import math
def litres_per_hour(litres, hours):
    if not math.isfinite(litres) or not math.isfinite(hours) or litres < 0 or hours <= 0:
        raise ValueError("nonnegative litres and positive finite hours required")
    return litres / hours
assert litres_per_hour(21., 3.) == 7.
try:
    litres_per_hour(21., 0.)
except ValueError:
    print("Rate 7.0 L/hour; zero hours rejected")
else:
    raise AssertionError("zero hours accepted")