Validation, authentication and strings

Validation in a program

Validation checks that input is sensible before the program uses it. It cannot check that the data is true.

• Presence check: something was entered: text != "".
• Length check: len(text) is within limits.
• Range check: a number is between two limits.
• Type check: the input is the right kind of data. code.isdigit() is True when every character is a digit, so check it before int(), which would crash on letters.
• Format check: the text follows a pattern, such as one @ with a full stop after it.

To keep asking until the input is valid, read once before the loop and again at the end of each pass:

n = int(input())
while n < 1 or n > 12:
    print("Out of range")
    n = int(input())