Quick notes so I don't forget:

The two main pure python equivalents in the docs would need to be
updated with the new special cases (currently they raise IndexError for
r>n).  The other two pure python equivalents would automatically handle
the proposed new cases.

The Mathematica definition of permutations does not discuss or allow for
the r>n case, http://mathworld.wolfram.com/Permutation.html .  One of
the other tools that actually lists permutations and takes a second
argument (most do not) is in Mathematica,
http://reference.wolfram.com/mathematica/ref/Permutations.html .  It is
unclear from their docs whether the r>n case is supported as a
zero-length output or as an error condition.