@antonymilne - here I wasn't sure whether the Optional was removed intentionally?

To be confirmed...

After reading through this section of the migration guide for pydantic V2 and this forum post I don't have a strong preference, both should work fine.

To sum up the relevant parts:

Runtime implications
In practice this means that for the following model all fields behave identically in the sense that neither is required during initialization because all of them will receive None as their value, when no other value is provided, and the type of each of them is set to be Optional[str].

from typing import Optional, Union
from pydantic import BaseModel, Field

class Model(BaseModel):
    a: str | None
    b: Union[str, None]
    c: Optional[str]
    d: Optional[str] = None
    e: Optional[str] = Field(default=None)
    f: Optional[str] = Field(default_factory=lambda: None)
    g: str = None

It seems that the g case, the one we are talking about here is under the hood converted to Optional[str] = None:

Note that even g is implicitly handled in a way that sets its type to be Optional[str], even though we annotate it with just str. You can verify this by doing the following:

for field in Model.__fields__.values():
    print(repr(field))

Migration to V2
What will change in pydantic V2 is the way how Optional[X] are handled if no defaults are provided as far as I understand e.g. Optional[str] and Optional[str] = None were both considered as non-required and a default of None was automatically set in V1.

This is not the case anymore with V2. Optional[str] without a default will be considered as a required field, while Optional[str] = None is not required, can be None, and is None by default. That being said you are right that str = None will result in the same, so that field being considered as non-required, can be None and is None by default. So functionally both are the same.

I have a slight preference for Optional[XXX] = None if we want to be more explicit, but as you already mentioned this sometimes leads to these mypy issues where we have to solve it via cast, making it a bit difficult to understand for new developers. So it’s questionable whether this improves it. So either solutions are actually fine for me :)

Interesting conversation. Optional[XXX] = None seems to be a little more what python developer are used to I guess, but if it causes other problems, happy to change

As discussed w/ @antonymilne : Changes will be reverted here and handled properly in separate PR

See #210