How is deserialization affected by only and partial?

[redlamb]

I was hoping to get some clarification on the differences between only and partial with respect to deserialization. (This question is similar to #509 which addresses serialization using the same fields.) Based on the docs and the response to that issue, I interpreted it to mean that only applies to serialization and partial applies to deserialization. However, in testing it looks like only also affects deserialization. What am I missing?

from marshmallow import Schema, fields

class MySchema(Schema):
    foo = fields.Field(required=True)
    bar = fields.Field(required=True)

only_schema = MySchema(only=('foo', ))
partial_schema = MySchema(partial=True)

print(only_schema.load({'foo': 42, 'bar': 24}))
print(only_schema.load({'foo': 42}))
print(partial_schema.load({'foo': 42, 'bar': 24}))
print(partial_schema.load({'foo': 42}))

Output:

UnmarshalResult(data={u'foo': 42}, errors={})
UnmarshalResult(data={u'foo': 42}, errors={})
UnmarshalResult(data={'foo': 42, 'bar': 24}, errors={})
UnmarshalResult(data={'foo': 42}, errors={})

I expected deserialization using only_schema to result in an error similar to the example below.

MySchema().load({'foo':42})

Output:

UnmarshalResult(data={'foo': 42}, errors={'bar': [u'Missing data for required field.']})

Ultimately, I'm trying to get away with using the same schema for (de)serialization, but maybe that's not the ideal way to use marshmallow.

[sabinem]

@redlamb:
I do not understand your problem with this: why would you expect an error with the only_schema: it filters, so it just takes the field, that are specified in your case 'foo', so why should it care for 'bar' as well?

[redlamb]

@sabinem: Thanks for the reply and sorry that my question wasn't very clear. I'm sure this is a misunderstanding of the docs, but I expected an error with the only_schema because only doesn't seem to apply to deserializing/loading an object. According the Schema API, it only appears to apply to serializing/dumping an object.

only (tuple) – A list or tuple of fields to serialize. If None, all fields will be serialized. Nested fields can be represented with dot delimiters.

This being the case, I expected a validation error when attempting to load data that doesn't meet the schema's requirements.

To further expand on what I was trying to do, I was trying to create a single schema that would only dump certain fields, but still meet validation requirements when loading.

[lafrech]

1/ When a field does not exist in a schema (or is excluded using only or exclude), corresponding key in deserialization input data is ignored. There is no error, unless you add a custom validator for this. See #524.

2/ Only instantiates a Schema with only those fields (and the others excluded). This affects serialization and deserialization. The doc is unclear (to say the least) about this. This should be fixed.

3/ You can get away with a single Schema by setting load_only=True on all the fields you want to exclude.