Hi all,
Excuse for TOFU. Thanks a lot Steven, Dave and Hugo.
Steven the explanation was really great. Thanks a lot for it.
Hugo, I was just curious, have no real need. Thanks.
With warm regards,
-Payal
--
On Tue, Jun 08, 2010 at 08:07:28PM +1000, Steven D'Aprano wrote:
> On Tue, 8 Jun 2010 04:08:15 pm Payal wrote:
> > Hi,
> > If I have a list (or a dict), is there any way of knowing how many
> > other variables are referencing the same object?
>
> Sort of. The question is simple, but the answer isn't. It depends what
> you mean by "variables", and it requires a good understanding of
> Python's programming model.
>
> Python doesn't have "variables" like C or Pascal, it has names and
> objects. Objects can be bound to no names at all:
>
> len([1,2,4])
>
> or to a single name:
>
> x = [1,2,4]
> len(x)
>
> or to multiple names:
>
> x = y = z = [1,2,4]
> w = y
> len(w)
>
>
> Objects can also be referenced by other objects, which in turn could
> have zero, one or more names. Consider this example:
>
> >>> a = [1,2,4]
> >>> b = a
> >>> c = {None: ('xyz', b)}
> >>> class K:
> ... pass
> ...
> >>> d = K()
> >>> d.attr = [c]
>
>
> Given your question, how many "variables" refer to the list [1,2,4]?,
> what answer would you expect? Depending on how I count them, I get
> either 4, 5 or 6:
>
> 4:
> "variables" (names) a, b, c and d
>
> 5:
> the dict globals() has two references to the list, using keys 'a'
> and 'b';
> the tuple ('xyz', b);
> the dict with key None and value the above tuple;
> the list [c];
> the instance d has a dict __dict__ with key 'attr'
>
> 6:
> same as five, but counting globals() twice
>
>
> Interestingly, Python has a standard tool for tracking referrers, the gc
> (garbage collector) module, and it disagrees with all of those counts:
>
> >>> import gc
> >>> len(gc.get_referrers(a))
> 2
> >>> print gc.get_referrers(a)
> [('xyz', [1, 2, 4]), {'a': [1, 2, 4], 'c': {None: ('xyz', [1, 2,
> 4])}, 'b': [1, 2, 4], 'd': <__main__.C instance at 0xb7f6008c>, 'gc':
> <module 'gc' (built-in)>, '__builtins__': <module '__builtin__'
> (built-in)>, 'C': <class __main__.C at
> 0xb7d0602c>, '__name__': '__main__', '__doc__': None}]
>
> So gc says two objects *directly* refer to the list: the tuple, and
> globals(). But of course there are multiple *indirect* references to
> the list as well. So the answer you get depends on the way you ask it.
>
>
> --
> Steven D'Aprano
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