Marshmallow How to Enforce a Required Field when Dumping?

Question

>>> class Foo(Schema):
...     id = fields.Int(dump_only=True, required=True)
...     name = fields.Str(required=True)
...
>>>
>>> Foo().dump({'asdf': 'abc'})
{}

I would have thought that this would raise a ValidationError.

Is there anyway to get a Marshmallow schema to enforce the required fields when calling dump ?

Answer 1

Marshmallow only validates on load. It is a design choice.

You can use a pre_load method for this.

I just realized this could also be achieved with a default callable raising a ValidationError.

Untested code:

class Foo(Schema):
    id = fields.Int(dump_only=True, required=True, default=lambda: raise ValidationError("missing id field")
    name = fields.Str(required=True)