The querystring template tag should render an empty query string result as "?" instead of ""

[Sarah Boyce]

Test to demonstrate the issue:

tests/template_tests/syntax_tests/test_querystring.py

@@ -56 +56 @@
                 self.assertRenderEqual(
                     "test_querystring_empty_params", context, expected=""
                 )
+        request = self.request_factory.get("/?a=1")
+        for param in cases:
+            with self.subTest(param=param, url="/?a=1"):
+                context = RequestContext(request, {"qd": param})
+                self.assertRenderEqual(
+                    "test_querystring_empty_params", context, expected="?"
+                )
 
     @setup({"querystring_replace": "{% querystring a=1 %}"})

In short, returning an empty string for querydict means that the page would not reload if this was used in a link, see #36182.
Therefore, it only makes sense to return an empty string when the query string that would be evaluated is equivalent to the current query string.

We are not supporting this in most cases e.g. if you are on /?color=green and in the template there is {% querystring color=green %} this evaluates to "?color=green" (not "").

In the niche case that you are on /?color=green and pass in an empty query dict rather than the default request.context.GET, and render {% querystring query_dict %} this returns "" and so the page would not reload and you would stay on /?color=green even though you probably expected this to blitz the query string and take you to / (which would be the case if "?" was returned).
I think this is niche because there is no reason for you not to hard code the url in these cases as you don't care about the current query string

For simplicity, I think the proceeding "?" should always be returned.
I think the "bug" I described is unlikely to happen in the wild, I am classing this as a cleanup. Other thoughts are welcome.

[Natalia Bidart]

Thank you Sarah for the report. As we discussed in the context of #35529, I agree.

[Natalia Bidart]

​PR

[David Feeley]

Hi, for whatever its worth, I like this change, conceptually an empty query string to me should be '?', and not , given the way empty hrefs mean to reload the current page.
thanks!
Dave