Schema dump method does not validate?

According to the docs:

Serialize objects by passing them to your schema’s dump method, which returns the formatted result (as well as a dictionary of validation errors…)

But as far as I can tell, the Schema.dump method does not perform validation. I have:

>>> from marshmallow import Schema
>>> from marshmallow.fields import String
>>> from marshmallow.validate import Regexp
>>> class TestSchema(Schema):
...     testfield1 = String(validate=Regexp('^foo'))
...     testfield2 = String(default='teststring')

>>> TestSchema().dump({'testfield1': 'bar'})
MarshalResult(data={'testfield2': 'teststring', 'testfield1': 'bar'}, errors={})

Based on the docs, I thought I would see validation errors in the errors key. The load method validates as expected:

>>> TestSchema().load({'testfield1': 'bar'})
UnmarshalResult(data={}, errors={'testfield1': ['String does not match expected pattern.']})

But the load message does not apply default values:

>>> TestSchema().load({})
UnmarshalResult(data={}, errors={})

So the only way to get a validated data structure with default values applied appears to be:

>>> result1 = TestSchema().load({'testfield1': 'foo'})
>>> result2 = TestSchema().dump(result1.data)
>>> result2.data
{'testfield2': 'teststring', 'testfield1': 'foo'}

The above is with:

>>> marshmallow.__version__
'2.13.6'
>>> sys.version
'3.5.4 (default, Aug 23 2017, 18:32:05) \n[GCC 6.4.1 20170727 (Red Hat 6.4.1-1)]'

Am I simply misreading the documentation?

[afallou]

For anyone finding this thread (like I have), note that @dnshio’s approach doesn’t always work.

validate ignores fields with dump_only=True (see here – load_fields are fields that are not dump_only, as opposed to dump_fields). As a result, those fields will be treated as unknown fields. For example, the following code:

class PersonSchema(Schema):
    name = fields.String(
        dump_only=True,
    )

schema = PersonSchema()
errors = schema.validate(dict(name="John"))
if errors:
    raise ValidationError(message=errors)

raises marshmallow.exceptions.ValidationError: {'name': ['Unknown field.']}

[dnshio]

I wanted to validate during serialisation time and was disappointed to find this thread.

However, it’s not that difficult to do what I wanted by validating the output from .dump(). e.g:

schema = SomethingSchema()
data, errors = schema.dump(some_object)

# call schema.validate to raise ValidationErrors 
schema.validate(data)