TheAlgorithms/Python · error · TypeError

recursive_lucas_number accepts only integer arguments.

Error message

recursive_lucas_number accepts only integer arguments.

What it means

Raised by recursive_lucas_number in maths/lucasas_series.py (maths/lucas_series.py) when n_th_number is not an int. The naive doubly-recursive Lucas implementation (L(0)=2, L(1)=1, L(n)=L(n-1)+L(n-2)) decrements its argument by 1 and 2 each step, which only terminates for integer arguments; the type guard raises TypeError before the first recursive call. Note there is no negative-argument guard, so negative ints recurse until a TypeError is eventually raised by hitting a non-int.

Source

Thrown at maths/lucas_series.py:23

def recursive_lucas_number(n_th_number: int) -> int:
    """
    Returns the nth lucas number
    >>> recursive_lucas_number(1)
    1
    >>> recursive_lucas_number(20)
    15127
    >>> recursive_lucas_number(0)
    2
    >>> recursive_lucas_number(25)
    167761
    >>> recursive_lucas_number(-1.5)
    Traceback (most recent call last):
        ...
    TypeError: recursive_lucas_number accepts only integer arguments.
    """
    if not isinstance(n_th_number, int):
        raise TypeError("recursive_lucas_number accepts only integer arguments.")
    if n_th_number == 0:
        return 2
    if n_th_number == 1:
        return 1

    return recursive_lucas_number(n_th_number - 1) + recursive_lucas_number(
        n_th_number - 2
    )


def dynamic_lucas_number(n_th_number: int) -> int:
    """
    Returns the nth lucas number
    >>> dynamic_lucas_number(1)
    1
    >>> dynamic_lucas_number(20)
    15127
    >>> dynamic_lucas_number(0)

View on GitHub (pinned to f5988cc097)

Solutions

1. Convert to non-negative int before calling: recursive_lucas_number(int(n)).
2. Prefer dynamic_lucas_number for larger n (the recursive version is exponential-time).
3. Guard n >= 0 at the call site since negative ints are not properly handled.

Example fix

// before
val = recursive_lucas_number(idx)  # idx is 20.0

// after
idx = int(idx)
val = recursive_lucas_number(idx) if idx >= 0 else None

n = int(n_th_number)
if n < 0:
    raise ValueError(f"Lucas index must be >= 0, got {n}")
val = dynamic_lucas_number(n)  # prefer dynamic for speed

def is_nonneg_int(v) -> bool:
    return isinstance(v, int) and not isinstance(v, bool) and v >= 0

try:
    val = recursive_lucas_number(n)
except TypeError:
    val = dynamic_lucas_number(int(n))

When it happens

Trigger: Calling recursive_lucas_number(-1.5), recursive_lucas_number(2.0), or recursive_lucas_number('5'). Also indirectly: calling with a negative int recurses down past 0 until n_th_number - 2 produces a non-int transition point and eventually trips the isinstance check.

Common situations: Float indices from formulas; string input from prompts (the __main__ block does int(input()) correctly, but reuse elsewhere may not); passing a negative index expecting generalized Lucas numbers.

Related errors

• dynamic_lucas_number accepts only integer arguments.
• Number should not be negative.
• find_max_recursive() arg is an empty sequence
• list index out of range
• find_min_recursive() arg is an empty sequence

AI-assisted analysis of TheAlgorithms/Python@f5988cc097 (2026-08-14). Data as JSON: /api/errors/031dba8539f99f94. Report an issue: GitHub.