TheAlgorithms/Python · error · ValueError

invalid k value

Error message

invalid k value

What it means

HarrisCorner.__init__ raises this ValueError when k is not one of exactly two floats: 0.04 or 0.06. The docstring says k is 'an empirically determined constant in [0.04,0.06]' (a range), but the implementation tests membership with `k in (0.04, 0.06)`, so any other value in the documented range — most notably 0.05 — is rejected. This is effectively a bug in the guard, not in the caller.

Source

Thrown at computer_vision/harris_corner.py:21

"""
Harris Corner Detector
https://en.wikipedia.org/wiki/Harris_Corner_Detector
"""


class HarrisCorner:
    def __init__(self, k: float, window_size: int):
        """
        k : is an empirically determined constant in [0.04,0.06]
        window_size : neighbourhoods considered
        """

        if k in (0.04, 0.06):
            self.k = k
            self.window_size = window_size
        else:
            raise ValueError("invalid k value")

    def __str__(self) -> str:
        return str(self.k)

    def detect(self, img_path: str) -> tuple[cv2.Mat, list[list[int]]]:
        """
        Returns the image with corners identified
        img_path  : path of the image
        output : list of the corner positions, image
        """

        img = cv2.imread(img_path, 0)
        h, w = img.shape
        corner_list: list[list[int]] = []
        color_img = img.copy()
        color_img = cv2.cvtColor(color_img, cv2.COLOR_GRAY2RGB)
        dy, dx = np.gradient(img)
        ixx = dx**2

View on GitHub (pinned to f5988cc097)

Solutions

1. Pass exactly 0.04 or 0.06: HarrisCorner(0.04, window_size)
2. Or fix the guard to match the documented range: `if 0.04 <= k <= 0.06:` (this is the correct upstream fix)
3. If you must keep the strict check, snap your tuned k to the nearest allowed value before constructing

Example fix

# before
corner = HarrisCorner(0.05, 3)
# ValueError: invalid k value

# after (use an accepted literal)
corner = HarrisCorner(0.04, 3)

# or fix the library guard
if 0.04 <= k <= 0.06:

K_VALUES = (0.04, 0.06)
k = 0.04 if k not in K_VALUES else k
hc = HarrisCorner(k, window_size)

def is_valid_k(k) -> bool:
    return isinstance(k, float) and k in (0.04, 0.06)

try:
    HarrisCorner(k, window_size)
except ValueError as e:
    if 'invalid k value' in str(e):
        HarrisCorner(0.04, window_size)  # fall back to accepted literal
    else:
        raise

When it happens

Trigger: HarrisCorner(0.05, 3); HarrisCorner(0.041, 5); HarrisCorner(0, 3); any k arrived at by tuning or random search inside [0.04, 0.06] other than the two endpoints.

Common situations: Following tutorials/textbooks that recommend k=0.05; hyperparameter sweeps; reading the docstring's [0.04,0.06] range and picking a midpoint. Only the literal values 0.04 and 0.06 pass.

Related errors

• The input array is not a square matrix
• number must be positive
• The value of input must be non-negative
• Input list must contain at least two elements
• Inputs and select signal must be 0 or 1

AI-assisted analysis of TheAlgorithms/Python@f5988cc097 (2026-08-14). Data as JSON: /api/errors/6b21b71e63d433da. Report an issue: GitHub.