How to detect and draw contours using OpenCV in Python?

I wrote the following code to detect and draw contours:

img = cv2.imread('test2.tif');

if not img is None:
    imgray = cv2.cvtColor(img,cv2.COLOR_BGR2GRAY);
    ret,thresh = cv2.threshold(imgray,127,255,0);
    contours,hierarchy = cv2.findContours(thresh,cv2.RETR_TREE,cv2.CHAIN_APPROX_SIMPLE);

    #draw a three pixel wide outline 
    cv2.drawContours(img,contours,-1,(0,255,0),3);

And here is the error I received:

Traceback (most recent call last): File “C:/Users/R.K.singh/Desktop/Image processing/intro-to-contours.py”, line 10, in contours,hierarchy = cv2.findContours(thresh,cv2.RETR_TREE,cv2.CHAIN_APPROX_SIMPLE); ValueError: too many values to unpack

What is wrong? I am using Python 2.7 and OpenCV 3.1.0

Answer

In order to emphasize Selchuk’s point, the syntax involving OpenCV 3.x has changed a little bit. It has a different return value when it comes to cv2.findContours. It returns the following image, contours, hierarchy.

Previous versions of OpenCV, however, return only contours, hierarchy. They do not return the image.