Question

I’ve got an XML file I want to parse with python. What is best way to do this? Taking into memory the entire document would be disastrous, I need to somehow read it a single node at a time.

Existing XML solutions I know of:

• element tree
• minixml

but I’m afraid they aren’t quite going to work because of the problem I mentioned. Also I can’t open it in a text editor – any good tips in generao for working with giant text files?

Answer 1

First, have you tried ElementTree (either the built-in pure-Python or C versions, or, better, the lxml version)? I’m pretty sure none of them actually read the whole file into memory.

The problem, of course, is that, whether or not it reads the whole file into memory, the resulting parsed tree ends up in memory.

ElementTree has a nifty solution that’s pretty simple, and often sufficient: iterparse.

for event, elem in ET.iterparse(xmlfile, events=('end')):
  ...

The key here is that you can modify the tree as it’s built up (by replacing the contents with a summary containing only what the parent node will need). By throwing out all the stuff you don’t need to keep in memory as it comes in, you can stick to parsing things in the usual order without running out of memory.

The linked page gives more details, including some examples for modifying XML-RPC and plist as they’re processed. (In those cases, it’s to make the resulting object simpler to use, not to save memory, but they should be enough to get the idea across.)

This only helps if you can think of a way to summarize as you go. (In the most trivial case, where the parent doesn’t need any info from its children, this is just elem.clear().) Otherwise, this won’t work for you.

The standard solution is SAX, which is a callback-based API that lets you operate on the tree a node at a time. You don’t need to worry about truncating nodes as you do with iterparse, because the nodes don’t exist after you’ve parsed them.

Most of the best SAX examples out there are for Java or Javascript, but they’re not too hard to figure out. For example, if you look at http://cs.au.dk/~amoeller/XML/programming/saxexample.html you should be able to figure out how to write it in Python (as long as you know where to find the documentation for xml.sax).

There are also some DOM-based libraries that work without reading everything into memory, but there aren’t any that I know of that I’d trust to handle a 40GB file with reasonable efficiency.