Python 作业题求助......

Write a program that repeatedly prompts a user for integer numbers until the user enters 'done'. Once 'done' is entered, print out the largest and smallest of the numbers. If the user enters anything other than a valid number catch it with a try/except and put out an appropriate message and ignore the number. Enter the numbers 4,5,6,7 and Match the desired output as shown below:

Invalid input
Maximum is 7
Minimum is 4

please build your program based on the sample code:

largest = None
smallest = None
while True:
num = raw_input("Enter a number: ")
if num == "done" : break
print num

print "Maximum", largest