Default values for webargs

[jmcarp]

Now that webargs is using marshmallow internally, we specify default argument values using the missing parameter:

arg = fields.Str(missing='foo')

But apispec doesn't check the missing key when introspective parameter defaults--it only checks default. To get the correct swagger default, we have to specify both parameters:

arg = fields.Str(missing='foo', default='foo')

Which isn't ideal.

Since we're using the same logic to introspect fields and schemas that are used for serialization and deserialization, one approach would be to tell field2property and related methods whether they're introspecting load or dump. For the specific case of parameter defaults, we'd check default for dump and missing for load.

[sloria]

I think the proposed approach--specifying whether we are dumping/loading--makes sense.

[lafrech]

I don't have the whole webargs history in mind, maybe this was correct back then, but I think current behaviour is partly wrong.

Here's what OpenAPI docs say about default parameters:

The default value is the one that the server uses if the client does not supply the parameter value in the request.

We should use the missing attribute in any case. The default attribute of a marshmallow field is internal detail the client shouldn't care about, I suppose.

Note that if used in a REST API, the result is pretty much the same from user perspective. If using missing, the default value is added at deserialization, saved in DB and returned. If using default, the default value is not saved in DB but added at serialization and returned. To me it makes more sense to use missing, in this case.

Anyway, when adding a schema definition, I think we should use missing. I prepared a PR for that.

I was about to remove the dump parameter but it is also used to manage dump_only/load_only attribute so extra care should be taken about this. Postponing.

[sloria]

I hadn't thought about it that way, but you're correct--missing is the correct thing to check when introspecting fields.