deriving from float or int

[Russ]

The problem is that when I derive a new class from float, the darn
thing won't let me create a constructor that accepts more than one
argument. I need two arguments: one for the numerical value and one for
the units. But when I try to give the constructor two arguments, I get
this when I call the constructor:

TypeError: float() takes at most 1 argument (2 given)

In other words, python doesn't seem to want to let me "extend" the
float type. I don't understand the reason for that, but I assume there
is a reason.