ubuntu18.04 cv2.VideoCapture无法读取视频

源代码：

# 读取视频

import cv2
video_file = 'cd ~/home/edit/Projects/test/test.mp4'
cap = cv2.VideoCapture(video_file)


# 读取摄像头,0为摄像头索引，当有多个摄像头时，从0开始编号

cap = cv2.VideoCapture(0)

# 从视频或摄像头中读取一帧（即一张图像）,返回是否成功标识ret(True代表成功，False代表失败），img为读取的视频帧


ret,frame = cap.read()

问题：

(python3.6:15946): GStreamer-CRITICAL **: 10:44:40.930: gst_element_make_from_uri: assertion 'gst_uri_is_valid (uri)' failed
[ WARN:0] global /home/nvidia/host/build_opencv/nv_opencv/modules/videoio/src/cap_gstreamer.cpp (711) open OpenCV | GStreamer warning: Error opening bin: no element "cd"
[ WARN:0] global /home/nvidia/host/build_opencv/nv_opencv/modules/videoio/src/cap_gstreamer.cpp (480) isPipelinePlaying OpenCV | GStreamer warning: GStreamer: pipeline have not been created
[ WARN:0] global /home/nvidia/host/build_opencv/nv_opencv/modules/videoio/src/cap_gstreamer.cpp (933) open OpenCV | GStreamer warning: Cannot query video position: status=0, value=-1, duration=-

状态：
未解决