Support delimited list parameters

[jmcarp]

Currently, webargs supports list parameters in query strings using repeated parameters: foo=bar&foo=baz. But it's also a common pattern to encode lists in query strings using delimited values: foo=bar,baz, foo=bar|baz, etc. This is described using the collectionFormat in swagger: https://github.com/swagger-api/swagger-spec/blob/master/versions/2.0.md#fixed-fields-8. It would be helpful to expose control over the formatting of list values to support both repeated and delimited parameters.

Here are a few thoughts about interfaces:

• Use a subclass of List, like DelimitedList, that splits and joins on a delimiter on de/serializing.
• Add an optional delimiter argument to List that has the same behavior as above.
• Add an option to Schema.Meta that controls de/serialization of lists:

class ArgSchema(Schema):
    class Meta:
        list_delimiter = ','

Not sure if I like any of these. What do you think @sloria?

[sloria]

The 1st option seems best. List is not meant to deserialize from strings, so that rules out 2. Option 3 doesn't seem ideal because you wouldn't be able to use the feature with dictionary argmaps.

[jmcarp]

Agreed with everything you said, but the first option strikes me as a little underpowered. In most use cases, all list parameters for a given resource will be encoded in the same way (all repeated parameters, or all delimited parameters), so I still like a higher-level option--what about an argument passed to parse and use_(kw)args? That said, I still think the lower-level option is independently useful, so I'm down to take a pass at it.

By the way, I don't particularly like delimited query parameters, but they seem to be pretty commonly used, and are even recommended by JSON-API, which is why I'd like to add support.

[sloria]

Good point. I want to avoid adding a custom Schema class in webargs for the time being. Would it be enough if the delimiter option were configurable at the Field class level and as a constructor argument?