Opencv+v4l2 set resolution

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Hello, I am using Opencv 2.4.9 on my Raspberry PI2 and I am trying to change the resolution width and height using v4l2 through command line.

command = 'v4l2-ctl -d --set-fmt-video=width= ,height= ';

The raspberry set the proper resolution but when I try to make a video stream using VideoCapture, the resolution returns to the default values. Has already someone encount and solve the same problem?

Thank you in advance.