Improve documentation on fields.DelimitedList vs fields.List

The docs for the fields.DelimitedList say it “can load from either a list or a delimited string” but I cannot get it to load from a list. It's possible I'm doing something wrong but When I swap out fields.DelimitedList for fields.List everything works as expected. I think either the docs should be changed, or we should fix the implementation of DelimitedList so that it works as expected.

[sloria]

5.4.0 removes the duck typing . I also added more docs re: parsing lists in query strings: https://webargs.readthedocs.io/en/latest/quickstart.html#parsing-lists-in-query-strings .

[sloria]

If we’re always expecting DelimitedList to handle the deserialization of a single value, why have it extend marshmallow.fields.List

Conceptually, DelimitedList is a List, evidenced by the fact that you can’t implement DelimitedList as a non-subclass of List without duplicating a bunch of List’s code. I suppose you could use composition, but that’s not clearly better design in this context.

Anything that extends marshmallow.fields.List and has a property delimiter will not behave as expected.

Yeah…I don’t recall why we decided to use duck-typing here. At the very least, it hurts readability. At worst, it leads to unexpected behavior like you ran into. I’ll change this.