>        def __init__(self, command, *, **callkwds):
Is the '*' marker needed?

>            self.callkwds = callkwds
These aren’t used in the module-level functions.  What is the use case?

If you forgive me for the nitpick, the docstrings have too much indenting.

> a "*.py" value interpolated with "{!u}" should indeed pick up all of those files,
> since the wildcard will be passed unmodified to the underlying shell
Great.

(This patch will also serve as a nice example of creating a string formatter.  I’ve seen the PyCon video about them, but I thought I was missing a piece since I didn’t see how you hook the format function to your custom formatter; it looks like the answer is that you don’t.)