urljoining with '?' will not clear a query string:

ACTUAL:
>>> import urllib.parse
>>> urllib.parse.urljoin('http://a/b/c?d=e', '?')
'http://a/b/c?d=e'

EXPECTED:
'http://a/b/c' (optionally, with a ? at the end)

WhatWG's URL standard expects a relative URL consisting of only a ? to replace a query string:

https://url.spec.whatwg.org/#relative-state

Seen in versions 3.6 and 2.7, but probably also affects later versions.

I'm working on a patch for this and can have one up in the next week or so, once I get the CLA signed and other boxes ticked.  I'm new to the Github process but hopefully it will be a good start for the discussion.

Python follows not WhatWG but RFC.
https://tools.ietf.org/html/rfc3986#section-5.2.2 is proper definition for url joining algorithm.

In this case, the RFC is mismatched from the actual behaviour of browsers (as described and codified by WhatWG).  It was surprising to me that urljoin() didn't do what I percieved as "the right thing" (and I expect other users would too).

I would personally expect urljoin to do "the thing that everybody else does".  Is there a sensible way to reduce this mismatch?

For reference, Java's stdlib does what I would expect here:

    URI base = URI.create("https://example.com/?a=b");
    URI rel = base.resolve("?");
    System.out.println(rel);

https://example.com/?

The relevant part in the RFC pseudo code is 

               if defined(R.query) then
                  T.query = R.query;
               else
                  T.query = Base.query;
               endif;

which is implemented in urljoin as:

        if not query:
            query = bquery


Is this correct? Should the code not say "if query is not None"?
(I can't see in the RFC a definition of defined()).

Sorry, urlparse returns '' rather than None when there is no query.
So we indeed need to check something like 
    if '?' not in url:
or what's in Paul's patch. 

However, my main point was to question whether fixing this is actually in contradiction with the RFC.

Reading more into this, from section 5.2,1:

> A component is undefined if its associated delimiter does not appear in the URI reference

So you could say that since there is a '?', the query component is *defined*, but *empty*. This would mean that assigning the target query to be '' has the desired effect as implemented by browsers and other languages' standard libraries.