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Here are comments from Guido, taken from a patch 
report that's been closed:
"""
In a discussion before lunch, Tim suggested that when 
Python exits (or better, in Py_Finalize()) a check 
could be made if there is any garbage in gc.garbage, 
and if so, a warning about this should be printed to 
stderr. That seems a nice feature.
In that same discussion, I realized that the module 
cleanup behavior (where all globals in a module are 
set to None when the module object is deallocated) is 
probably no longer necessary now that we have GC.
"""
The point to #1 is that we let objects in unreachable 
cycles leak when they have __del__ methods. While we 
give the user ways to know about that and to clean 
them up (via exposing the trash in gc.garbage), a non-
expert user (or an expert who simply isn't thinking 
about this -- whatever, you don't know unless you 
specifically look for it) may never know that they're 
leaking.

Although this dates back to 2001, I think this might still be useful.

Displaying the full list may be unwieldy if it's very large. Instead, we could by default display the following kind of message:
Python warning: 5 uncollectable objects remaining at interpreter shutdown. Use gc.DEBUG_UNCOLLECTABLE to list them.
And list gc.garbage only if gc.DEBUG_UNCOLLECTABLE is defined.

Here is a patch. Perhaps I should add tests too.

New patch with tests.

More thorough tests.

Patch looks basically OK on an eyeball scan. A couple of suggestions:
- use test.script_helper.run_python rather than rolling your own run_command (or, at least, just make run_command a thin wrapper around run_python). I've been trying to trim down the number of different ways the test suite launches interpreter subprocesses for testing purposes, and script_helper also does a bit of extra cleanup to try to eliminate false alarms regarding reference leaks. That said, I do like the way you're checking that the output went to the correct stream, so perhaps instead add a run_python variant to script_helper that returns a 3-tuple containing the exit code, stdout and stderr and make your run_command a wrapper around that (then create an issue to migrate the rest of the script_helper.run_python based tests to switch to the new version so they will also check that the output is on the correct stream rather than lumping the two streams together).
- update the description of gc.DEBUG_UNCOLLECTABLE in the docs to specifically mention that the complete list of uncollectable objects will also be printed at interpreter shutdown.

> - use test.script_helper.run_python rather than rolling your own
> run_command (or, at least, just make run_command a thin wrapper around
> run_python). I've been trying to trim down the number of different
> ways the test suite launches interpreter subprocesses for testing
> purposes, and script_helper also does a bit of extra cleanup to try to
> eliminate false alarms regarding reference leaks.
After a quick grep, it seems script_helper is really in the minority
(it's only used in three test files) while ad hoc calls to subprocess
with sys.executable are all over the place. I'd rather open a separate
feature request for that new script_helper function than convert all
uses of subprocess in this issue.
> - update the description of gc.DEBUG_UNCOLLECTABLE in the docs to
> specifically mention that the complete list of uncollectable objects
> will also be printed at interpreter shutdown.
Ah, indeed. Thanks!

Here is a new patch with doc.

I went forward and committed the patch in r83861.