Remove validation on serialization

When using Dict(values=Nested()) I expect errors in dictionary values to be nested in such a way that I can locate exactly what element is causing the problem. This seems to work as expected for load(), but not for dump().

Minimal example:

• Marshmallow version: 3.0.0rc4
• Python version: 3.7.2

from marshmallow import Schema, ValidationError, fields


class InnerSchema(Schema):
    x = fields.Int()


class OuterSchema(Schema):
    inner = fields.Dict(keys=fields.Str(), values=fields.Nested(InnerSchema))


foo = {"inner": {"some-key": {"x": "abc"}}}

try:
    OuterSchema().load(foo)
except ValidationError as e:
    print(e)
    # >>> {'inner': defaultdict(<class 'dict'>, {'some-key': {'value': {'x': ['Not a valid integer.']}}})}
    # Very helpful - and what I expect

class Inner:
    def __init__(self, x):
        self.x = x


class Outer:
    def __init__(self, inner):
        self.inner = inner


bar = Outer(inner={"some-key": Inner(x="abc")})

try:
    OuterSchema().dump(bar)
except ValidationError as e:
    print(e)
    # >>> {'inner': {'x': ['Not a valid integer.']}}
    # Surprisingly - 'some-key' and 'value' are not present here,
    # which makes it harder to pinpoint the problem in large objects

[sloria]

Probably not? handle_error is meant for handling ValidationError, which we’re proposing should only get raised on deserialization.

[sloria]

Should we get rid of the code that rewrites the errors in _serialize()? If we aren’t going to pack the errors into the same data structure as load(), should we let the first error bubble to the top uninterrupted?

Yes, that’s what I’m proposing.