Errors & debugging Medium

Raise a clear custom error

Validate input early and raise ValueError with a helpful message.

12 lines 3 views 1 copies Updated May 23, 2026

Try in editor

errors raise validation

How it works

Validate function inputs at the boundary — fail fast with a message that explains the fix.

raise ValueError('...') is appropriate for bad arguments that callers can correct.

Document expected types and ranges in the docstring so IDE hints and readers stay aligned.

Python

def positive_only(n):
    """Return n if it is strictly positive."""
    if n <= 0:
        raise ValueError(f"Expected a positive number, got {n}")
    return n


print(positive_only(5))
try:
    positive_only(-1)
except ValueError as exc:
    print(exc)

How it works

More from Errors & debugging