It appears that you’re using OpenCV version 3.x, while writing code intended for the 2.x branch. There were some API changes between those two branches. Since you’re using Python, you have a handy help available — make sure to use it, along with the documentation.
OpenCV 2.x:
>>> import cv2
>>> help(cv2.findContours)
Help on built-in function findContours in module cv2:
findContours(...)
findContours(image, mode, method[, contours[, hierarchy[, offset]]]) -> contours, hierarchy
OpenCV 3.x:
>>> import cv2
>>> help(cv2.findContours)
Help on built-in function findContours:
findContours(...)
findContours(image, mode, method[, contours[, hierarchy[, offset]]]) -> image, contours, hierarchy
This means that in your script the correct way to call findContours
when using OpenCV 3.x would be something like
(_, cnts, _) = cv2.findContours(closed.copy(), cv2.RETR_EXTERNAL, cv2.CHAIN_APPROX_SIMPLE)
UPDATE (Dec 2018)
In OpenCV 4.x, findContours
returns 2 values only.
>>> help(cv2.findContours)
Help on built-in function findContours:
findContours(...)
findContours(image, mode, method[, contours[, hierarchy[, offset]]]) -> contours, hierarchy
. @brief Finds contours in a binary image.