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From: Antoine Pitrou <solipsis@pitrou.net>
Subject: Re: What does =?utf-8?b?Z2MuZ2V0X29iamVjdHMoKQ==?= return?
Date: Mon, 17 Mar 2014 17:18:05 +0000 (UTC)
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Xref: csiph.com comp.lang.python:68439

Chris Angelico <rosuav <at> gmail.com> writes:
> 
> It's not strictly an implementation detail, beyond that there are
> certain optimizations. For instance...
> 
> >   For CPython 3.4 I guess strings and other atomic types such as ints are
> > not, as well as raw object() instances. Custom class instances on the other
> > hand seem to be under GC control.
> 
> ... strings and ints should never be listed, and custom objects should
> always be listed, but I'd say the non-tracking of object() would be an
> implementation-specific optimization.

These are all implementation details, tied to the fact that the primary
object reclaim mechanism in CPython is reference counting. Other 
implementations may use a full GC and gc.get_objects() may then also
return strings and other "atomic" objects (but the implementation may
also elicit to hack get_objects() in order to closely mimick CPython).

All in all, though, gc.get_objects() is an expensive function call (it
will walk the entire graph of objects tracked by the GC, which can be very
large in non-trivial applications), so it's really only useful for
debugging (and, I'd add, for low-level debugging). In most situations,
gc.get_objects() is certainly the wrong tool to use.

Regards

Antoine.