Serialization bug using the Union registry

[priba]

With the last release, there is a RecursionError: maximum recursion depth exceeded during the serialisation of dataclasses using the Union decorator. The issue comes from the changes introduced in #77. Union is passed down through the MRO and creates an infinite recursion.

A simple solution is to add a check at:

python-databind/databind/src/databind/core/settings.py

Line 206 in a11f4a4

if isinstance(item, setting_type):

We should avoid yielding a Union. Possibly adding a inheritable attribute to ClassDecoratorSetting that identifies which decorators should be inherited. Then the previous line becomes something like: if isinstance(item, setting_type) and (klass is type_ or item.inheritable):.

Small example to reproduce the error:

from dataclasses import dataclass

from databind.core import ObjectMapper, Union
from databind.json import JsonModule

@Union(style=Union.FLAT)
@dataclass
class Base:
    x: float = 0.0

@Union.register(Base, "a")
@dataclass
class A(Base):
    pass

@Union.register(Base, "b")
@dataclass
class B(Base):
    y: float = 0.0

mapper = ObjectMapper()
mapper.module.register(JsonModule())

try:
    result = mapper.deserialize({"type": "a", "x": 1.0}, Base)
    print(f"OK   deserialize Base: {result}")
except RecursionError:
    print("FAIL deserialize Base: RecursionError")

try:
    result = mapper.serialize(A(x=1.0), Base)
    print(f"OK   serialize Base: {result}")
except RecursionError:
    print("FAIL serialize Base: RecursionError")

try:
    result = mapper.deserialize([{"type": "a", "x": 1.0}, {"type": "b", "x": 2.0, "y": 3.0}], list[Base])
    print(f"OK   deserialize list[Base]: {result}")
except RecursionError:
    print("FAIL deserialize list[Base]: RecursionError")

try:
    result = mapper.serialize([A(x=1.0), B(x=2.0, y=3.0)], list[Base])
    print(f"OK   serialize list[Base]: {result}")
except RecursionError:
    print("FAIL serialize list[Base]: RecursionError")

nit: The version in the __init__ files still point to 3.5.2.