issue2756: urllib2 add_header fails with existing unredirected_header

>>> import urllib2
>>> url = 'http://www.whompbox.com/headertest.php'
>>> request = urllib2.Request(url)
>>> request.add_data("Spam")
>>> f = urllib2.urlopen(url)
>>> request.header_items()
[]
>>> request.unredirected_hdrs.items()
[]
>>> f = urllib2.urlopen(request)
>>> request.header_items()

[('Content-length', '4'), ('Content-type', 'application/x-www-form-urlencoded'), ('Host', 'www.whompbox.com'), ('User-agent', 'Python-urllib/2.6')]

>>> request.unredirected_hdrs.items()

[('Content-length', '4'), ('Content-type', 'application/x-www-form-urlencoded'), ('Host', 'www.whompbox.com'), ('User-agent', 'Python-urllib/2.6')]


Comment: This is fine. What is actually happening is do_request_ method in the http_open() is setting the unredirected_hdrs to above items.

>>> request.add_header('Content-type','application/xml')
>>> f = urllib2.urlopen(request)
>>> request.header_items()

[('Content-length', '4'), ('Content-type', 'application/xml'), ('Host', 'www.whompbox.com'), ('User-agent', 'Python-urllib/2.6')]

Comment: When we add_header() the headers are indeed changed. Correct behavior. 
>>> request.unredirected_hdrs.items()

[('Content-length', '4'), ('Content-type', 'application/x-www-form-urlencoded'), ('Host', 'www.whompbox.com'), ('User-agent', 'Python-urllib/2.6')]

Comment: add_header() has not modified the unredirected_hdr.
Is this the whole purpose of issue2756? If yes, then better understanding of unredirected_hdr is needed and in the do_request_ method of AbstractHTTPHandler, where it changes unredirected_hdrs based on the logic of "not request.has_header(...)" what is actually aimed for checking that.

If add_header() is not supposed to change unredirected_hdrs but, add_unredirected_header() is the call to change unredirected_hdrs then, it is working fine and as expected.

(This is an undocumented interface, items() call was used for viewing the headers, tough actual code might not be using it.

>>>request.add_unredirected_header('Content-type','application/xml')
>>> request.unredirected_hdrs.items()

[('Content-length', '4'), ('Content-type', 'application/xml'), ('Host', 'www.whompbox.com'), ('User-agent', 'Python-urllib/2.6')]
>>>
Comment: add_unredirected_header() has correctly affected.

After application of the attached patch in issue report which modifies the add_header() and add_unredirected_header() method to remove the existing header of the same name. We will observe that the unredirected_hdr itself is removed and it is never added back.

After application of attached patch:

>>> url = 'http://www.whompbox.com/headertest.php'
>>> request = urllib2.Request(url)
>>> request.add_data("Spam")
>>> f = urllib2.urlopen(request)
>>> request.header_items()

[('Content-length', '4'), ('Content-type', 'application/x-www-form-urlencoded'), ('Host', 'www.whompbox.com'), ('User-agent', 'Python-urllib/2.6')]

>>> request.unredirected_hdrs.items()

[('Content-length', '4'), ('Content-type', 'application/x-www-form-urlencoded'), ('Host', 'www.whompbox.com'), ('User-agent', 'Python-urllib/2.6')]

>>> request.add_header('Content-type','application/xml')
>>> f = urllib2.urlopen(request)
>>> request.header_items()

[('Content-length', '4'), ('Content-type', 'application/xml'), ('Host', 'www.whompbox.com'), ('User-agent', 'Python-urllib/2.6')]

>>> request.unredirected_hdrs.items()

[('Content-length', '4'), ('Host', 'www.whompbox.com'), ('User-agent', 'Python-urllib/2.6')]
>>>
Comment: Notice the absense of Content-type header.