url_for produces relative links with nontrivial APPLICATION_ROOT

[rosekunkel]

Expected Behavior

url_for should consistently produce urls which are absolute with regard to the server root, i.e., they should start with a /.

app = Flask(__name__, static_url_path='')
url_for('static', filename='some_static_file')

Actual Behavior

When running under gnunicorn with APPLICATION_ROOT='/myapp/', the url_for call produces the URL myapp/some_static_file, while when running under werkzeug with APPLICATION_ROOT='/', the url_for call produces the URL /some_static_file.

This is a problem, because, combined with this werkzeug issue, there's no straightforward way to use url_for in templates for both development and production: {{ url_for('static', filename='some_static_file') }} produces myapp/some_static_file in production, which fails, while /{{ url_for('static', filename='some_static_file') }} produces //some_static_file in development, which fails.

Environment

• Python version: 3.6.5
• Flask version: 1.0.2
• Werkzeug version: 0.14.1

[davidism]

APPLICATION_ROOT, along with SERVER_NAME, is only used when generating URLs outside a request (such as generating external URLs for an email). It appears to still be working as expected.

app.config["SERVER_NAME"] = "example.com"
app.config["APPLICATION_ROOT"] = "/app"

with app.app_context():
    print(url_for("static", filename="test.txt", _external=True))
    # http://example.com/app/static/test.txt

If you're using Gunicorn to serve a Flask app under a non-root path, you need to configure Gunicorn to set the WSGI SCRIPT_NAME correctly. One way to do this is to set the SCRIPT_NAME environment variable.

SCRIPT_NAME=/app gunicorn -b 127.0.0.1:5000 -w 2 example:app

If you're using the dev server, you can use DispatcherMiddleware to configure the SCRIPT_NAME in a similar way. You can also use this with Gunicorn, if you don't want Gunicorn to handle SCRIPT_NAME itself.

dev_app.py:

from werkzeug.exceptions import NotFound
from werkzeug.wsgi import DispatcherMiddleware
from myproject import app

app.wsgi_app = DispatcherMiddlware(NotFound(), {"/app": app.wsgi_app})

export FLASK_APP=dev_app.py
export FLASK_ENV=development
flask run

[davidism]

#2759 is open for better documentation about this.

[bsolomon1124]

Not sure if gevent as wsgi server supports SCRIPT_NAME. For some reason it looks to simply set that value empty.

[davidism]

I'm not sure what part of Gunicorn that is, but it does indeed support SCRIPT_NAME, as shown above. How it picks it up must be happening elsewhere in the code. https://docs.gunicorn.org/en/latest/faq.html?highlight=script_name#how-do-i-set-script-name

[bsolomon1124]

@davidism I am actually talking about using gevent as standalone wsgi app server (as you allude to, it is more commonly used as a gunicorn worker class):

from flask import Flask, redirect, url_for
from gevent.pywsgi import WSGIServer

app = Flask(__name__)

@app.route('/bar')
def hello_world():
    return "hi"

@app.route("/favicon.ico")
def favicon():
    return redirect(url_for("static", filename="favicon.ico"))

if __name__ == "__main__":
    WSGIServer(('127.0.0.1', 5000), application=app).serve_forever()

$ SCRIPT_NAME='/foo' python hello.py

In this case url_for("static", filename="favicon.ico") will still generate the absolute path /static/favicon.ico. If the app is reverse-proxied by something like nginx which proxy-passes /foo to gevent, that path should be /foo/static/favicon.ico.

It is the end of a long day and I am probably not thinking about this the right way, though.

[davidism]

Oh, that's gevent. This sounds like an issue with gevent then, or with your setup of gunicorn and gevent. An old closed issue on the Flask issue tracker is not the best place to get answers about that. 😉

[bsolomon1124]

@davidism Please let me give another example here.

Say that a template contains

<link rel="shortcut icon" href="{{ url_for('static', filename='favicon.ico') }}">

In this case (using example above) you have href="/static/favicon.ico" in HTML with no other contextual information for the browser to go off of. If your reverse proxy server is proxy-passing requests to www.host.example.com/foo to your application's root, then this href should be /foo/static/favicon.ico. That seems to be independent of gunicorn vs gevent, no? Taking gunicorn as an example, could you help me understand through what mechanism SCRIPT_NAME makes its way into url_for()?

[davidism]

When Flask.url_map is bound to the current request.environ, it will append SCRIPT_NAME to any URL it generates. Whether SCRIPT_NAME is set is not something Flask controls, only uses.

Please use Stack Overflow or https://discord.gg/pallets for questions about your own code. This tracker is for issues related to the project itself