> But there's no builtin function for r'foo\bar' that gives you 'foo\\bar'.

I'm confused about what's being requested here. r'foo\bar' and 'foo\\bar' are different source code representations of the exact same string (same type, same contents), so the identity function is such a function.

>>> f(r'foo\bar') == 'foo\\bar'
True

Nicholas: presumably you're after something more than the identity function. Can you clarify what the input and output types to your proposed function would be, and then give example input and output values?