When subclassing gst.BaseTransform with passthrough disabled and in_place enabled the buffer passed to do_transform_ip is not writtable. A code snippet is attached to show the bug:

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import gst, gobject, traceback

class MyTransform(gst.BaseTransform):    
    __gsttemplates__ = (
     
        gst.PadTemplate("src",
                    gst.PAD_SRC,
                    gst.PAD_ALWAYS,
                    gst.caps_new_any()),
        
        gst.PadTemplate("sink",
                    gst.PAD_SINK,
                    gst.PAD_ALWAYS,
                    gst.caps_new_any())
    )

    def __init__(self, name):
            self.__gobject_init__()
            self.set_name(name)
            self.set_passthrough(False)
            self.set_in_place(True)
          
        
    def do_transform_ip(self, buffer):
            buffer[0]=1 #This should work
            return gst.FLOW_OK
    
    def set_property(self, key, value):
            if key == "name":
                self.set_name(name)
            else:
                print "Property %s does not exist" % key
    
    def get_property(self, key):
            if key == "name":
                return self.get_name()
            else:
                print "Property %s does not exist" % key

gobject.type_register(MyTransform)
gobject.threads_init()

src = gst.element_factory_make("fakesrc", "src")
sink = gst.element_factory_make("fakesink", "sink")
trans = MyTransform("trans")

pipe = gst.Pipeline("MyPipeline")
pipe.add(src,trans, sink)
gst.element_link_many(src, trans, sink)

def bus_event(bus, message):

    t = message.type
    print message
    if t == gst.MESSAGE_EOS:

        loop.quit()

    return True

pipe.get_bus().add_watch(bus_event)

pipe.set_state(gst.STATE_PLAYING)

loop = gobject.MainLoop()

try:
    loop.run()
except:
    traceback.print_exc()

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The non-working line is marked with #This should work