how to plot a histogram for various images displayed in a imageviewer

[ishani]

hello frns
i am displaying an image in a image viewer using qlabel . now i want to plot histogram for the displayed image in the imageviewr .Below is the attached code I have made which is not working and gives the following error
img = cv2.imread(self.lbl,0)
TypeError: expected string or Unicode object, QLabel found

I know I am passing Qlabel and its expecting string or unicode object. But, I don't know how to resolve this issue. Please can anyone tell me what to do or If you have better solution please put forward.

P.S: I am using PyQt4 and OpenCV and python 2.7
plzz help me to get out of this.