dmalcolm (dmalcolm) wrote,

Python 3 support for GTK via PyGI: proof-of-concept

Screenshot showing GTK window created by Python 3 via PyGI

I spent much of last week taking part in a GTK/Python hackfest.

My particular interest is in Python 3 support, so I spent some time helping John Ehresman clean up pygobject, and some time with John Palmieri on PyGI

I'm somewhat new to gobject-introspection, so I created an ASCII-art diagram to try to help figure out how it works:

In the old GTK approach to binding native libraries for use by other language runtimes (such as Python's), a .defs file provided metadata on the API, which had to be kept in-sync with the code. An example can be seen here. For Python, this was used to generate .c code which could then be compiled into an extension module. A problem with this approach is that you typically need to write numerous .overrides files to handle the awkward cases, and these are specific to Python.

This means that for every N libraries (gtk, atk, gstreamer, dbus, telepathy, etc) and M runtimes (python 2, python 3, javascript, java, etc) you potentially need to handle N*M bindings, and each one could involve specialcasing.

In the new approach, "gobject-introspection" defines a simple textual format for source-code comments, containing similar information to a .defs file, but (I hope) rich enough to handle more of the special cases. This is scraped from the source into an XML file (e.g. Foo.gir), then compiled into an efficient binary format (e.g. Foo.typelib) which can be mapped into memory at runtime using a library (

The idea is that each language runtime can define a bridge, which calls into libgirepositrory, mapping between the language and the various libraries, using libffi, dynamically lazy-creating the glue code.

My hope is that we now need to handle only N + M cases, and so hopefully we have less of a combinatorial explosion, and potentially, faster start-up times and less RAM usage.

I've now got a py3k branch of PyGI in a Fedora git repository here.

(the tracking bug is

This code is able to be compiled against both Python 2.6 and Python 3.1 (in each case using the py3k branch of pygobject), and all tests pass with both versions of Python.

I can now write something like this:
from gi.repository import Gtk
w = Gtk.Window()
w.set_title('\u6587\u5b57\u5316\u3051') # 3 Kanji and a hiragana: "Mojibake"
b = Gtk.Button()
b.set_label('I am a GtkButton created by Python 3 via PyGI')

and have Python 3 dynamically load the Gtk typelib, dynamically load the underlying C libraries, and generate the machine code glue to call into the GTK library and create a Gtk Window. In this case a unicode string is correctly marshalled from Python 3, converted into UTF 8 internally within PyGI, and used to invoke gtk_window_set_title dynamically.

Early days yet (still seeing far too many errors when spelunking around inside the API), but at least it works well enough for a screenshot!

Thanks to the other hackfest attendees for answering my many dumb questions about PyGI, to Red Hat for paying me to work on Python 3 porting, and for sponsoring our food at the event, to One Laptop per Child for letting us use a room in their office as hacking space, and to Canonical for buying us coffee.
Tags: fedora, python
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