$ ./python
Python 3.4.0a0 (default:801567d6302c+, May 23 2013, 14:22:00) 
[GCC 4.7.2] on linux
Type "help", "copyright", "credits" or "license" for more information.
>>> import gc
>>> gc.set_debug(gc.DEBUG_UNCOLLECTABLE)
>>> import antigravity
>>> Fontconfig warning: "/etc/fonts/conf.d/50-user.conf", line 9: reading configurations from ~/.fonts.conf is deprecated.

>>> 
gc: uncollectable <Popen 0x7faa2e37bd58>
gc: uncollectable <dict 0x7faa2f535f38>
gc: uncollectable <type 0x26e3198>
gc: uncollectable <list 0x7faa2e36c138>
gc: uncollectable <dict 0x7faa2e366f38>
gc: uncollectable <tuple 0x7faa2e3699f8>
gc: uncollectable <function 0x7faa2e36ee00>
gc: uncollectable <function 0x7faa2e382a68>
gc: uncollectable <function 0x7faa2e382280>
gc: uncollectable <function 0x7faa2e3821c8>
gc: uncollectable <function 0x7faa2e382618>
gc: uncollectable <getset_descriptor 0x7faa2e37d1c0>
gc: uncollectable <function 0x7faa2e36eeb8>
gc: uncollectable <function 0x7faa2e382840>
gc: uncollectable <function 0x7faa2e382bd8>
gc: uncollectable <getset_descriptor 0x7faa2e37d2b0>
gc: uncollectable <function 0x7faa2e36ed48>
gc: uncollectable <function 0x7faa2e382b20>
gc: uncollectable <function 0x7faa2e382c90>
gc: uncollectable <function 0x7faa2e3823f0>
gc: uncollectable <function 0x7faa2e382110>
gc: uncollectable <function 0x7faa2e3824a8>
gc: uncollectable <function 0x7faa2e3826d0>
gc: uncollectable <function 0x7faa2e36ef70>
gc: uncollectable <function 0x7faa2e382058>
gc: uncollectable <function 0x7faa2e382788>
gc: uncollectable <function 0x7faa2e3829b0>
gc: uncollectable <function 0x7faa2e382e00>
gc: uncollectable <function 0x7faa2e3828f8>
gc: uncollectable <function 0x7faa2e382d48>
gc: uncollectable <function 0x7faa2e382560>
gc: uncollectable <function 0x7faa2e382338>
gc: uncollectable <dict 0x7faa2e35bf38>
gc: uncollectable <tuple 0x7faa2e37fed8>
gc: uncollectable <tuple 0x7faa2e369b48>
gc: uncollectable <tuple 0x7faa2e37fe58>
gc: uncollectable <list 0x7faa2e366138>
gc:0: ResourceWarning: gc: 1 uncollectable objects at shutdown
      [<subprocess.Popen object at 0x7faa2e37bd58>]

platform: Ubuntu 12.10 AMD64
browser: Firefox 20.0

Presumably this is caused by the fact that Popen.__del__() ressurects self by appending self to _active if the process is still alive.

On Windows this is unnecessary.  On Unix it would be more sensible to just append the *pid* to _active.

See also issue 5993.

Outdated by PEP 442.