ValueError: distinguish format, range and type

What fails and when

ValueError usually means the type is acceptable but its value violates the operation's requirements. int('12x') accepts a string argument but not that string's content. Similarly, an out-of-range port number can be an int yet invalid. Validation should preserve the source of the bad value for a useful message without echoing secrets.

Self-contained executable example

Save this as 007_ValueError.py and run python3 007_ValueError.py. The example checks both the error and a valid case.

def parse_port(text):
    try:
        port = int(text, 10)
    except ValueError as exc:
        raise ValueError("port must be a decimal integer") from exc
    if not 1 <= port <= 65535:
        raise ValueError("port outside 1..65535")
    return port
assert parse_port("443") == 443
for invalid in ("abc", "0", "65536"):
    try:
        parse_port(invalid)
    except ValueError:
        pass
    else:
        raise AssertionError(invalid)

Diagnostic sequence and safe repair

Separate parsing from semantic validation: first interpret input, then check its permitted domain. A broad except Exception clause would also swallow programming bugs; catch ValueError at the boundary you can handle. If zero, whitespace and signs have special meaning in your protocol, write those decisions down rather than relying on int() defaults.

Practical lab and boundary test

Decide whether ' +80 ' is permitted and whether '80.0' should be rejected. Implement the policy without changing the accepted numeric range; test min, max, just below and just above.

Strict port input: grammar before integer conversion

A permissive int() call accepts whitespace and signs. For a network configuration file that promises one to five ASCII digits, enforce that grammar before conversion; the length limit also bounds the work done on hostile input. Leading zeros are allowed here only while the total input remains at most five characters.

def parse_port(text):
    if not isinstance(text, str):
        raise TypeError('port must be text')
    if len(text) > 5 or not text.isascii() or not text.isdecimal():
        raise ValueError('port must be 1..5 ASCII digits')
    port = int(text)
    if not 1 <= port <= 65535:
        raise ValueError('port outside 1..65535')
    return port

assert parse_port('080') == 80
assert parse_port('65535') == 65535
for bad in ('', '000080', '65536', '８０', '-1', ' 80 '):
    try: parse_port(bad)
    except ValueError: pass
    else: raise AssertionError(bad)

Diagnosis and mastery check

If the conversion fails, distinguish an invalid character from an out-of-range number; do not log secret configuration values. Verify that '0' and '65536' are range failures, while '８０' and ' 80 ' are grammar failures. The exercise is complete when a test suite covers both bounds, non-ASCII digits, leading zeros, the empty string and non-string inputs, without catching unrelated exceptions.