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

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.

Well, when using an API, you have to check the requirement so of that API.

Maybe that function requires a filename of an image?

Cheers,
_