missing vs default

Marshmallow library has two different parameter of the field types are used for default value: missing and default.

• Missing is used for deserialization if the field is not found in the input data.
• Default is used for serialization if the input value is missing.

Example using dict:

import marshmallow_objects as marshmallow


class MySchema(marshmallow.Schema):
    field = marshmallow.fields.Str(default='field_default_value',
                                   missing='field_missing_value')


print(MySchema().load({}))
print(MySchema().dump({}))

# {'field': 'field_missing_value'}
# {'field': 'field_default_value'}

The field was not found in raw dat ({}) and the value from missing parameter was used at load (deserialization) and the value from default was used at dump (serialization). It really works with a dictionary, but there is a totally different behavior with an object.

Example using object:

class MyObject(object):
    def __init__(self, field=None):
        self.field = field

    def __repr__(self):
        return '<MyObject(field={self.field!r})>'.format(self=self)


class MySchema(marshmallow.Schema):
    field = marshmallow.fields.Str(default='field_default_value',
                                   missing='field_missing_value')

    @marshmallow.post_load
    def make_object(self, data):
        return MyObject(**data)


print(MySchema().load({}))
print(MySchema().dump(MyObject(**{})))

# <MyObject(field='field_missing_value')>
# {'field': None}

load function works fine, there is no field in the raw data ({}) and the value from missing parameter was used, but dump function didn't used the value from default parameter, because It found field field in the object.

So with marshmallow-objects models the missing parameter should be used, not default.

class MyModel(marshmallow.Model):
    field = marshmallow.fields.Str(default='field_default_value',
                                   missing='field_missing_value')


print(MyModel.load({}))
print(MyModel(**{}).dump())

# {'field': 'field_missing_value'}
# {'field': 'field_missing_value'}