According to the RFC, the server is allowed to send back any encoding it likes when no Accept-Encoding header is supplied, but all the examples I can find of urllib2.urlopen usage assume they're getting plain text back.  I think it would be better to inject an Accept-Encoding header when none is explicitly supplied so that nobody else trips over this issue.

See http://support.github.com/discussions/site/1510

HTTP Ref says that Server can send any encoding, if client does not
specify Accept-Encoding header. But if 'identity' is one of the
encoding that server recognizes (?), then it should send it as
identity, which indicates untransformed content.

I also see in the httplib that Accept-Encoding = 'identity' is added in the
request level to the headers. I shall see what is missing here, if it
is not being sent for all requests.

BTW, I could not figure out the problem you are facing from the url
mentioned. I specifically do not see any interleaving gzip and no-gzip
request behaviours at different points.

How many tests did you run?  My two tests were minutes apart.  I have the feeling that this has something to do with cacheing behavior on the server.

What was the content of http://support.github.com/discussions/site/1510
I can't find it. Is the issue still going on?

This doesn't seem to be an issue in 3.4+, the following headers are injected in a call to urlopen():

GET / HTTP/1.1
Accept-Encoding: identity
Host: example.com
User-Agent: Python-urllib/3.4
Connection: close

However, this is not the same behaviour in 2.7:

GET / HTTP/1.0
Host: example.com
User-Agent: Python-urllib/1.17

That said, I wouldn't see this as a bug but a feature request, so it should be invalid for 2.7.

Setting this to pending to close unless anyone has any objections or further details.

I suspect for Demian’s 2.7 experiment, he used the older urllib.urlopen(), rather than urllib2.urlopen() as given in the original description. When I use urllib2.urlopen("http://localhost/"), I see

GET / HTTP/1.1
Accept-Encoding: identity
Host: localhost
Connection: close
User-Agent: Python-urllib/2.7

Even in the urllib (no 2) case, since it is using HTTP 1.0, I suspect not having Accept-Encoding is not such a problem.

The underlying HTTP library has always added “Accept-Encoding: identity” for HTTP 1.1 by default (https://hg.python.org/cpython/annotate/4a3e9871b41b/Lib/httplib.py#l444), so I am closing this.