TheAlgorithms/Python · error · ValueError

Monogons and Digons are not polygons in the Euclidean space

Error message

Monogons and Digons are not polygons in the Euclidean space

What it means

Raised by check_polygon() in maths/check_polygon.py when the input list of side lengths has fewer than 2 entries. The function decides whether side lengths can form a polygon; with 0 or 1 sides there is no polygon in Euclidean space (a monogon/digon is degenerate), so it raises ValueError before checking the triangle-inequality-style condition.

Source

Thrown at maths/check_polygon.py:33

    >>> check_polygon([3, 7, 13, 2])
    False
    >>> check_polygon([1, 4.3, 5.2, 12.2])
    False
    >>> nums = [3, 7, 13, 2]
    >>> _ = check_polygon(nums) #   Run function, do not show answer in output
    >>> nums #  Check numbers are not reordered
    [3, 7, 13, 2]
    >>> check_polygon([])
    Traceback (most recent call last):
        ...
    ValueError: Monogons and Digons are not polygons in the Euclidean space
    >>> check_polygon([-2, 5, 6])
    Traceback (most recent call last):
        ...
    ValueError: All values must be greater than 0
    """
    if len(nums) < 2:
        raise ValueError("Monogons and Digons are not polygons in the Euclidean space")
    if any(i <= 0 for i in nums):
        raise ValueError("All values must be greater than 0")
    copy_nums = nums.copy()
    copy_nums.sort()
    return copy_nums[-1] < sum(copy_nums[:-1])


if __name__ == "__main__":
    import doctest

    doctest.testmod()

View on GitHub (pinned to f5988cc097)

Solutions

1. Ensure the input list contains at least 2 side lengths (in practice at least 3 for a real polygon) before calling.
2. Fix the upstream collection step that produced an empty or single-element list of side lengths.
3. Validate parsed input (e.g. after split/strip of a user string) before passing to check_polygon.

Example fix

# before
sides = []
check_polygon(sides)  # ValueError

# after
sides = [3, 7, 13, 2]
if len(sides) < 3:
    raise SystemExit('need at least 3 side lengths')
check_polygon(sides)

if len(sides) < 3:
    raise ValueError(f'need at least 3 sides, got {len(sides)}')

def is_polygon_input(nums) -> bool:
    return isinstance(nums, (list, tuple)) and len(nums) >= 3

try:
    check_polygon(sides)
except ValueError as e:
    if 'Monogons' in str(e):
        sides = collect_more_sides()  # re-prompt / re-parse
    else:
        raise

When it happens

Trigger: Calling check_polygon([]) (empty list) or check_polygon([5]) (single side). The guard is len(nums) < 2.

Common situations: Feeding an empty dataset or a filtered list that ended up empty into a geometry pipeline; parsing side lengths from user input or a file where only one value was extracted; forgetting that a triangle needs 3 sides and testing with fewer.

Related errors

• All values must be greater than 0
• Length must be a positive.
• 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/e07f6f82b2763fd5. Report an issue: GitHub.