$ rpm -q mock
mock-1.3.5-1.el6.noarch

$ mock -r fedora-rawhide-x86_64 --init --new-chroot
INFO: mock.py version 1.3.5 starting (python version = 2.6.6)...
Start: init plugins
INFO: selinux disabled
Finish: init plugins
Start: run
Start: clean chroot
ERROR: 'module' object has no attribute 'check_output'
Traceback (most recent call last):
  File "/usr/libexec/mock/mock", line 886, in <module>
    main()
  File "/usr/lib/python2.6/site-packages/mockbuild/trace_decorator.py", line 89, in trace
    result = func(*args, **kw)
  File "/usr/libexec/mock/mock", line 704, in main
    buildroot.finalize()
  File "/usr/lib/python2.6/site-packages/mockbuild/trace_decorator.py", line 89, in trace
    result = func(*args, **kw)
  File "/usr/lib/python2.6/site-packages/mockbuild/buildroot.py", line 544, in finalize
    util.orphansKill(self.make_chroot_path())
  File "/usr/lib/python2.6/site-packages/mockbuild/trace_decorator.py", line 89, in trace
    result = func(*args, **kw)
  File "/usr/lib/python2.6/site-packages/mockbuild/util.py", line 202, in orphansKill
    vm_list = subprocess.check_output(["/usr/bin/machinectl", "list", "--no-pager"])
AttributeError: 'module' object has no attribute 'check_output'



The code in there doesn't appear to be Python 2.6 compatible.