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.