TheAlgorithms/Python · error · ValueError

The entered barcode has a negative value. Try again.

Error message

The entered barcode has a negative value. Try again.

What it means

Raised by get_barcode in strings/barcode_validator.py when the barcode parses to a negative integer (int(barcode) < 0). Barcodes are conventionally non-negative digit strings, and the checksum validation downstream assumes that, so values like '-123' (where int() succeeds but is negative) are rejected with ValueError. It only fires after the isalpha() check, i.e. the input parsed successfully as an int.

Source

Thrown at strings/barcode_validator.py:71

    return len(str(barcode)) == 13 and get_check_digit(barcode) == barcode % 10


def get_barcode(barcode: str) -> int:
    """
    Returns the barcode as an integer

    >>> get_barcode("8718452538119")
    8718452538119
    >>> get_barcode("dwefgiweuf")
    Traceback (most recent call last):
        ...
    ValueError: Barcode 'dwefgiweuf' has alphabetic characters.
    """
    if str(barcode).isalpha():
        msg = f"Barcode '{barcode}' has alphabetic characters."
        raise ValueError(msg)
    elif int(barcode) < 0:
        raise ValueError("The entered barcode has a negative value. Try again.")
    else:
        return int(barcode)


if __name__ == "__main__":
    import doctest

    doctest.testmod()
    """
    Enter a barcode.

    """
    barcode = get_barcode(input("Barcode: ").strip())

    if is_valid(barcode):
        print(f"'{barcode}' is a valid barcode.")
    else:
        print(f"'{barcode}' is NOT a valid barcode.")

View on GitHub (pinned to f5988cc097)

Solutions

  1. Reject or correct negative values at your input boundary before calling get_barcode.
  2. Strip leading '+'/'-' and validate digits-only: if not s.isdigit(): reject.
  3. If a negative value indicates bad upstream data, log and re-prompt rather than silently taking abs().

Example fix

# before
get_barcode(code)  # code = '-8718452538119'

# after
if not code.isdigit():
    raise ValueError('Barcode must be a non-negative digit string')
get_barcode(code)
Defensive patterns

Strategy: validation

Validate before calling

code = str(barcode).strip()
if not code.isdigit():  # rejects '-', '+', '.', and letters in one check
    raise ValueError('Barcode must be a non-negative digit string')
value = get_barcode(code)

Try / catch

try:
    value = get_barcode(code)
except ValueError as e:
    if 'negative' in str(e):
        raise ValueError('Signed values are not valid barcodes') from e
    raise

Prevention

When it happens

Trigger: get_barcode('-123'); get_barcode('−8718452538119' with a leading minus); programmatic input where a signed field is passed through. Note '12-3' or '1.2' would instead crash at int() with an uncaught ValueError, not this error.

Common situations: Numeric IDs stored as signed integers elsewhere in the pipeline; CSV columns with minus signs; users typing a dash before a digit sequence.

Related errors

AI-assisted analysis of TheAlgorithms/Python@f5988cc097 (2026-08-14). Data as JSON: /api/errors/f4d59b992a1f985d. Report an issue: GitHub.