Detecting circle using hough transform - Python(OpenCv)

import numpy as np
import cv2 as cv
#smarties.png
img = cv.imread('shapes6.png')
output = img.copy()
gray = cv.cvtColor(img, cv.COLOR_BGR2GRAY)
gray = cv.medianBlur(gray, 5)
circles = cv.HoughCircles(gray, cv.HOUGH_GRADIENT, 1, 20, param1=50,
                          param2=30, minRadius=0, maxRadius=0)
detected_circles = np.uint16(np.around(circles))
for(x, y, r) in detected_circles[0, :]:
    cv.circle(output, (x, y), r, (0, 255, 0), 3)
    cv.circle(output, (x, y), 2, (0, 255, 255), 3)

cv.imshow('output', output)
cv.waitKey(0)
cv.destroyAllWindows()