TheAlgorithms/Python · error · ValueError

Shift must be non-negative

Error message

Shift must be non-negative

What it means

Thrown by left_rotate_32() when the shift amount is negative. A negative rotation is undefined for the bit trick used ((i << shift) ^ (i >> (32 - shift))) and Python's arbitrary-precision ints would silently produce wrong results, so shift < 0 is rejected. Note the value guard (error 388) fires first if both are invalid.

Source

Thrown at hashes/md5.py:293

    >>> left_rotate_32(4294967295, 4)
    4294967295
    >>> left_rotate_32(1234, 0)
    1234
    >>> left_rotate_32(0, 0)
    0
    >>> left_rotate_32(-1, 0)
    Traceback (most recent call last):
    ...
    ValueError: Input must be non-negative
    >>> left_rotate_32(0, -1)
    Traceback (most recent call last):
    ...
    ValueError: Shift must be non-negative
    """
    if i < 0:
        raise ValueError("Input must be non-negative")
    if shift < 0:
        raise ValueError("Shift must be non-negative")
    return ((i << shift) ^ (i >> (32 - shift))) % 2**32


def md5_me(message: bytes) -> bytes:
    """
    Returns the 32-char MD5 hash of a given message.

    Reference: https://en.wikipedia.org/wiki/MD5#Algorithm

    Arguments:
        message {[string]} -- [message]

    Returns:
        32-char MD5 hash string

    >>> md5_me(b"")
    b'd41d8cd98f00b204e9800998ecf8427e'
    >>> md5_me(b"The quick brown fox jumps over the lazy dog")

View on GitHub (pinned to f5988cc097)

Solutions

1. Pass shifts in [0, 31] per the MD5 specification; use the fixed shift table verbatim.
2. Normalize computed shifts: shift % 32.
3. Validate your shift table length/contents once at module load, not per call.

Example fix

# before
left_rotate_32(x, s - t)  # s - t may be negative

# after
left_rotate_32(x, (s - t) % 32)

assert 0 <= shift < 32, f"shift must be in [0, 32), got {shift}"

def is_valid_shift(shift: int) -> bool:
    return isinstance(shift, int) and 0 <= shift < 32

try:
    r = left_rotate_32(x, s)
except ValueError as e:
    if "Shift" in str(e):
        r = left_rotate_32(x, s % 32)
    else:
        raise

When it happens

Trigger: Calling left_rotate_32(0, -1), or computing the shift from an expression like (s - t) that can go negative, or indexing a shift table out of range with a negative index.

Common situations: Parameterized rotation schedules where the shift is derived at runtime (e.g. variable round constants) and can underflow; typos in shift tables copied from the MD5 spec.

Related errors

• Input must be of length 32
• Input must be non-negative
• Input must have length that's a multiple of 512
• number must be positive
• The value of input must be non-negative

AI-assisted analysis of TheAlgorithms/Python@f5988cc097 (2026-08-14). Data as JSON: /api/errors/7e68445afd0d7f67. Report an issue: GitHub.