[gst-devel] Re: making Python GStreamer plugins work
otaylor at redhat.com
Mon May 2 02:56:18 CEST 2005
On Sat, 2005-04-30 at 12:25 -0600, John Ehresman wrote:
> Owen Taylor wrote:
> > I think it should be possible to do basically the same thing in Python
> > by creating the GObject out of GObject.__new__() rather than out of
> > GObject.__init__().
> This would break backwards compatibility.
> > __new__() calls g_object_new() calls ... ->init() which allocates
> > the python object and binds it to the GObject
> > This is not 101% compatible with the current system but all the
> > incompatible cases I can come up with are pretty artificial (*)
> It would break this:
> class MyButton(gtk.Button):
> def __init__(self):
> gtk.Button.__init__(self, 'My Default Text', True)
> This could be rewritten to use __new__, but it would need to be rewritten.
I think this still works ... since the GObject is *always* created in
the __new__ callback, GtkButton's __init__ looks like:
def __init__(self, label=None):
Constructor arguments that set CONSTRUCT_ONLY properties may be a
problem (when aren't they...)
> > But in such cases all GObject additions would be would be a slightly
> > cleaner version of John's technique of passing data to the constructor
> > via magic properties... whether the constructor triggers GObject
> > creation or whether GObject creation triggers the constructor is
> > going to make a difference in how things are ordered.
> Yes, using a magic property is a bad idea -- there's a reason the patch
> was never committed. It's better to use thread local storage, although
> that depends on no callback occurring between the call to g_object_new
> and the GType constructor.
I don't think you can make that assumption ... the init() function of
a superclass might result in the creation of another Python object.
(It's a little contrived, but
> For bindings that have the language specific
> constructor trigger GObject creation (like pygtk), it would be better to
> have a factory function that would get all the properties passed in and
> be responsible for returning a valid GObject.
That's basically what constructor() is ... all it seems you might want
gpointer g_object_new_with_data (GType object_type,
const gchar *first_property_name,
and a hidden fourth parameter to constructor(). (Compatible by the
"OK to call with more parameters" quasi-standard that we depend upon in
various places. *Except* that any classes deriving from the class
have to provide the fourth argument when chaining up. Not a problem
But while this would allow you to do the GObject/Python hookup to
either a newly created or existing Python object it has certain ordering
- The naive implementation would try to set implemented-in-Python
CONSTRUCT properties before the Python/GObject hookup was done.
You'd have to strip them out from the list of construct properties
before chaining up, chain up, do the hookup, then set the Python
- The problem with chaining up to the superclass's __init__ with
parameters exists if you call __init__ at any point, since the
__init__ function might attempt to chain up with parameters
even if no parameters were passed in.
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