[5.1.x] Fixed #36182 -- Returned "?" if all parameters are removed in querystring template tag.

Thank you to David Feeley for the report and Natalia Bidart for the review.

Backport of 05002c153c5018e4429a326a6699c7c45e5ea957 from main.

4 files changed, 20 insertions, 9 deletions

diff --git a/django/template/defaulttags.py b/django/template/defaulttags.py
index ae74679ec6..1152452081 100644
--- a/django/template/defaulttags.py
+++ b/django/template/defaulttags.py
@@ -1194,18 +1194,18 @@ def querystring(context, query_dict=None, **kwargs):
     """
     if query_dict is None:
         query_dict = context.request.GET
-    query_dict = query_dict.copy()
+    params = query_dict.copy()
     for key, value in kwargs.items():
         if value is None:
-            if key in query_dict:
-                del query_dict[key]
+            if key in params:
+                del params[key]
         elif isinstance(value, Iterable) and not isinstance(value, str):
-            query_dict.setlist(key, value)
+            params.setlist(key, value)
         else:
-            query_dict[key] = value
-    if not query_dict:
+            params[key] = value
+    if not params and not query_dict:
         return ""
-    query_string = query_dict.urlencode()
+    query_string = params.urlencode()
     return f"?{query_string}"
 
 
diff --git a/docs/ref/templates/builtins.txt b/docs/ref/templates/builtins.txt
index c00baf3a07..7d2cf93085 100644
--- a/docs/ref/templates/builtins.txt
+++ b/docs/ref/templates/builtins.txt
@@ -965,7 +965,7 @@ This tag requires a :class:`~django.http.QueryDict` instance, which defaults to
 :attr:`request.GET <django.http.HttpRequest.GET>` if none is provided.
 
 If the :class:`~django.http.QueryDict` is empty and no additional parameters
-are provided, an empty string is returned. A non-empty result includes a
+are provided, an empty string is returned. Otherwise, the result includes a
 leading ``"?"``.
 
 .. admonition:: Using ``request.GET`` as default
diff --git a/docs/releases/5.1.7.txt b/docs/releases/5.1.7.txt
index 8143305a96..e184da6aca 100644
--- a/docs/releases/5.1.7.txt
+++ b/docs/releases/5.1.7.txt
@@ -9,4 +9,6 @@ Django 5.1.7 fixes several bugs in 5.1.6.
 Bugfixes
 ========
 
-* ...
+* Fixed a bug in Django 5.1 where the ``{% querystring %}`` template tag
+  returned an empty string rather than ``"?"`` when all parameters had been
+  removed from the query string (:ticket:`36182`).
diff --git a/tests/template_tests/syntax_tests/test_querystring.py b/tests/template_tests/syntax_tests/test_querystring.py
index 3f1cf3d281..099be96f9e 100644
--- a/tests/template_tests/syntax_tests/test_querystring.py
+++ b/tests/template_tests/syntax_tests/test_querystring.py
@@ -17,6 +17,15 @@ class QueryStringTagTests(SimpleTestCase):
         output = template.render(context)
         self.assertEqual(output, "")
 
+    @setup({"test_querystring_remove_all_params": "{% querystring a=None %}"})
+    def test_querystring_remove_all_params(self):
+        non_empty_context = RequestContext(self.request_factory.get("/?a=b"))
+        empty_context = RequestContext(self.request_factory.get("/"))
+        template = self.engine.get_template("test_querystring_remove_all_params")
+        for context, expected in [(non_empty_context, "?"), (empty_context, "")]:
+            with self.subTest(expected=expected):
+                self.assertEqual(template.render(context), expected)
+
     @setup({"querystring_non_empty": "{% querystring %}"})
     def test_querystring_non_empty(self):
         request = self.request_factory.get("/", {"a": "b"})