Generate a list in a URL with Flask's url_for

Question

I am using webargs to parse parameters from request.args with Marshmallow and pass them as arguments to a Flask view. My client uses a comma separated list to represent multiple values for a key:

/queues/generate?queue_id=4&include_ids=1,2,3

To parse this I use Marshmallow's DelimitedList field.

from marshmallow import Schema, fields
from webargs import use_args
from webargs.fields import DelimitedList

class GenerationParamsSchema(Schema):
    queue_id = fields.Integer(required=True)
    include_ids = DelimitedList(fields.Integer(), required=False)

@queues.route('/generate_content', methods=['GET'])
@use_args(GenerationParamsSchema(strict=True))
def generate_content_count(generation_params):
    ...

However, if I generate the URL with Flask's url_for, it produces duplicate keys for each value:

url_for('queues.generate', queue_id=4, include_ids=[1, 2, 3])

/queues/generate?queue_id=4&include_ids=1&include_ids=2&include_ids=3

Parsing this with a DelimitedList field only captures the first value. Changing to a List field correctly captures the values again. So either my Flask URLs fail, or my client URLs fail.

I can't change how my client generates URLs, so I'd like to stick with parsing using the DelimitedField. How can I make url_for generate the same style?

Answer 1

There's no standard for specifying multiple values for a key in a query string. Flask, browsers, and many other web technologies use the "repeat key" style that you see with url_for and request.args. Your client has chosen to use a different style.

If you want url_for to generate the delimited style, you'll need to pre-process the values you pass to url_for. Write a wrapper around url_for and use it instead.

from flask import url_for as _url_for

@app.template_global()
def url_for(endpoint, **values):
    for key, value in values.items():
        if isinstance(value, (tuple, list)):
            values[key] = ','.join(value)

    return _url_for(endpoint, **values)

Keep in mind that requests.args only understands the repeat key style, so you'll have to parse, with webargs or otherwise, any incoming comma separated values. It may be easier to generate the repeat key style from your client instead.