We want to add Length objects to integers:
Z = 1 + Length(4.5, "yd")
def __radd__(self, other):
z = self + other
z.unit = "m"
z.value *= Length.__metric[self.unit]
return z
5 + Length(3, "yd")
__radd__(self,other)
Length(3, "yd") + 5
__add__(self,other)