Validation Patterns

Real programs check inputs before trusting them. Validation is just decisions with a job:

hour = 25          # from user input somewhere
if not 0 <= hour <= 23:
    print("invalid hour")

The validation pattern

1. Check the shape of the data (is it present? the right type?).
2. Check the range/domain (is it sensible?).
3. Fail early with a clear message; otherwise proceed.

username = "ab"
if len(username) < 3:
    print("too short: need at least 3 characters")
elif " " in username:
    print("no spaces allowed")
else:
    print("welcome,", username)

A validator is the perfect mini project: pure decisions, immediate feedback, and it is exactly what every real backend does before touching a database.