gstreamer critical error when trying to capture video using webcam python opencv

i'm trying to take a video with webcam using opencv and python with a simple code

import numpy as np
import cv2

cap = cv2.VideoCapture(0)
print('cap.isOpened')
if cap.isOpened():
    print ('cap is opened')
    while(True):
        re,img=cap.read()
        cv2.imshow("video output", img)
        k = cv2.waitKey(10)&0xFF
        if k==27:
            break
cap.release()
cv2.destroyAllWindows()

it's working fine if i try to play an existing video such as .mp4 file. but when i try using a webcam i got an error

GStreamer-CRITICAL **: gst_element_get_state: assertion 'GST_IS_ELEMENT (element)' failed cap.isOpened

for more information i'm using odroid xu4 with ubuntu 16.04, webcam i use logitech c170 ( it work properly in webcamtest and using guvcview) thought it doesn't workon cheese and camorama.

need help about this please..

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