Python opencv detect black squares

1. How to detect squares in OpenCV?
2. What is cv2 arcLength?

How to detect squares in OpenCV?

Compute the aspect ratio of the contour cnt. Set a range of aspect ratios to detect the square. We set it [0.9, 1.1]. If the ratio is between 0.9 and 1.1, the detected contour is a square else it is a rectangle.

What is cv2 arcLength?

cv2. arcLength() is used to calculate the perimeter of the contour. If the second argument is True then it considers the contour to be closed. Then this perimeter is used to calculate the epsilon value for cv2. approxPolyDP() function with a precision factor for approximating a shape.