Using isdigit for floats?
The existing answers are correct in that the more Pythonic way is usually to try...except (i.e. EAFP).
However, if you really want to do the validation, you could remove exactly 1 decimal point before using isdigit().
>>> "124".replace(".", "", 1).isdigit()True>>> "12.4".replace(".", "", 1).isdigit()True>>> "12..4".replace(".", "", 1).isdigit()False>>> "192.168.1.1".replace(".", "", 1).isdigit()FalseNotice that this does not treat floats any different from ints however. You could add that check if you really need it though.
Use regular expressions.
import rep = re.compile('\d+(\.\d+)?')a = raw_input('How much is 1 share in that company? ')while p.match(a) == None: print "You need to write a number!\n" a = raw_input('How much is 1 share in that company? ')