Python Errors and Exceptions: How to Use try, except, else and finally

P8.5 How to Raise an Error in Python for Bad Data

By Ravindra Bagale, trainer LinkedIn Last updated: 27 October 2026

Why. Sometimes the data is technically valid but wrong for the business: a negative quantity, an age of 250. Stop early with a clear message.

What. raise ValueError("message") creates an error on purpose.

How.

Validate input

Check the value with if.
If it is wrong, raise an error with a clear message.
Callers can catch it with try/except.

Worked example A — Reject a negative quantity.

Step 1 — What we do. An order system for a warehouse in Hamburg.

Step 2 — Type and run this code.

def check_qty(q):
    if q < 0:
        raise ValueError(f"Quantity cannot be negative: {q}")
    return q

print(check_qty(5))
check_qty(-2)

Step 3 — Output you should see.

5
ValueError: Quantity cannot be negative: -2

Step 4 — What it means. The first call passes. The second stops with our own clear message.

Worked example B — Catch your own error.

Step 1 — What we do. We validate a list of ages from a clinic and collect problems.

Step 2 — Type and run this code.

def check_age(a):
    if not 0 <= a <= 120:
        raise ValueError(f"Bad age: {a}")
    return a

problems = []
for a in [34, 250, 61, -1]:
    try:
        check_age(a)
    except ValueError as e:
        problems.append(str(e))
print(problems)

Step 3 — Output you should see.

['Bad age: 250', 'Bad age: -1']

Step 4 — What it means. as e gives the error object; str(e) is its message. Now you have a data quality report.

Ravindra Bagale's Tip

Fail loudly on impossible values. A silent wrong number in a report is worse than a clear error.

Samajla ka? If yes, take the quiz for this chapter. If not, run every example again and change one value each time.