Python OpenCV convert image to byte string?

I'm working with PyOpenCV. How to convert cv2 image (numpy) to binary string for writing to MySQL db without a temporary file and imwrite?

I googled it but found nothing...

I'm trying imencode, but it doesn't work.

capture = cv2.VideoCapture(url.path)
capture.set(cv2.cv.CV_CAP_PROP_POS_MSEC, float(url.query))
self.wfile.write(cv2.imencode('png', capture.read()))

Error:

  File "server.py", line 16, in do_GET
  self.wfile.write(cv2.imencode('png', capture.read()))
  TypeError: img is not a numerical tuple

Help somebody!