Help with Gst.Iterator.find_custom() in python

[Mathieu Duponchelle]

Looks like a bug in pygobject, I tried to pinpoint it but that code base is
a bit of a maze :(

And I think there's actually two bugs: one is the parameters passed back
to the closure as you noticed, the other one is the final return value when
no match was found, these are two separate issues.

Note that the gst-python overrides let you iterate the GstIterator as a normal
python iterator, why don't you simply loop over it to find your object?

> Hi, I'm trying to create a function in python to find elements in a bin based
> on their type (not name).


On 5/21/21 9:24 AM, Marianna S. Buschle via gstreamer-devel wrote:
> Hi, I'm trying to create a function in python to find elements in a bin based
> on their type (not name).
>
> I have such a function in C which works and I'm trying to convert it to
> python but I'm experiencing some issues.
>
> def compare_gtype(element, type):
>     print(element)
>     print(type)
>     
>     return 1
>
> def gst_bin_get_by_gtype(bin, type_name):
>     factory = Gst.ElementFactory.find(type_name)
>     type = factory.get_element_type()
>     print(type_name)
>     print(type)
>     
>     print("[" + bin.get_name() + "]: looking up child element of type " +
> type.name)
>
>     children = bin.iterate_recurse()        
>     if children:
>         print(children)
>         print(children.type)
>         found, elem = children.find_custom(compare_gtype, type)
>     else:
>         print("Error")
>
>     if found:
>         print("Found")
>         return elem
>     else:
>         print("Not Found")
>         return None
>
> This is the output I get:
>
> filesink
> <GType GstFileSink (140449056935968)>
> [pipeline0]: looking up child element of type GstFileSink
> <Gst.Iterator object at 0x7fbcded2bdc0 (GstIterator at 0x7fbcd0009220)>
> <GType GstElement (140449054520272)>
> <GType GstFileSink (140449056935968)>
> 140448920294640
> <GType GstFileSink (140449056935968)>
> 140448920294640
> <GType GstFileSink (140449056935968)>
> 140448920294640
> <GType GstFileSink (140449056935968)>
> 140448920294640
> <GType GstFileSink (140449056935968)>
> 140448920294640
> API Exception: {'type': 'TypeError', 'args': ('unknown type (null)',),
> 'message': 'unknown type (null)'}
> Traceback (most recent call last):
>   File "/usr/lib/python3/dist-packages/jsonrpc/manager.py", line 113, in
> _get_responses
>     result = method(*request.args, **request.kwargs)
>   File "/home/msb/QtecGit/python-tests/gst_ctrl.py", line 248, in play
>     gst_handler.play(index)
>   File "/home/msb/QtecGit/python-tests/gst_ctrl.py", line 69, in play
>     elem = gst_bin_get_by_gtype(self.pipeline[index], "filesink")
>   File "/home/msb/QtecGit/python-tests/gst_ctrl.py", line 42, in
> gst_bin_get_by_gtype
>     found, elem = children.find_custom(compare_gtype, type)
> TypeError: unknown type (null)
>
> So for me it seems like the find_custom() is not passing the GstElement from
> the iterator to the compare_gtype() function...
>
> And this is the pipeline it is searching (no filesink):
> filesrc name=replay location=/home/msb/test.ts ! decodebin ! videoconvert !
> ximagesink sync=1
>
> And just for reference the code in C:
>
> static gint
> compare_gtype(const GValue *velement, GType type)
> {
> 	gint eq = 1;
> 	GstElement *element = g_value_get_object(velement);
>
> 	GST_OBJECT_LOCK(element);
> 	if (G_OBJECT_TYPE(element) == type)
> 		eq = 0;
> 	GST_OBJECT_UNLOCK(element);
>
> 	return eq;
> }
>
> static GstElement *
> gst_bin_get_by_gtype(GstBin *bin, GType type)
> {
> 	GstIterator *children;
> 	GValue result = { 0, };
> 	GstElement *element;
> 	gboolean found;
>
> 	g_return_val_if_fail(GST_IS_BIN(bin), NULL);
>
> 	QTEC_DEBUG("[%s]: looking up child element of type %s",
> 		   GST_ELEMENT_NAME(bin), g_type_name(type));
>
> 	children = gst_bin_iterate_recurse(bin);
> 	found = gst_iterator_find_custom(children,
> 					 (GCompareFunc) compare_gtype,
> 					 &result, (gpointer) type);
> 	gst_iterator_free(children);
>
> 	if (found) {
> 		element = g_value_dup_object(&result);
> 		g_value_unset(&result);
> 	} else {
> 		element = NULL;
> 	}
>
> 	return element;
> }
>
>         GstElementFactory * proxysrc_factory =
> gst_element_factory_find("proxysrc");
>
> 	if (!proxysrc_factory)
> 		return FALSE;
>
> 	GType proxysrc_type =
> gst_element_factory_get_element_type(proxysrc_factory);
>
> 	GstElement *psrc = gst_bin_get_by_gtype(GST_BIN(bin), proxysrc_type);
>
>
>
> --
> Sent from: http://gstreamer-devel.966125.n4.nabble.com/
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