Objects associated to the panel with panel.set_userptr() are never DECREF()ed. Attached file is script to reproduce the leak.

Confirmed with Python2.7/3.3.

New changeset aff0bdab358e by Andrew Kuchling in branch '2.7':
#18113: Objects associated to a curses.panel object with set_userptr() were leaked.
http://hg.python.org/cpython/rev/aff0bdab358e

New changeset 3d75bd69e5a9 by Andrew Kuchling in branch '3.3':
#18113: Objects associated to a curses.panel object with set_userptr() were leaked.
http://hg.python.org/cpython/rev/3d75bd69e5a9

Thanks for the bug report!  I've committed a fix to the 2.7 and 3.3 branches that retrieves the existing userptr and Py_DECREFs it if it's not null.  This seems to fix the leak.

There is a problem with this patch. Py_XDECREF can execute arbitrary Python code and this code can call set_panel_userptr. Here is a reproducer (it causes segfault).

And here is a patch which fixes a segfault. But I can't write a test for it.

serhiy.storchaka: good point!  I wonder if, for strict correctness, we should only incref obj (the new object) if set_panel_userptr() returns OK and not an error code.  I've attached a new version of the patch that does this check, and also adds a test.

(OTOH, looking at the ncurses 5.9 source code, set_panel_userptr() only returns an error if the panel object is NULL, which should never happen because Python reports an error if the panel creation fails.  So maybe the rc == ERR check is pointless.)

> I've attached a new version of the patch that does this check, and also adds a test.

You are right. Your patch LGTM.

> (OTOH, looking at the ncurses 5.9 source code, set_panel_userptr() only returns an error if the panel object is NULL, which should never happen because Python reports an error if the panel creation fails.  So maybe the rc == ERR check is pointless.)

Future versions or alternative implementations can returns an error in other circumstances.

New changeset 99733ff98a50 by Andrew Kuchling in branch '2.7':
#18113: avoid segfault if Py_XDECREF triggers code that calls set_panel_userptr again
http://hg.python.org/cpython/rev/99733ff98a50

New changeset 61fafef4c8a2 by Andrew Kuchling in branch '3.3':
#18113: avoid segfault if Py_XDECREF triggers code that calls set_panel_userptr again
http://hg.python.org/cpython/rev/61fafef4c8a2

I believe the most recent 2 commits fix the segfault problem, so I'll now close this again.  Please re-open if there are further issues with the bugfix.