Support dataclass default None as allowed type

Hi, thanks for the nice library.

The following use case triggers an error in desert:

from dataclasses import dataclass

import desert

@dataclass
class WebhooksInfoItem:
    url: str = None

WebhooksInfoItemSchema = desert.schema(WebhooksInfoItem)

WebhooksInfoItemSchema.loads('{}')
# Ok: WebhooksInfoItem(url=None)

WebhooksInfoItemSchema.loads('{"url": null}')
# marshmallow.exceptions.ValidationError: {'url': ['Field may not be null.']}

marshmallow_dataclass seems to support optional declaration through default None in both cases. Is it forbidden in desert by design? It does not look like it according to the first usecase.

I feel that = None to be an implicit equivalent of Optional[] to be a good shortcut. Maybe I do not see a potential drawback?

[wanderrful]

For what it’s worth, my random two cents is that when we instantiate the desert.Schema in the first place, just as we can add meta={"unknown": marshmallow.EXCLUDE} we should also be able to add a "partial": True", which should implicitly make all fields in the Dataclass schema have from typing import Optional wrapped around each member field of the Dataclass if it doesn’t already have it when it gets processed internally.

My personal inuitition when I encountered this problem was that the partial=True kwarg in the schema’s instantiation or load methods should have done that, but it didn’t, which led me to this thread. Thanks for reading! 🚀

[remidebette]

I am still discovering the way to use the lib.

I do think Optional is actually more suited to my purpose. As it works as expected I already reworked my code to use Optional instead.

(It also has the advantage that I do not need to bubble down all optional parameters to the bottom of my dataclass.)

But I still think the error does not protect against much.

Very interestingly, when using the python dataclass constructor itself the behavior is exactly reversed since it is = None that makes it a python keyword (hence optional):

from dataclasses import dataclass
from typing import Optional

import desert

@dataclass
class WebhooksInfoItem:
    url: str = None

WebhooksInfoItemSchema = desert.schema(WebhooksInfoItem)

WebhooksInfoItemSchema.load({})
# Ok: WebhooksInfoItem(url=None)   as there is a default value
WebhooksInfoItem()
# Ok: WebhooksInfoItem(url=None)  as there is a default value

@dataclass
class WebhooksInfoItem:
    url: Optional[str]

WebhooksInfoItemSchema = desert.schema(WebhooksInfoItem)

WebhooksInfoItemSchema.load({})
# Ok: WebhooksInfoItem(url=None)   since it is Optional
# also WebhooksInfoItemSchema.dump(WebhooksInfoItemSchema.load({}))
# {"url":None}

WebhooksInfoItem()
# Error: TypeError: __init__() missing 1 required positional argument: 'url'

This all might confuse a python beginner starting to use desert.