Max and min initializing with None

gistfile1.txt

max = None
min= None
while True:
    num = raw_input("Enter a number: ")
    if num == "done" : break
    if len(num)<1 : break
    try:
        num = float(num)
    except :
        print "Invalid input"
        continue
    if num > max or max == None:
        max= num
    if num < min or min == None:
        min = num
print "Maximum is", max
print "Minimum is", min