Develop Reference

Retrieve only a portion of an XML feed

I'm using Scrapy XMLFeedSpider to parse a big XML feed(60MB) from a website, and i was just wondering if there is a way to retrieve only a portion of it instead of all 60MB because right now the RAM consumed is pretty high, maybe something to put in the link like:
"http://site/feed.xml?limit=10", i've searched if there is something similar to this but i haven't found anything.
Another option would be limit the items parsed by scrapy, but i don't know how to do that.Right now once the XMLFeedSpider parsed the whole document the bot will analyze only the first ten items, but i supposes that the whole feed will still be in the memory.
Have you any idea on how to improve the bot's performance , diminishing the RAM and CPU consumption? Thanks

When you are processing large xml documents and you don't want to load the whole thing in memory as DOM parsers do. You need to switch to a SAX parser.
SAX parsers have some benefits over DOM-style parsers. A SAX parser
only needs to report each parsing event as it happens, and normally
discards almost all of that information once reported (it does,
however, keep some things, for example a list of all elements that
have not been closed yet, in order to catch later errors such as
end-tags in the wrong order). Thus, the minimum memory required for a
SAX parser is proportional to the maximum depth of the XML file (i.e.,
of the XML tree) and the maximum data involved in a single XML event
(such as the name and attributes of a single start-tag, or the content
of a processing instruction, etc.).
For a 60 MB XML document, this is likely to be very low compared to the requirments for creating a DOM. Most DOM based systems actually use at a much lower level to build up the tree.
In order to create make use of sax, subclass xml.sax.saxutils.XMLGenerator and overrider endElement, startElement and characters. Then call xml.sax.parse with it. I am sorry I don't have a detailed example at hand to share with you, but I am sure you will find plenty online.

You should set the iterator mode of your XMLFeedSpider to iternodes (see here):
It’s recommended to use the iternodes iterator for performance reasons
After doing so, you should be able to iterate over your feed and stop at any point.

How to pretty print xml in python without generating a DOM tree?

Generating a DOM tree is too expensive for very large xml data. Is there a method to accomplish the printing without generating it? I am using python-2.7.

Whatever the language, the way to parse a XML document without generating a tree is to use event-oriented parsers. With these kinds of parser, you give to the parser some event handlers that the parser will call at specific points of the processing: beginning of a node, end of a node, beginning of data, etc.
So you can use that kind of parser and go to a new line each time there is a new node, and increase indentation where you are entering a node and decrease indentation when you are exiting a node.
Because of the way these parsers work, it will be tricky to look ahead to see for example if a node fit in a line, so the pretty print may not be as pretty as when working with a tree (or you can, but it would be complicated).
In python, there are 3 event-driven parsers that come with the standard library (in no particular order):
ElementTree.iterparse()
pyexpat
sax (SAX is a well-known event-driven XML parsing API)
I suggest you have a look at them and try to play with.

Parse XML with lxml, and then manipulate it with cElementTree

I have an app which constantly reloads a large amount of XML data from a file, and then performs manipulations, and then writes back to file.
The lxml library is proven much faster for parsing and un-parsing XML, but cElementTree is much faster for certain kinds of manipulation. Both have an almost identical API.
How can I parse an XML file with lxml, and then manipulate it with cElementTree?
This is what I've tried, but the object produced by lxml parse methods inherently use it's own manipulative methods.
import xml.etree.cElementTree as ET
from lxml import etree as lxmlET

This question is perhaps the Python equivalent of "My friend has a fast car and I just have a clunker. How can I make my car go as fast as hers?"
I'm not saying this couldn't be done, but I should call call such an enterprise either ambitious or foolhardy, depending on your level of programming skill. The point is that each system has, as you have discovered, its own internal representation of the parsed XML.
While it might be possible to write code to take the parsed object produced by lxml and re-create or wrap it as ElementTree elements, it's probably going to a) take as long as parsing with ElementTree in the first place, and b) be a maintenance nightmare.
So do yourself a favor and choose one technology then stick with it (at least for each individual program).
I would also point out that XML was intended primarily as a data interchange language. The fact that you seem to be using it as a structured data repository inevitably introduces large inefficiencies in the processing, particularly as data volumes go up. Might it be better to choose some more amenable representation and then only convert it to XML for output and usage by other systems?

Python xml marshalling

I'm learning Python, my background is Java EE. I have used JAXB before, where I can basically define a regular class, throw some annotations in there and then use JAXB to marshall objects to xml. This means I am not concerned with creating root elements, nodes, etc. but merely writing the Java class and anotating it here and there. Is there anything like this for Python?

Here are a few:
lxml.objectify
gnosis.xml.objecity
pyxser seems pretty cool
Pickle to XML - uses Python's pickle and xml.dom.minidom
pyxml -from xml import marshal (might be buggy)
Amara might be worth looking into.

PyXB seems to be the closest thing to JAXB although I haven't used it yet. I use lxml at the moment and find it works well.
Amara was promising but seemed to stagnate.

Parsing a large (~40GB) XML text file in python

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?

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.

The best solution will depend in part on what you are trying to do, and how free your system resources are. Converting it to a postgresql or similar database might not be a bad first goal; on the other hand, if you just need to pull data out once, it's probably not needed. When I have to parse large XML files, especially when the goal is to process the data for graphs or the like, I usually convert the xml to S-expressions, and then use an S-expression interpreter (implemented in python) to analyse the tags in order and build the tabulated data. Since it can read the file in a line at a time, the length of the file doesn't matter, so long as the resulting tabulated data all fits in memory.