Python Secret Weblog for 2005 January 13 (entry 0)

Thu Jan 13 2005 20:33 lxml findall and xpath performance:

Update: lxml got quite a bit faster since this entry, see here

I've been testing findall() performance on etree versus ElementTree/cElementTree. cElementTree and even ElementTree are quite a bit faster than lxml.etree at this stage. Possible causes of performance loss:

• lxml.etree has to maintain proxy objects over the underlying libxml2 C tree.
• lxml.etree uses a weak value dictionary to maintain weak references to all proxies in use. This seems particularly slow.
• There's also a lot of UTF-8 to unicode conversion involved, as libxml2 uses UTF-8 strings throughout, and Python uses double-byte unicode strings.

Unfortunately the Python profilers don't profile C functions called in the extension module, which makes my measuring job somewhat harder.

Anyway:

findall('//v') on ot.xml

ElementTree: 0.13 s
cElementTree: 0.11 s
lxml.etree: 1.9 s

Whoah, not good for lxml.etree to lose out to pure Python that badly!

I also tested libxml2 xpath, which I added to lxml.etree today, and even this is quite a bit slower at simple operations like (//v), somewhat more to my surprise:

xpath('//v')

lxml.etree: 0.76 s

I think in part the large result set slows it down, as Element proxies have to be created for all elements in it.

As an example of that, this is actually faster (as it only makes strings):

xpath('//v/text()')
lxml.etree: 0.34

Of course, xpath is not only about raw performance, but also about features, like this:

>> t = parse('ot.xml')
>> self.t.xpath('(//v)[5].text()')
[u'And God called the light Day, and the darkness he called Night. And the evening and the morning were the first day.\n']

This happens in about 0.25 seconds, and is not something cElementTree can do with its findall(), though I expect the cElementtree Python equivalent of that would be quite a bit faster, I expect.

Oh well, it was a bit of a bummer that Fredrik released something insanely much faster just as I was finally getting somewhere with lxml.etree.. :)