Expected Behavior

If url_for is called with a query argument that is an empty list (and no other query arguments), the URL should have no query part at all, including no trailing question mark. (When called with a nonempty list, it adds several query arguments with the same name, which is what I expect; when called with None, it omits the question mark already.)

@app.route('/')
def index():
    return flask.url_for('index', extra_param=[])

Full example: https://github.com/lucaswerkmeister/repro-flask-url_for-empty

Actual Behavior

The URL has a question mark at the end (in the above example: /?).

Environment

• Python version: 3.9.1
• Flask version: 1.1.2
• Werkzeug version: 1.0.1

Workaround

Replace the empty list with None by appending or None to the argument. (This also works in Jinja2 templates.)