missing and default values should be specified in deserialized form

Marshmallow expects missing and default values to be specified in a pre-serialized form, which is inconvenient and unintuitive.

Here is a schema which I would expect to work:

import datetime
import uuid

class TestSchema(Schema):
    id = fields.UUID(
        missing=uuid.uuid1)
    ts = fields.DateTime(
        format='rfc',
        missing=datetime.datetime.now)

However, it does not:

schema = TestSchema()
pprint.pprint(tuple(schema.load({})))
({}, {'id': [u'Not a valid UUID.'], 'ts': [u'Not a valid datetime.']})

Instead, I have to convert my default and missing values to primitive types, which undermines the purpose of a library like marshmallow that already knows how to do this. For data types like datetimes, which have many possible serialized formats, I have to keep the formatting of my default and missing values in sync with my field properties, which is fragile and convoluted:

from email.Utils import formatdate

class TestSchema(Schema):
    id = fields.UUID(
        missing=lambda: str(uuid.uuid1()))
    ts = fields.DateTime(
        format='rfc',
        missing=formatdate)

In the case of default values, marshmallow does not attempt to validate them, so no error is raised, but the resulting document is not deserializable.

[lafrech]

I just sent a PR to achieve this.

@chadrik, @costrouc, @sloria, feedback welcome.

[Kareeeeem]

That workaround works but then context is not there. I get the idea for the change but the defaulting nested schemas just became a hell of a lot more convoluted.