Preserve blank values when extending queries

src/rfc3986/builder.py

@@ -325,7 +325,9 @@ def extend_query_with(self, query_items: _QueryType) -> "URIBuilder":
             URIBuilder(scheme=None, userinfo=None, host=None, port=None,
                     path=None, query='a=b+c&a=b+c', fragment=None)
         """
-        original_query_items = parse_qsl(self.query or "")
+        original_query_items = parse_qsl(
+            self.query or "", keep_blank_values=True
+        )
         if not isinstance(query_items, list):
             query_items = list(query_items.items())
 

tests/test_builder.py

@@ -323,3 +323,19 @@ def test_geturl():
         "sigmavirus24/rfc3986"
     )
     assert expected == uri
+
+
+@pytest.mark.parametrize(
+    "original, expected",
+    [
+        ("a=", "a=&new=value"),
+        ("a=&a=1", "a=&a=1&new=value"),
+        ("flag&b=", "flag=&b=&new=value"),
+    ],
+)
+def test_extend_query_preserves_existing_blank_values(original, expected):
+    """Extending a query retains empty values and repeated parameters."""
+    original_builder = builder.URIBuilder(query=original)
+    extended = original_builder.extend_query_with({"new": "value"})
+    assert extended.query == expected
+    assert original_builder.query == original