from fractions import Fraction

def line(self, x0, y0, x1, y1):
    rev = reversed
    if abs(y1 - y0) <= abs(x1 - x0):
        x0, y0, x1, y1 = y0, x0, y1, x1
        rev = lambda x: x
    if x1 < x0:
        x0, y0, x1, y1 = x1, y1, x0, y0
    leny = abs(y1 - y0)
    for i in range(leny + 1):
        self.set(*rev((round(Fraction(i, leny) * (x1 - x0)) + x0, (1 if y1 > y0 else -1) * i + y0)))

Bitmap.line = line

# see test code above