prevent double slash /a//b when blueprint prefix ends with slash

CHANGES.rst

@@ -143,6 +143,8 @@ unreleased
   :data:`SERVER_NAME` does not implicily enable it. It can be enabled by
   passing ``subdomain_matching=True`` to the ``Flask`` constructor.
   (`#2635`_)
+- A single trailing slash is stripped from the blueprint ``url_prefix``
+  when it is registered with the app. (`#2629`_)
 
 .. _pallets/meta#24: https://github.com/pallets/meta/issues/24
 .. _#1421: https://github.com/pallets/flask/issues/1421
@@ -186,6 +188,7 @@ unreleased
 .. _#2607: https://github.com/pallets/flask/pull/2607
 .. _#2636: https://github.com/pallets/flask/pull/2636
 .. _#2635: https://github.com/pallets/flask/pull/2635
+.. _#2629: https://github.com/pallets/flask/pull/2629
 
 
 Version 0.12.2

flask/blueprints.py

@@ -9,7 +9,6 @@
     :copyright: © 2010 by the Pallets team.
     :license: BSD, see LICENSE for more details.
 """
-
 from functools import update_wrapper
 
 from .helpers import _PackageBoundObject, _endpoint_from_view_func
@@ -53,6 +52,9 @@ def __init__(self, blueprint, app, options, first_registration):
 
         #: The prefix that should be used for all URLs defined on the
         #: blueprint.
+        if url_prefix and url_prefix[-1] == '/':
+            url_prefix = url_prefix[:-1]
+
         self.url_prefix = url_prefix
 
         #: A dictionary with URL defaults that is added to each and every

tests/test_blueprints.py

@@ -114,7 +114,20 @@ def bp_forbidden():
     assert client.get('/nope').data == b'you shall not pass'
 
 
-def test_blueprint_url_definitions(app, client):
+def test_blueprint_prefix_slash(app, client):
+    bp = flask.Blueprint('test', __name__, url_prefix='/bar/')
+
+    @bp.route('/foo')
+    def foo():
+        return '', 204
+
+    app.register_blueprint(bp)
+    app.register_blueprint(bp, url_prefix='/spam/')
+    assert client.get('/bar/foo').status_code == 204
+    assert client.get('/spam/foo').status_code == 204
+
+
+def test_blueprint_url_defaults(app, client):
     bp = flask.Blueprint('test', __name__)
 
     @bp.route('/foo', defaults={'baz': 42})