fix url_prefix argument when nesting blueprints

CHANGES.rst

@@ -18,6 +18,8 @@ Unreleased
 -   Show an error when a blueprint name contains a dot. The ``.`` has
     special meaning, it is used to separate (nested) blueprint names and
     the endpoint name. :issue:`4041`
+-   Combine URL prefixes when nesting blueprints that were created with
+    a ``url_prefix`` value. :issue:`4037`
 
 
 Version 2.0.0

src/flask/blueprints.py

@@ -260,7 +260,7 @@ class Blueprint(Scaffold):
         """Called by :meth:`Flask.register_blueprint` to register all
         views and callbacks registered on the blueprint with the
         application. Creates a :class:`.BlueprintSetupState` and calls
-        each :meth:`record` callbackwith it.
+        each :meth:`record` callback with it.
 
         :param app: The application this blueprint is being registered
             with.
@@ -344,13 +344,17 @@ class Blueprint(Scaffold):
                 app.cli.add_command(self.cli)
 
         for blueprint, bp_options in self._blueprints:
-            url_prefix = options.get("url_prefix", "")
-            if "url_prefix" in bp_options:
-                url_prefix = (
-                    url_prefix.rstrip("/") + "/" + bp_options["url_prefix"].lstrip("/")
+            bp_options = bp_options.copy()
+            bp_url_prefix = bp_options.get("url_prefix")
+
+            if bp_url_prefix is None:
+                bp_url_prefix = blueprint.url_prefix
+
+            if state.url_prefix is not None and bp_url_prefix is not None:
+                bp_options["url_prefix"] = (
+                    state.url_prefix.rstrip("/") + "/" + bp_url_prefix.lstrip("/")
                 )
 
-            bp_options["url_prefix"] = url_prefix
             bp_options["name_prefix"] = options.get("name_prefix", "") + self.name + "."
             blueprint.register(app, bp_options)
 

tests/test_blueprints.py

@@ -835,3 +835,36 @@ def test_nested_blueprint(app, client):
     assert client.get("/parent/no").data == b"Parent no"
     assert client.get("/parent/child/no").data == b"Parent no"
     assert client.get("/parent/child/grandchild/no").data == b"Grandchild no"
+
+
+def test_nested_blueprint_url_prefix(app, client):
+    parent = flask.Blueprint("parent", __name__, url_prefix="/parent")
+    child = flask.Blueprint("child", __name__, url_prefix="/child")
+    grandchild = flask.Blueprint("grandchild", __name__, url_prefix="/grandchild")
+    apple = flask.Blueprint("apple", __name__, url_prefix="/apple")
+
+    @parent.route("/")
+    def parent_index():
+        return "Parent"
+
+    @child.route("/")
+    def child_index():
+        return "Child"
+
+    @grandchild.route("/")
+    def grandchild_index():
+        return "Grandchild"
+
+    @apple.route("/")
+    def apple_index():
+        return "Apple"
+
+    child.register_blueprint(grandchild)
+    child.register_blueprint(apple, url_prefix="/orange")  # test overwrite
+    parent.register_blueprint(child)
+    app.register_blueprint(parent)
+
+    assert client.get("/parent/").data == b"Parent"
+    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"