Can't you do it like so: def cmp_long_vs_float(l, f): try: lf = float(l) except OverflowError: if special_float_value(f): # NaN, Inf return cmp(0.0, f) return cmp(l, 0L) else: return cmp(lf, f) The try/else control flow seems to be what Python does today. If the OverflowError is triggered, then the magnitude of l is bigger than any finite float so you might just as well compare it against 0L. Infinite floats are handled by the "if special_float_value(f):" branch, or just ignored if you prefer.. (but having 2L**2000 compare greater than +INF would bring complaints)
If it turns out to be more efficient, the implementation could LBYL to see whether l can convert to float without overflow.
Jeff