Errors when working with marshmallow_dataclass

[csghone]

I am able to use marshmallow_dataclass perfectly when using with dataclasses.dataclass

However, I want to use it with attrs module instead - use the converters/validators functionalities.
It works well most of the time. However, I am facing problems when I use marshmallow_dataclass + attrs with inheritance.

Sample code is given below.

• Scenario1 : ENABLE_DC = True, ENABLE_ATTRS = False
  • Works as expected. Validator is inactive
• Scenario2 : ENABLE_DC = True, ENABLE_ATTRS = True
  • Works as expected. Validator also works.
• Scenario3 : ENABLE_DC = False, ENABLE_ATTRS = True
  • Crashes.
  • Fails in B = ClsB.Schema().load({'a': 123, 'b': 'bb', 'c': 4, 'd': "123"})
    • Error: marshmallow.exceptions.ValidationError: {'a': ['Unknown field.'], 'b': ['Unknown field.']}

Questions:

• Is it possible to fix Scenario 3 ?
• Scenario 2 seems to be ok for my requirements. But I am not sure what weird stuff might happen by enabling both dataclasses and attrs together. If you are aware of possible problems I might get into, please let me know.

I have raised this query with marshmallow_dataclass as well.
lovasoa/marshmallow_dataclass#236

I hope the first question is answered by marshmallow_dataclass team and second by the attrs team.

import dataclasses
import marshmallow
from marshmallow import Schema
import marshmallow_dataclass
from marshmallow_dataclass import class_schema
import attr
from typing import ClassVar, Type

ENABLE_DC    = True
ENABLE_ATTRS = True

# Conditional decorator
def conditionally(dec, cond):
    def resdec(f):
        if not cond:
            return f
        return dec(f)
    return resdec


@marshmallow_dataclass.add_schema
@conditionally(dataclasses.dataclass, ENABLE_DC)
@conditionally(attr.s(auto_attribs=True), ENABLE_ATTRS)
class ClsA:
    a: int
    b: str
    Schema : ClassVar[Type[Schema]] = Schema


@marshmallow_dataclass.add_schema
@conditionally(dataclasses.dataclass, ENABLE_DC)
@conditionally(attr.s(auto_attribs=True), ENABLE_ATTRS)
class ClsB(ClsA):
    if ENABLE_ATTRS:
        c: float = attr.ib(validator=attr.validators.lt(5))
    else:
        c: float
    d: str
    Schema : ClassVar[Type[Schema]] = Schema


B = ClsB.Schema().load({'a': 123, 'b': 'bb', 'c': 4, 'd': "123"})
print(B)
# ClsB(a=123, b='bb', c=4.0, d='123')

B = ClsB.Schema().load({'a': 123, 'b': 'bb', 'c': "3", 'd': 'dd'})
print(B)
# ClsB(a=123, b='bb', c=3.0, d='dd')

try:
    B = ClsB.Schema().load({'a': "asdad", 'b': 'bb', 'c': 2, 'd': 'dd'})
    print(B)
except Exception as e:
    print(e)
# {'a': ['Not a valid integer.']}

try:
    B = ClsB.Schema().load({'a': 123, 'b': 'bb', 'c': '123123asd', 'd': 'dd'})
    print(B)
except Exception as e:
    print(e)
# {'c': ['Not a valid number.']}

try:
    B = ClsB.Schema().load({'a': 123, 'b': 'bb', 'c': '123123', 'd': 'dd'})
    print(B)
except Exception as e:
    print(e)
# "ClsB(a=123, b='bb', c=123123.0, d='dd')" if not ENABLE_ATTRS else "'c' must be < 5: 123123.0"

[hynek]

I've simplified your breaking example to:

import attr, marshmallow_dataclass
from marshmallow import Schema
from typing import ClassVar, Type


@marshmallow_dataclass.add_schema
@attr.s(auto_attribs=True)
class ClsA:
    a: int
    b: str
    Schema: ClassVar[Type[Schema]] = Schema


@marshmallow_dataclass.add_schema
@attr.s(auto_attribs=True)
class ClsB(ClsA):
    c: float = attr.ib(validator=attr.validators.lt(5))
    d: str
    Schema: ClassVar[Type[Schema]] = Schema

b = ClsB(1, "2", 3.0, "4")

print(ClsB.Schema().dumps(b))

B = ClsB.Schema().load({"a": 123, "b": "bb", "c": 4, "d": "123"})

Sadly, given the output of the print (only c and d), my guess is that marshmallow-dataclass simply doesn't support attrs. All attributes are accessible at __attrs_attrs__ (or preferably via attrs.fields()) but it seems to only know about the fields that are defined within the body using type annotations.

I suspect it would be rather trivial for marshmallow-dataclass to support attrs if they cared for it, but it's nothing we can do anything about here at attrs.

[csghone]

@hynek - Thank you for the response.
I'll try out some experiments to find some better workaround based on your explanations.