scipy/scipy — pull request #3902 by trevorcampbell (2014-08-25 19:26:40+00:00)
Title: BUG: psi(x) should return -inf at x=0, not inf

See http://mathworld.wolfram.com/DigammaFunction.html

As x -> 0, psi(x) -> (-inf). Returning infinity is nonintuitive behaviour.