth(self):
pass
print dir()
t = Test()
print dir()
del t
print dir()
t2 = Test2()
print dir()
del t2
print dir()
--- Output ---
$ python deltest.py
['Test', 'Test2', '__builtins__', '__doc__', '__file__', '__name__', 'func']
Initializing test object.
['Test', 'Test2', '__builtins__', '__doc__', '__file__', '__name__', 'func',
't']
Freeing test object.
['Test', 'Test2', '__builtins__', '__doc__', '__file__', '__name__', 'func']
Initializing test object.
['Test', 'Test2', '__builtins__', '__doc__', '__file__', '__name__', 'func',
't2']
['Test', 'Test2', '__builtins__', '__doc__', '__file__', '__name__', 'func']
----------------------------------------------------------------------
>Comment By: Peter Donis (peterdonis)
Date: 2006-09-07 23:05
Message:
Logged In: YES
user_id=1592444
mwh's comment is correct. See attached script deltest2.py,
showing the contents of gc.garbage and how the reference
cycle can be cleared (script output shown below). Note that
I eliminated the base class and the masking of its class
field; that was a red herring. My only other question is
whether it might be worthwhile to add a sentence or two to
the description of bound method objects in the
documentation to make it clearer that you're creating a
reference to the class instance.
--- script output ---
$ python deltest2.py
['Test', '__builtins__', '__doc__', '__file__', '__name__',
'gc']
Initializing test object.
['Test', '__builtins__', '__doc__', '__file__', '__name__',
'gc', 't']
['Test', '__builtins__', '__doc__', '__file__', '__name__',
'gc']
[]
gc: uncollectable <Test 0xb7c3f34c>
gc: uncollectable <dict 0xb7c4c1c4>
gc: uncollectable <instancemethod 0xb7cb86bc>
3
[<__main__.Test object at 0xb7c3f34c>, {'testfield': <bound
method Test.meth of <__main__.Test object at 0xb7c3f34c>>},
<bound method Test.meth of <__main__.Test object at
0xb7c3f34c>>]
[]
0
['Test', '__builtins__', '__doc__', '__file__', '__name__',
'g', 'gc']
Freeing test object.
----------------------------------------------------------------------
Comment By: Michael Hudson (mwh)
Date: 2006-09-07 10:06
Message:
Logged In: YES
user_id=6656
It's a reference cycle featuring an object with a __del__ --> it ends up in
gc.garbage.
More coffee for Neal?
----------------------------------------------------------------------
Comment By: Neal Norwitz (nnorwitz)
Date: 2006-09-07 03:19
Message:
Logged In: YES
user_id=33168
The attached variant puts this into a function and shows ref
leaks. It requires a debug build.
----------------------------------------------------------------------
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