Python leaks in cyclic garbage collection

[moerchendiser2k3]

Hi, I have some problems with Python and the garbage collection. In
the following piece of code I create a simple gargabe collection but I
am still wondering why the finalizers are never called - at least on
exit of Py they should be called somehow. What do I miss here? I know,
there is no deterministic way how to resolve this though.

class Foo():

def __init__(self):
self.b=Bar(self)

def __del__(self):
print "Free Foo"

class Bar():
def __init__(self, f):
self.f=f

def __del__(self):
print "Free Bar"

f=Foo()
print f

 

[moerchendiser2k3]

Thanks a lot for any help!!!

Bye,
moerchendiser2k3

 

[Chris Rebert]

Hi, I have some problems with Python and the garbage collection. In
the following piece of code I create a simple gargabe collection but I
am still wondering why the finalizers are never called - at least on
exit of Py they should be called somehow. What do I miss here?

<code snippet involving circularly-referenced objects with __del__
methods snipped>

Read The Fine Manual (all emphases added):

http://docs.python.org/reference/datamodel.html#object.__del__ :
"***It is not guaranteed that __del__() methods are called for objects
that still exist when the interpreter exits.***"
"Note: [...] x.__del__() — [...] is only called when x‘s reference
count reaches zero. Some common situations that may prevent the
reference count of an object from going to zero include: circular
references between objects [...] Circular references which are garbage
are detected when the option cycle detector is enabled (it’s on by
default), ***but can only be cleaned up if there are no Python-level
__del__() methods involved***. Refer to the documentation for the `gc`
module for more information about how __del__() methods are handled by
the cycle detector, particularly the description of the `garbage`
value."

And following the pointer to gc.garbage's docs:
http://docs.python.org/library/gc.html#gc.garbage :
"Objects that have __del__() methods and are part of a reference cycle
***cause the entire reference cycle to be uncollectable*** [...]
Python doesn’t collect such cycles automatically because, in general,
it isn’t possible for Python to guess a safe order in which to run the
__del__() methods. [...] It’s generally better to avoid the issue by
not creating cycles containing objects with __del__() methods"

Cheers,
Chris

 

[Steven D'Aprano]

And following the pointer to gc.garbage's docs:
http://docs.python.org/library/gc.html#gc.garbage : "Objects that have
__del__() methods and are part of a reference cycle ***cause the entire
reference cycle to be uncollectable*** [...] Python doesn’t collect such
cycles automatically because, in general, it isn’t possible for Python
to guess a safe order in which to run the __del__() methods. [...] It’s
generally better to avoid the issue by not creating cycles containing
objects with __del__() methods"

Another solution is to manually break the cycle, perhaps by providing a
"close" method, and requiring the caller to use it.

 

[Andrea Crotti]

Il giorno 19/feb/2011, alle ore 05.10, moerchendiser2k3 ha scritto:

Hi, I have some problems with Python and the garbage collection. In
the following piece of code I create a simple gargabe collection but I
am still wondering why the finalizers are never called - at least on
exit of Py they should be called somehow. What do I miss here? I know,
there is no deterministic way how to resolve this though.

class Foo():

def __init__(self):
self.b=Bar(self)

def __del__(self):
print "Free Foo"

class Bar():
def __init__(self, f):
self.f=f

def __del__(self):
print "Free Bar"

f=Foo()
print f

Wild guess:
maybe when python exits they are called but sys.stdout has already been closed and nothing gets written on it anymore.

 

[moerchendiser2k3]

Thanks for your answers! They really helped me out!!

 

[n00m]

Wild guess:

maybe when python exits they are called but sys.stdout has already been closed and nothing gets written on it anymore.

Certainly NOT.


class Foo():
def __init__(self):
self.b = Bar(self)
def __del__(self):
print "Free Foo"

class Bar():
def __init__(self, f):
self.f = f
def __del__(self):
print "Free Bar"


f = Foo()
print f.b.f.b.f.b.f.b.f.b.f.b.f.b.f.b.f
print f.b.f.b.f.b.f.b.f.b.f.b.f.b.f.b.f.b



*********Console output:***********

<__main__.Foo instance at 0x00AF7328>
<__main__.Bar instance at 0x00AF7350>

D:\v3>