Wandering Thoughts: Recent Entries For 2008/06/08

Thinking about Python's inheritance model

I recently read an entry by someone who was surprised that Python subclasses did not automatically call their parent class's 'constructor' method, by which he meant __init__. This is a straightforward consequence of Python's inheritance model, but Python's inheritance model is slightly odd.

The straightforward way to explain Python's inheritance model is that it uses what I could call 'lookup based' or 'name based' subclassing, where what subclassing does is change what binding will be returned when you look up a given name on an instance of the (sub)class. And that's pretty much it as for what subclassing does.

(Technically subclassing has some additional effects when you use __slots__, but that's another discussion.)

The next thing to know is that all of Python's special operations, including __init__, work by normal method calls; they look up the binding for the name of the special method and then call what they get back (if anything). So in this model it makes perfect sense that there is no upcall to the parent class's __init__ method; your subclass overrode the binding of the name, so your method function gets called instead of the parent's. Doing an upcall would make __init__ a magic special operation, unlike all of the others.

(Note that multiple inheritance does not preclude magic upcalls, since there is a defined method resolution order that the upcalling could follow.)

One consequence of this simple model of subclassing is that you can fake subclassed instances by using other methods and outside people generally won't notice anything. (They can tell if they use isinstance() or the like, but most code shouldn't.)

Recovering my Eee PC from a post-update problem

I recently applied a bunch of pending Eee updates from Asus, and suddenly the full desktop mode that I prefer stopped working. The Eee always booted into the basic interface, and when I switched back to advanced mode everything in my Desktop directory had disappeared (which is bad, because I have some customized launchers there)

(Fortunately I had an off-machine backup of my Desktop directory so I could restore my customizations.)

I enabled advanced desktop mode the easy way, namely by installing the advanced-desktop-eeepc package. After poking around and trying several things, what fixed the problem problem was to apt-get remove and then apt-get install the package again.

(I assume that something in one of the Asus updates overwrote a customization that the advanced desktop stuff required, although I couldn't spot anything obvious. It is a little disturbing to me that something was apparently deleting all additions to my Desktop folder, although I don't know whether this is something in the basic desktop or in the advanced desktop setup stuff.)

Sidebar: the launchers I've found useful

By 'launchers' I mean icons on the desktop that start various programs when clicked, which are created by .desktop files in your $HOME/Desktop directory. The ones I use are: blank the screen, start konsole, start xterm, and start a black-on-white xterm (instead of the default white on black colour scheme).