Flask Restful accept a list in a post request

What are your parser add_argument options? Is products_in_basket an actual key to the data that is requested? Or are you trying to provide an arbitrary name and/or rename the dict?

Take a look at Multiple Values & Lists from the Request Parsing documentation.

You may want to be doing something like this instead...

parser = reqparse.RequestParser()
parser.add_argument('my_list', action='append')