Considering this further, I've realised that the idea of implicit quoting for this style of helper function is misguided on another level - the parameters to be interpolated may not even be strings yet, so attempting to quote them would fail:

>>> subprocess.call("exit {}".format(1), shell=True)
1
>>> shlex.quote(1)
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "/home/ncoghlan/devel/py3k/Lib/shlex.py", line 285, in quote
    if _find_unsafe(s) is None:
TypeError: expected string or buffer

I'll note that none of these problems will be unique to the new convenience API - they're all a general reflection of the impedance mismatch between typical shell interfaces and filenames that can contain spaces.