TheAlgorithms/Python · error · ValueError

Length must be a positive.

Error message

Length must be a positive.

What it means

Raised by dodecahedron_surface_area() in maths/dodecahedron.py when edge <= 0 or edge is not an int. A regular dodecahedron's surface area 3*sqrt(25+10*sqrt(5))*e^2 only makes sense for a positive edge length, so the function validates it up front. Note the guard is stricter than the message implies: it also rejects floats, so dodecahedron_surface_area(2.5) raises even though 2.5 is a positive length.

Source

Thrown at maths/dodecahedron.py:36

    :param edge: length of the edge of the dodecahedron
    :type edge: float
    :return: the surface area of the dodecahedron as a float


    Tests:
    >>> dodecahedron_surface_area(5)
    516.1432201766901
    >>> dodecahedron_surface_area(10)
    2064.5728807067603
    >>> dodecahedron_surface_area(-1)
    Traceback (most recent call last):
      ...
    ValueError: Length must be a positive.
    """

    if edge <= 0 or not isinstance(edge, int):
        raise ValueError("Length must be a positive.")
    return 3 * ((25 + 10 * (5 ** (1 / 2))) ** (1 / 2)) * (edge**2)


def dodecahedron_volume(edge: float) -> float:
    """
    Calculates the volume of a regular dodecahedron
    v = ((15 + (7 * (5** (1 / 2)))) / 4) * (e**3)
    where:
    v --> is the volume of the dodecahedron
    e --> is the length of the edge
    reference-->"Dodecahedron" Study.com
    <https://study.com/academy/lesson/dodecahedron-volume-surface-area-formulas.html>

    :param edge: length of the edge of the dodecahedron
    :type edge: float
    :return: the volume of the dodecahedron as a float

    Tests:

View on GitHub (pinned to f5988cc097)

Solutions

1. Pass an int edge, or verify whole-number values first: int(edge) if edge == int(edge).
2. If you need fractional edge lengths, compute the formula directly with floats: 3 * ((25 + 10 * 5**0.5) ** 0.5) * edge**2.
3. Fix upstream units so edge lengths are integral (e.g. work in millimetres instead of metres).

Example fix

# before
dodecahedron_surface_area(2.5)  # ValueError: Length must be a positive.

# after
edge = 2.5
area = 3 * ((25 + 10 * 5 ** 0.5) ** 0.5) * edge ** 2  # float formula
# or, for whole values: dodecahedron_surface_area(int(edge))

if not isinstance(edge, int) or isinstance(edge, bool) or edge <= 0:
    raise ValueError(f'edge must be a positive int, got {edge!r}')

def is_positive_int_edge(e) -> bool:
    return isinstance(e, int) and not isinstance(e, bool) and e > 0

try:
    area = dodecahedron_surface_area(edge)
except ValueError:
    area = 3 * ((25 + 10 * 5 ** 0.5) ** 0.5) * float(edge) ** 2  # float fallback formula

When it happens

Trigger: Calling dodecahedron_surface_area(-1), dodecahedron_surface_area(0), or dodecahedron_surface_area(5.0) / any float edge. The guard is edge <= 0 or not isinstance(edge, int).

Common situations: Edge lengths computed from geometry that are naturally floats (hypotenuse, scaling factors); CSV/JSON numeric input parsed as float; negative values from a subtraction; bool edges (True passes isinstance int and equals 1).

Related errors

• Monogons and Digons are not polygons in the Euclidean space
• All values must be greater than 0
• surface_area_cube() only accepts non-negative values
• surface_area_cuboid() only accepts non-negative values
• surface_area_sphere() only accepts non-negative values

AI-assisted analysis of TheAlgorithms/Python@f5988cc097 (2026-08-14). Data as JSON: /api/errors/22022085f108cdca. Report an issue: GitHub.