There is a difference in what HTTPResponse.getheaders() returns.

Python 2.7.2 (default, Jun 12 2011, 14:24:46) [MSC v.1500 64 bit (AMD64)] on win32
Type "help", "copyright", "credits" or "license" for more information.
>>> import httplib
>>> c = httplib.HTTPConnection('www.joelverhagen.com')
>>> c.request('GET', '/sandbox/tests/cookies.php')
>>> c.getresponse().getheaders()
[('content-length', '0'), ('set-cookie', 'test_cookie1=foobar; expires=Fri, 02-Mar-2012 16:54:15 GMT, test_cookie2=barfoo; expires=Fri, 02-Mar-2012 16:54:15 GMT'), ('vary', 'Accept-Encoding'), ('server', 'Apache'), ('date', 'Fri, 02 Mar 2012 16:53:15 GMT'), ('content-type', 'text/html')]

Python 3.2.2 (default, Sep  4 2011, 09:07:29) [MSC v.1500 64 bit (AMD64)] on win32
Type "help", "copyright", "credits" or "license" for more information.
>>> from http import client
>>> c = client.HTTPConnection('www.joelverhagen.com')
>>> c.request('GET', '/sandbox/tests/cookies.php')
>>> c.getresponse().getheaders()
[('Date', 'Fri, 02 Mar 2012 16:56:40 GMT'), ('Server', 'Apache'), ('Set-Cookie', 'test_cookie1=foobar; expires=Fri, 02-Mar-2012 16:57:40 GMT'), ('Set-Cookie', 'test_cookie2=barfoo; expires=Fri, 02-Mar-2012 16:57:40 GMT'), ('Vary', 'Accept-Encoding'), ('Content-Length', '0'), ('Content-Type', 'text/html')]

As you can see, in 2.7.2 HTTPResponse.getheaders() in 2.7.2 joins headers with the same name by ", ". In 3.2.2, the headers are kept separate and two or more 2-tuples.

This causes problems if you convert the list of 2-tuples to a dict, because the keys collide (causing all but one of the values associated the non-unique keys to be overwritten).  It looks like this problem is caused by using the email header parser (which keeps the keys and values as separate 2-tuples). In Python 2.7.2, the HTTPMessage.addheader(...) function does the comma-separating.

Is this API change intentional? Should HTTPResponse.getheaders() comma-separate the values like the HTTPResponse.getheader(...) function (in both 2.7.2 and 3.2.2)?

See also:
https://github.com/shazow/urllib3/issues/3#issuecomment-3008415