Python VideoCapture Isopened always return false

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I am using python 2.7 and opencv 3.0.0beta. However, when I load a .mov and .mp4 video using opencv, it always returns false. I have not tried other type of videos yet. I searched a lot of solutions online and changed the opencv_ffmpeg.dll toopencv_ffmpeg300.dll and moved them to the folder of python and even added path in system variables. Butttt, it still does not work.