Unabled to defind metadata with Nested types

[boomjuice]

I defined a dataclass as follow.

@dataclass
class ImportData:
      data_list: t.List[t.List[str]] = field(metadata={'allow_none': True})

and how do i defind the attrs of str in nested types?
with older version, i defind the attrs like this.

class ImportDataSchema(Schema):
    data_list = fields.List(fields.List(fields.Str(allow_none=True)))

[dairiki]

For your specific example, I think this will work:

@dataclass
class ImportData:
      data_list: t.List[t.List[t.Optional[str]]]

Using the Optional inner type will automatically set allow_none on the field.
(You could equivalently write t.Union[str, None] — or, in python 3.10 or above, str | None — in place of t.Optional[str].)