Dataclasses do not implement __iter__, causing webargs.use_kwargs to fail

webargs will attempt to combine the results of schema.load into the resulting kwargs to be passed into a function, as seen here:

https://github.com/marshmallow-code/webargs/blob/5.5.1/src/webargs/core.py#L446-L448

In order to repair this assumption, while still using dataclasses, I had to implement a mixin for my dataclasses to allow them to appear as iterables for the purposes of kwargs.update(dataclass)

class Iterable:
    def __iter__(self):
        """When extending a dataclass, yields all key value pairs as if it were a dictionary"""
        for attr, value in self.__dict__.items():
            if value is not None:
                yield attr, value

@dataclass
class Building(Iterable):
    # field metadata is used to instantiate the marshmallow field
    height: float = field(metadata={"validate": marshmallow.validate.Range(min=0)})
    name: str = field(default="anonymous")

Should I go make noise over in webargs saying they should accept dataclasses? or is this something that we can safely hack in here, as we’re are actively transforming the resulting dictionary INTO a dataclass.

Another acceptable solution would be to add a meta argument to allow a developer to skip the post_load step, and allow load to return a dictionary and NOT a dataclass, which is honestly more optimal IMO.

[CptSpaceToaster]

It looks fine for now!

from flask import Flask
from marshmallow_dataclass import dataclass
from webargs.flaskparser import use_args
from typing import Optional

app = Flask(__name__)

@dataclass
class Person:
  name: str
  greeting: Optional[str] = "Hello"

@app.route("/", methods=['POST'])
@use_args(Person.Schema)
def index(person: Person):
    return f"{person.greeting} {person.name}"

It definitely wasn’t on my radar to move away from use_kwargs but I can’t come up with anything that would be break this yet. I guess you’ll hear from me if there’s an issue.

[sloria]

That should already work. @CptSpaceToaster can you try that out?