Union dump not working

[elvinlemmens]

Hi,
I have noticed the dumping of a Union type is returning an empty object:

from marshmallow_dataclass import dataclass
from typing import List, Union

@dataclass
class Vector:
    x: float
    y: float

@dataclass
class Point:
    x: float
    z: float


@dataclass
class Geometries:
    elements: List[Union[Point, Vector]]

pts = Geometries.Schema().load({"elements": [
    {"x": 1, "y": 1},
    {"x": 1, "z": 1},
]})
schema=Geometries.Schema()
print(schema.dump(schema.load({"elements": [
    {"x": 1, "y": 1},
    {"x": 1, "z": 1},
]})))

gives

{"elements": [{}, {}]}

but should again return

{"elements": [
    {"x": 1, "y": 1},
    {"x": 1, "z": 1},
]}

[YBadiss]

As far as I can tell, the issue is not the Union but the List:

from marshmallow_dataclass import dataclass
from typing import Union, List

@dataclass
class Vector:
    x: float
    y: float

@dataclass
class Point:
    x: float
    z: float


@dataclass
class Geometries:
    single: Union[Point, Vector]
    multi: List[Union[Point, Vector]]

schema=Geometries.Schema()
pts = schema.load({"multi": [
    {"x": 1, "y": 1},
    {"x": 1, "z": 1}
], "single": {"x": 1, "z": 1}})
print(schema.dump(pts))  # {'single': {'z': 1.0, 'x': 1.0}, 'multi': [{}, {}]}

[elvinlemmens]

Indeed, thanks for noticing. A normal List[Point] works fine for example. It's the combo of Union and List that is causing th eissue.

[YBadiss]

Actually it seems like there are multiple issues, the one above with List[Union], but also with simple Union. For example doing the following will fail on the second assert

@dataclass
class Geometries:
    single: Union[Point, Vector]

schema = Geometries.Schema()

res = schema.dump(Geometries(single=Point(x=1, z=2)))
# OK
assert res == {"single": {"x": 1.0, "z": 2.0}}, res

res = schema.dump(Geometries(single=Vector(x=1, y=2)))
# NOK, only "x" is dumped but that's just because both Vector and Point have an "x" attribute
assert res == {"single": {"x": 1.0, "y": 2.0}}, res

Changing the order of Vector and Point in the Union will change which assert succeeds and which one fails. So it's always the first type that is used in the Union when dumping.

[YBadiss]

It is actually probably linked to adamboche/python-marshmallow-union#27 not this repo

[lovasoa]

This issue is caused by a bug in marshmallow-union. I opened a pull request to fix it: adamboche/python-marshmallow-union#33

[ammerzon]

I tried the example from @YBadiss after upgrading to marshmallow-union v0.1.15.post1 but the serialization is still not working.

@dataclass
class Vector:
    x: float
    y: float


@dataclass
class Point:
    x: float
    z: float


@dataclass
class Geometries:
    single: Union[Point, Vector]


GeometriesSchema = marshmallow_dataclass.class_schema(Geometries)

res = GeometriesSchema().dump(Geometries(Point(x=1, z=2)))
# OK
assert res == {"single": {"x": 1.0, "z": 2.0}}, res

res = GeometriesSchema().dump(Geometries(Vector(x=1, y=2)))
# NOK, only "x" is dumped but that's just because both Vector and Point have an "x" attribute
assert res == {"single": {"x": 1.0, "y": 2.0}}, res

requirements.txt

marshmallow-dataclass[enum,union]==7.6.0
marshmallow-enum==1.5.1
marshmallow-union==0.1.15.post1