isNatural = lambda x: int(x)==x and x>0

Actually, sorry. This is crashy; what I mean is:

from numbers import Number

isNatural = lambda x: isinstance(x, Number) and int(x)==x and x>0

--
Keeping medicines from the bloodstreams of the sick; food
from the bellies of the hungry; books from the hands of the
uneducated; technology from the underdeveloped; and putting
advocates of freedom in prisons. Intellectual property is
to the 21st century what the slave trade was to the 16th.