Tracking down memory leaks?

[Jean-Paul Calderone]

On 12 Feb 2006 10:13:02 -0800, "dmsbox2000-list1 at yahoo.com" <dstark7 at gmail.com> wrote:
>>> I'm not an expert on python internals, and it is possible that they have
>>> a way of checking for cases like this.   I think the deepcopy method
>>> catches this, but I don't *think* basic garbage collection look for this
>>> sort of thing.
>>
>> deepcopy has nothing to do with garbage collection.
>>
>> This is where you use deepcopy:
>>
>> py> a = [2, 4, [0, 1, 2], 8]  # note the nested list
>> py> b = a         # b and a both are bound to the same list
>> py> b is a        # b is the same list as a, not just a copy
>> True
>> py> c = a[:]      # make a shallow copy of a
>> py> c is a        # c is a copy of a, not a itself
>> False
>> py> c[2] is a[2]  # but both a and c include the same nested list
>> True
>>
>> What if you want c to include a copy of the nested list? That's where you
>> use deepcopy:
>>
>> py> d = copy.deepcopy(a)
>> py> d[2] is a[2]
>> False
>
>What I ment is that deepcopy is recursive, and if you have a circular
>reference in your data structure a recursive copy will become infinite.
> I think deepcopy has the ability to detect this situation.  So if it
>could be detected for deepcopy, I don't see why it could not be
>detected for garbage collection purposes.

It's moot, since the garbage collector can collect cycles:

  Make a cycle:

    >>> a = []
    >>> b = []
    >>> a.append(b)
    >>> b.append(a)

  Get rid of all references to all objects participating in it:

    >>> del a, b

  Explicitly invoke the garbage collector in order to observe the number of objects it cleans up:

    >>> gc.collect()
    2

Jean-Paul