If you are doing a simple terminal script in Python, and you need to validate some user-supplied input, you can put the assignment and validation in a try block inside a while loop.

while: # variable not assigned
    try:
        # input, assignment, validation
    except:
        continue

If the assignment or validation fails, the loop repeats and the user has another opportunity to enter input. (You could even add a message to except if you wanted to.)

This came up for me in a little script I wrote to semi-automate a task. I needed to rate the quality of something I was looking at, and I was going for speed: Glance at the item, make a snap quality judgment, type a number.

But, of course, sometimes I typed the number incorrectly, hitting a letter instead of a number. This screwed things up latter, when I tried to order all my items by quality rating.

So, I cast the input to an integer inside the try block, and used the while loop to repeat until the quality rating was assigned.

while 'quality' not in lnk:
    try:
        lnk['quality'] = int(input("Quality (0-9): "))
    except:
        continue

This approach could be expanded for more complicated types of validation, and messages to the user could be added. I just wanted a quick way to deal with my fat fingers.