python - Parsing compressed xml feed into ElementTree

Question

I'm trying to parse the following feed into ElementTree in python: "http://smarkets.s3.amazonaws.com/oddsfeed.xml" (warning large file)

Here is what I have tried so far:

feed = urllib.urlopen("http://smarkets.s3.amazonaws.com/oddsfeed.xml")

# feed is compressed
compressed_data = feed.read()
import StringIO
compressedstream = StringIO.StringIO(compressed_data)
import gzip
gzipper = gzip.GzipFile(fileobj=compressedstream)
data = gzipper.read()

# Parse XML
tree = ET.parse(data)

but it seems to just hang on compressed_data = feed.read(), infinitely maybe?? (I know it's a big file, but seems too long compared to other non-compressed feeds I parsed, and this large is killing any bandwidth gains from the gzip compression in the first place).

Next I tried requests, with

url = "http://smarkets.s3.amazonaws.com/oddsfeed.xml"
headers = {'accept-encoding': 'gzip, deflate'}
r = requests.get(url, headers=headers, stream=True)

but now

tree=ET.parse(r.content)

or

tree=ET.parse(r.text)

but these raise exceptions.

What's the proper way to do this?

Answer 1

You can pass the value returned by urlopen() directly to GzipFile() and in turn you can pass it to ElementTree methods such as iterparse():

#!/usr/bin/env python3
import xml.etree.ElementTree as etree
from gzip import GzipFile
from urllib.request import urlopen, Request

with urlopen(Request("http://smarkets.s3.amazonaws.com/oddsfeed.xml",
                     headers={"Accept-Encoding": "gzip"})) as response, 
     GzipFile(fileobj=response) as xml_file:
    for elem in getelements(xml_file, 'interesting_tag'):
        process(elem)

where getelements() allows to parse files that do not fit in memory.

def getelements(filename_or_file, tag):
    """Yield *tag* elements from *filename_or_file* xml incrementaly."""
    context = iter(etree.iterparse(filename_or_file, events=('start', 'end')))
    _, root = next(context) # get root element
    for event, elem in context:
        if event == 'end' and elem.tag == tag:
            yield elem
            root.clear() # free memory

To preserve memory, the constructed xml tree is cleared on each tag element.