'cv2' has no attribute 'CV_HOUGH_GRADIENT'

edit

the CV_ prefix has been removed with opencv3, and all constants are in the cv2 submodule now

also cv2.cv was removed entirely.

what you want is simply:

cv2.HOUGH_GRADIENT

in the future, try to do a :

>>> help(cv2)

press space, until you reach the bottom (LOOONG list), the constants are all at the end of it