“because it’s not compiled, it’s also very quick” [1]

It’s a different definition of quick of course, but somehow I begin to feel disconnected and trolling mode wells up from the darkness within.

But… even in that sense, is it really as quick as we think it is? In my experience it certainly makes it quicker to discover new and interesting types of bugs.

The quote relates to Ruby, but really, be it Ruby, Perl, or Python it seems much the same.

[1] I’ve quoted from the 3rd page of an article there. I’m still not quite sure why these sites bother to separate their articles into “pages.” Even SMH used to be one-page-per-article before this pagination fad caught on.

I play with the snake from time to time, Python having now almost entirely supplanted Perl as my hak-n-slash language. But I’m certainly still learning as I go. Today I’ve been trying to work out what’s gone wrong with something I’m doing. I thought I’d done something strange with inheritance, not understanding some case where inheritance causes data to become static (in the sense of static members in C++.) What was really the problem is that I didn’t realise that default parameters to members are kept and reused, and if you go and use a parameter you can end up with state being held on to where you don’t (well, I didn’t) expect it.

I’ve hit this at least once before I think, it is vaguely familiar. Anyway, it’s a bit of a tricky gotcha as far as I’m concerned. Intuitive? I wouldn’t say so. Here’s the example:

#!/usr/bin/python

class Parent:
    def __init__(self, stuff = []):
        self.stuff = stuff
    def doIt(self):
        print " ".join(self.stuff)
    def addStuff(self, stuff):
        self.stuff.append(stuff)

class Child(Parent):
    def __init__(self, stuff = []):
        Parent.__init__(self)
    def doIt(self):
        self.addStuff("foo")
        Parent.doIt(self)

c1 = Child()
c1.doIt()
c2 = Child()
c2.doIt()
c3 = Child()
c3.doIt()

Execute this and we see:

foo
foo foo
foo foo foo

What? Why is it accumulating “foo”s? The answer is in the “stuff = []” argument to __init__. What is going on, as far as I can garner from the documentation, is that that default argument (a list) is being instantiated once and then kept for future use. What’s more, I’m assigning self.stuff to this, which is kept as a reference and doesn’t create a copy. So I have ended up with a static value for self.stuff, well, within the functionality of this code – it isn’t entirely congruous to a static member.

How to fix it? I don’t know the definitive approach, but here’s a couple of ways. To achieve complete deep copying of the argument whether it be default or passed in use copy.deepcopy:

#!/usr/bin/python
import copy

class Parent:
    def __init__(self, stuff = []):
        self.stuff = copy.deepcopy(stuff)
...

Alternatively, you could use copy.copy for a shallow copy and I guess that might be analogous to this:

#!/usr/bin/python

class Parent:
    def __init__(self, stuff = []):
        self.stuff = []
        self.stuff.extend(stuff)
...

I’m sure there are great uses for this behaviour in Python. What are they? Something more fundamental than “neat tricks” involving incremental/changing default state? Am I the only one to think this somewhat of a gotcha?

Now that I’ve worked out what was wrong I can retrospectively build the right Google magic to get straight to the answer.

No time to read up on more casual chatter about it though. The documented formula is something like:

#!/usr/bin/python

class Parent:
    def __init__(self, stuff = None):
        if stuff is None:
            stuff = []
        self.stuff = stuff
...

In theory my ADSL finally goes live tomorrow. The frustration of trying to work without a reliable ‘net connection has been high. The O2 mobile broadband account has been a life-saver, although it certainly has its failings.

There’s one thing that has been rather a bother. While SSH connections are fairly manageable, the only problem being intermittent disconnections lasting 5 to 10 minutes, HTTP behaves very badly. Even when I did have an apparent working ‘net link, evidenced by the fact that my SSH sessions were still live, HTTP connections would time out and fail regularly.

I have HTTP working pretty well now, here’s what I did:

1. Forward all HTTP(S) to a remote proxy, in my case running on a machine I have in Germany.
2. Stop using the O2 allocated DNS addresses, I’m using OpenDNS‘s 208.67.222.220 and 208.67.222.222 instead, though I’m considering just using my own remote DNS server.

The former should be all that is required, however it looks like Opera uses local name resolution for something even when a proxy is set up. The latter alone may be sufficient too, since the ‘net is certainly still there but it looks like name resolution times out or fails.

I’ve been using Linux for quite some time now. It’s been more than 10 years since my first hesitant install of Debian (from the front cover of a tech mag) in early 1998, and a little longer since I first poked at my cousin’s Linux computers. In the earlier days of my Linux use there was rarely a time I had to power-cycle a locked up machine when I was running a ‘stable’ distro. Sure, sometimes a bleeding-edge X, or somesuch, would lock up – but I was nearly always able to switch to a console or get a serial terminal and kick it in the arse.

Things seem to be a bit different now. More and more often I’ll have to hold down that power button. Sometimes I can magic-sysreq a reboot, but often not. The culprit is usually web browsers, but sometimes other X applications. It does seem to be an X thing. I use an RC Firefox with a beta Flash, so I expect breakage. What I don’t expect is breakage in my userspace apps to lock up my machine irretrievably. Especially since I’m using a stable distro, aside from the Flash and Firefox my machine is 100% Ubuntu ‘gutsy’ (no proper ‘net yet so I haven’t done my dist-upgrade, the 13th I’m told now, Friday the 13th! Joy.)

Linux never used to do this to me. Has robustness suffered in the quest for a snappier user experience? Am I just unlucky? It could be my non-free video driver (nvidia) I guess. I haven’t had the time to try debugging the problem, I should really, since it can be replicated.

Maybe I’m just seeing the past through rose coloured glasses, could it have been worse than I recall? I’ve had to reboot twice this morning, so now I’ll stop trying to play with the Firefox RC and get back to browsing with Opera.

That aside, it looks like Firefox 3 could win me back from being an Opera user. If my bloody Linux stops hanging when I use it!

My donglenet is cool because I can post this from my laptop while travelling home on the train while passing between a couple of fields.

OK, so it cuts out from time to time, well… a lot, and can’t handle tunnels at all.

Yeah, it isn’t really that good.

But it’s still cool. 🙂

Of course, funny to think that I had access to this very same coolness nearly 5 years ago when I got my first 3G handset back in Sydney. Old tech in new packaging (though definitely higher bandwidth.)