Issue passing a missing value to a ListField about umongo

[touilleMan]

I think your usecase is totally legit so we should add some checks in the __init__ for the missing value:

• Make sure missing param provides an iterable or a callable
• if iterable, convert it to List : missing = List(self.container, missing)
• if callable, wrap it : missing = lambda *args, **kwargs: List(missing(*args, **kwargs))

On top of that we should add a word about this in ListField's documentation

[lafrech]

DictField is also affected.

I pushed a fix for this in #171.

It is along the lines of your proposal above.

The checks are performed after the call to super().__init__ to avoid leaking data_object structures (Dict and List) to the pure marshmallow Schema. And because self.container is not defined before that.

At this point, I was tempted to call self.deserialize rather than Dict or List, and even to factorize than in Field itself. I don't think it would hurt. In fact, it would allow the user to pass default values in both serialized form and deserialized form (assuming the field can swallow its deserialized form, which I think is the case as this is the intent of the _deserialize override in some fields, such as DateTime). Since this might have other consequences, I left it aside for a moment.

Maybe ReferenceField should be addressed as well. It is a bit different and I don't really see the use case, so I'm leaving it as is.