Does python urllib2 automatically uncompress gzip data fetched from webpage?

1. How can I tell if the data at a URL is gzipped?

This checks if the content is gzipped and decompresses it:

from StringIO import StringIO
import gzip

request = urllib2.Request('http://example.com/')
request.add_header('Accept-encoding', 'gzip')
response = urllib2.urlopen(request)
if response.info().get('Content-Encoding') == 'gzip':
    buf = StringIO(response.read())
    f = gzip.GzipFile(fileobj=buf)
    data = f.read()

2. Does urllib2 automatically uncompress the data if it is gzipped? Will the data always be a string?

No. The urllib2 doesn't automatically uncompress the data because the 'Accept-Encoding' header is not set by the urllib2 but by you using: request.add_header('Accept-Encoding','gzip, deflate')

If you are talking about a simple .gz file, no, urllib2 will not decode it, you will get the unchanged .gz file as output.

If you are talking about automatic HTTP-level compression using Content-Encoding: gzip or deflate, then that has to be deliberately requested by the client using an Accept-Encoding header.

urllib2 doesn't set this header, so the response it gets back will not be compressed. You can safely fetch the resource without having to worry about compression (though since compression isn't supported the request may take longer).