gstreamer - caps modify

#!/usr/bin/env python3
# -*- coding:utf-8 -*-

import sys
import gi
gi.require_version('Gst', '1.0')
from gi.repository import Gst
Gst.init(None)

pipeline = Gst.parse_launch("v4l2src device=/dev/video0 ! capsfilter name=mycapsfilter caps=video/x-raw,width=1280,height=720 ! x264enc tune=zerolatency ! mp4mux ! filesink location=/tmp/file.mp4")

pipeline.set_state(Gst.State.PLAYING)

aaa = pipeline.get_by_name('mycapsfilter')

while True:
    print "+ [1], - [2], end [0]"
    x = raw_input()

    if x=='0':
        print "END"
        bus = pipeline.get_bus()
        bus.add_signal_watch()
        pipeline.send_event(Gst.Event.new_eos())
        msg = bus.timed_pop_filtered(Gst.CLOCK_TIME_NONE, Gst.MessageType.ERROR | Gst.MessageType.EOS)
        pipeline.set_state(Gst.State.NULL)
        sys.exit()

    elif x=='1':
        print "+"
        aaa.set_property("caps", 'video/x-raw,width=1280,height=720')

    elif x=='2':
        print "-"
        aaa.set_property("caps", 'video/x-raw,width=320,height=240')


    else:
        print "wtf"









/usr/bin/python2.7 /home/martin/PycharmProjects/gstreamer/e.py
+ [1], - [2], end [0]
1
+
Traceback (most recent call last):
  File "/home/martin/PycharmProjects/gstreamer/e.py", line 31, in <module>
    aaa.set_property("caps", 'video/x-raw,width=1280,height=720')
TypeError: could not convert 'video/x-raw,width=1280,height=720' to type 'GstCaps' when setting property 'GstCapsFilter.caps'

Process finished with exit code 1