Archive for February, 2010

Released django-taxonomy on github

Sunday, February 14th, 2010

Hi all,

I did a post several months ago about creating a generic taxonomy app for Django that was loosely coupled, unintrusive, and could evolve with an app that needed categories today, but then tags later, or labels later, or some other classification mechanism later. I wanted one app to just be generic enough to deal with it, so I created django-taxonomy…. and then did nothing with it.

Well, *I* did stuff with it, but I never put it anywhere where anyone else could do anything with it. So now I have: django-taxonomy is up on github. Please fork it and send me merge requests, because this app is not super high on my priority list, which is why I’m releasing it: it’ll get further and be more useful to you with the help of the community 🙂

Seeking Elegant Pythonic Solution

Monday, February 1st, 2010

So, I have some code that queries a data source, and that data source sends me back an XML message. I have to parse the XML message so I can store information from it into a relational database. So, let’s say my XML response looks like this:


So, as you can see, the attributes for each result returned for a query can differ, and if a result doesn’t have a value for some attribute, the corresponding xml element isn’t included at all for that result. If it were just 2 or 3 attributes, I could easily enough get around it by doing something like this:

def __init__(self, xmlresult):
  self.xmlresult = xmlresult
  if self.xmlresult.xpath('fname') is not None:
    self.fname = self.xmlresult.xpath('fname')
  if self.xmlresult.xpath('lname') is not None:
    self.lname = self.xmlresult.xpath('lname')

Like I said, if it were just a few things I needed to check for, I’d do it this way and be done with it. It’s not just a few though — it’s like 50 attributes. Now what?

I decided lxml.objectify would be a great way to go. It would allow me to access these things as object attributes, which should mean I can do something like this:

self.fname = getattr(self.xmlresult, 'fname', None)
self.lname = getattr(self.xmlresult, 'lname', None)

So, you *can* do this, technically speaking. Trouble is, you’re asking for an attribute of an ObjectifiedElement object, and when you do that, it returns an object that is not a native Python datatype, which I did not realize when I first started using lxml.objectify. So, in the above, ‘self.fname’ will not be a Python string — it’ll be an lxml.objectify.StringElement object. Of course, my database driver, my ‘join()’ operations, and everything else in my code that relies on native Python datatypes is now broken.

What I actually need to do is get the ‘.pyval’ attribute of self.xmlresult.fname, if that attribute exists at all. So, something that does what I mean, which is “self.fname = getattr(self.xmlresult, ‘fname.pyval’, None). And, of course, doing ‘getattr(self.xmlresult, ‘fname’, None).pyval’ doesn’t work because None has no attribute ‘pyval’. I’ve tried a couple of other hacks too, but I’ve learned enough Python to know that if it feels like a hack, there’s probably a better way. But I can’t find that better way. Ideas?