Note that this is a regression relative to 2.6, where the same call returns '' (which is different from what it returns on linux, where the result would be '%f', or OSX, where the result would be 'f').  (Tests done on windows XP using pythons installed from the python.org installers.)

My guess is that the difference between python 2.6 and python 2.7+ is issue 4804.  I'm therefore adding the nosy list from that issue to this one.

For the OP's benefit: my guess is that this is a bug in the Microsoft C runtime, but we probably need to provide a workaround for it in Python.