Merge pull request #4063 from pgjones/bp-fix

Bp fix

src/flask/blueprints.py

@@ -354,6 +354,8 @@ class Blueprint(Scaffold):
                 bp_options["url_prefix"] = (
                     state.url_prefix.rstrip("/") + "/" + bp_url_prefix.lstrip("/")
                 )
+            else:
+                bp_options["url_prefix"] = state.url_prefix
 
             bp_options["name_prefix"] = options.get("name_prefix", "") + self.name + "."
             blueprint.register(app, bp_options)

tests/test_blueprints.py

@@ -868,3 +868,17 @@ def test_nested_blueprint_url_prefix(app, client):
     assert client.get("/parent/child/").data == b"Child"
     assert client.get("/parent/child/grandchild/").data == b"Grandchild"
     assert client.get("/parent/child/orange/").data == b"Apple"
+
+
+def test_nested_blueprint_url_prefix_only_parent_prefix(app, client):
+    parent = flask.Blueprint("parent", __name__)
+    child = flask.Blueprint("child", __name__)
+
+    @child.route("/child-endpoint")
+    def child_index():
+        return "Child"
+
+    parent.register_blueprint(child)
+    app.register_blueprint(parent, url_prefix="/parent")
+
+    assert client.get("/parent/child-endpoint").data == b"Child"