Flask 1.0 backwards incompatible root url_prefix

[albertyw]

Some of the recent PRs in blueprint.py seem to make Flask 1.0 not backwards compatible with 0.x versions. When url_prefix='', url_prefix='/', or url_prefix is not specified, a ValueError('urls must start with a leading slash') is now raised.

Expected Behavior

I would have assumed, given the discussion in #2629, that '' and maybe / would be acceptable url_prefixes to indicate the blueprint is meant for the root.

An example of a broken library is flask-sitemap (https://github.com/inveniosoftware/flask-sitemap/blob/master/flask_sitemap/__init__.py#L120) which uses

            app.register_blueprint(
                self.blueprint,
                url_prefix=app.config.get('SITEMAP_BLUEPRINT_URL_PREFIX') # default is '/'
            )

Actual Behavior

In my testing, I continually get

    self.init_app(app)
  File "/home/albertyw/.virtualenvs/baseflask/local/lib/python2.7/site-packages/flask_sitemap/__init__.py", line 120, in init_app
    url_prefix='' # app.config.get('SITEMAP_BLUEPRINT_URL_PREFIX')
  File "/home/albertyw/.virtualenvs/baseflask/local/lib/python2.7/site-packages/flask/app.py", line 64, in wrapper_func
    return f(self, *args, **kwargs)
  File "/home/albertyw/.virtualenvs/baseflask/local/lib/python2.7/site-packages/flask/app.py", line 1113, in register_blueprint
    blueprint.register(self, options, first_registration)
  File "/home/albertyw/.virtualenvs/baseflask/local/lib/python2.7/site-packages/flask/blueprints.py", line 186, in register
    deferred(state)
  File "/home/albertyw/.virtualenvs/baseflask/local/lib/python2.7/site-packages/flask/blueprints.py", line 207, in <lambda>
    s.add_url_rule(rule, endpoint, view_func, **options))
  File "/home/albertyw/.virtualenvs/baseflask/local/lib/python2.7/site-packages/flask/blueprints.py", line 79, in add_url_rule
    view_func, defaults=defaults, **options)
  File "/home/albertyw/.virtualenvs/baseflask/local/lib/python2.7/site-packages/flask/app.py", line 64, in wrapper_func
    return f(self, *args, **kwargs)
  File "/home/albertyw/.virtualenvs/baseflask/local/lib/python2.7/site-packages/flask/app.py", line 1211, in add_url_rule
    rule = self.url_rule_class(rule, methods=methods, **options)
  File "/home/albertyw/.virtualenvs/baseflask/local/lib/python2.7/site-packages/werkzeug/routing.py", line 603, in __init__
    raise ValueError('urls must start with a leading slash')
ValueError: urls must start with a leading slash

unless I set url_prefix='//', in which case the url_prefix is the root.

Environment

• Python version: python 2.7 and 3.6
• Flask version: 1.0
• Werkzeug version: 0.14.1

[davidism]

The real error here is that Flask-Sitemap doesn't use a leading slash for the rules, and relied on the prefix being prepended. Flask-Sitemap should be fixed to use an empty prefix and start the rules with leading slashes.

We just never added the leading slash check to Blueprint.route, and there should be a corresponding "can't end with trailing slash" check for the prefix.

It's not a big deal to fix in Flask right now, but don't rely on that behavior, write your routes with leading slashes.