TheAlgorithms/Python · error · ValueError

Negative arguments are not supported

Error message

Negative arguments are not supported

What it means

Raised by fibonacci(n) (fast doubling implementation) when n is negative. The fast-doubling identities compute F(2n) and F(2n+1) from F(n), which is undefined for negative indices in this implementation, so the guard rejects them up front with a ValueError. Valid calls return F(n) in O(log n) time.

Source

Thrown at dynamic_programming/fast_fibonacci.py:20

"""
This program calculates the nth Fibonacci number in O(log(n)).
It's possible to calculate F(1_000_000) in less than a second.
"""

from __future__ import annotations

import sys


def fibonacci(n: int) -> int:
    """
    return F(n)
    >>> [fibonacci(i) for i in range(13)]
    [0, 1, 1, 2, 3, 5, 8, 13, 21, 34, 55, 89, 144]
    """
    if n < 0:
        raise ValueError("Negative arguments are not supported")
    return _fib(n)[0]


# returns (F(n), F(n-1))
def _fib(n: int) -> tuple[int, int]:
    if n == 0:  # (F(0), F(1))
        return (0, 1)

    # F(2n) = F(n)[2F(n+1) - F(n)]
    # F(2n+1) = F(n+1)^2+F(n)^2
    a, b = _fib(n // 2)
    c = a * (b * 2 - a)
    d = a * a + b * b
    return (d, c + d) if n % 2 else (c, d)


if __name__ == "__main__":
    n = int(sys.argv[1])

View on GitHub (pinned to f5988cc097)

Solutions

1. Guard the call site: use fibonacci(n) only when n >= 0, e.g. fibonacci(max(0, n)) if clamping is acceptable.
2. Fix the upstream index computation that produced the negative value.
3. If negative indices (negafibonacci) are genuinely needed, implement F(-n) = (-1)^(n+1) * F(n) yourself around this function.

Example fix

# before
value = fibonacci(count - 2)  # count == 1 -> -1 -> ValueError

# after
value = fibonacci(count - 2) if count >= 2 else 0

if not isinstance(n, int) or n < 0:
    raise ValueError('n must be a non-negative integer')
value = fibonacci(n)

def is_non_negative_int(value: object) -> bool:
    return isinstance(value, int) and value >= 0

try:
    value = fibonacci(n)
except ValueError:
    raise ValueError(f'fibonacci index {n!r} invalid; must be >= 0') from None

When it happens

Trigger: fibonacci(-1) or any negative n, typically from n = len(seq) - 2 style computations on short sequences, or from signed offsets/index arithmetic that underflows past zero.

Common situations: Index math on lists shorter than expected (e.g. fibonacci(len(items) - 3) with 2 items); CLI/config values parsed as negative; passing a negative Fibonacci index expecting negafibonacci support, which this function does not implement.

Related errors

• Limit for the Catalan sequence must be ≥ 0
• the value of input must be a natural number
• Number should not be negative.
• iterations must be defined as integers
• starting number must be and integer

AI-assisted analysis of TheAlgorithms/Python@f5988cc097 (2026-08-14). Data as JSON: /api/errors/d1158c526a4ba0fc. Report an issue: GitHub.