Cannot add marshmallow metadata to list items

Issue

I can’t seem to find any way to apply validation to items of a list type object, and upon inspection of the field_for_schema implementation I don’t think this is possible, as no metadata is forwarded to the schema creation for the list item type:

marshmallow_dataclass: init.py

239:  def field_for_schema(
240:          typ: type,
241:          default=marshmallow.missing,
242:          metadata: Mapping[str, Any] = None
243:  ) -> marshmallow.fields.Field:
...
303:      if origin in (list, List):
304:          list_elements_type = typing_inspect.get_args(typ, True)[0]
305:          return marshmallow.fields.List(
306:              field_for_schema(list_elements_type),  # <<< No metadata passed
307:              **metadata
308:          )

Current State

Let’s say I have a User class and and want to maintain a list of email addresses for each user. Currently, the only way I’d be able to validate the IP addresses is with a callable that iterated over the list and manually invoked Marshmallow’s validators:

from typing import List
from marshmallow.validate import Email
from marshmallow_dataclass import dataclass

@dataclass
class User:
    name: str
    emails: List[str] = field(metadata={
        'validate': lambda l: all(Email()(i) for i in l)
    })

…which, admittedly, is pretty simple, but it’s more weakly representative of the validator’s intent, particularly for tools that post-process the schema (i.e. - the fact that it’s a list validator means that tools aren’t able to recognize that the validation actually applies to the list item).

Proposal

I’d like to add a mechanism for explicitly passing metadata to the list item schema. I figure using a key in the metadata dict that doesn’t conflict with Marshmallow schema kwargs would work just fine for this purpose:

from typing import List
from marshmallow.validate import Email
from marshmallow_dataclass import dataclass

@dataclass
class User:
    name: str
    emails: List[str] = field(metadata={
        'item_metadata': {
            'validate': Email()
        }
    })

If this sounds like something of interest, I’ll gladly whip it up and put out a PR.

[rusnyder]

Sorry for the delay in getting back, here, but this works perfectly. Thanks for the update!

[lovasoa]

Version 6.1.0 was just released with support for NewType, so I am closing this. https://pypi.org/project/marshmallow-dataclass/6.1.0/