A clearer `ValidationError` should be provided when an `Optional` type hint is used

[Andrew-S-Rosen]

Initial Checks

• I have searched Google & GitHub for similar requests and couldn't find anything
• I have read and followed the docs and still think this feature is missing

Description

When using an Optional type hint, the Pydantic ValidationError does not mention that "None" is valid.

from typing import Literal, Optional

from pydantic import BaseModel


class Settings(BaseModel):
    sample: Optional[Literal["test1", "test2"]]

Settings(sample=2)

ValidationError: 1 validation error for Settings
sample
  Input should be 'test1' or 'test2' [type=literal_error, input_value=2, input_type=int]
    For further information visit https://errors.pydantic.dev/2.5/v/literal_error

From a user's perspective, this can be confusing because sample doesn't only take "test" or "test2" as inputs but also None, and this is not mentioned in the returned ValidationError.

Affected Components

• Compatibility between releases
• Data validation/parsing
• Data serialization - .model_dump() and .model_dump_json()
• JSON Schema
• Dataclasses
• Model Config
• Field Types - adding or changing a particular data type
• Function validation decorator
• Generic Models
• Other Model behaviour - model_construct(), pickling, private attributes, ORM mode
• Plugins and integration with other tools - mypy, FastAPI, python-devtools, Hypothesis, VS Code, PyCharm, etc.

[sydney-runkle]

@Andrew-S-Rosen,

Thanks for the feature request! Certainly seems like a reasonable change. Are you interested in opening a PR to add support for this?

[Andrew-S-Rosen]

@sydney-runkle: Unfortunately, I don't have the bandwidth to pick up this PR but wanted to pitch the idea anyway. Hopefully someone else will be interested.

[christianzzz]

How about adding an none_required error?

ValidationError: 1 validation error for Settings
sample
  Input should be 'test1' or 'test2' [type=literal_error, input_value=2, input_type=int]
    For further information visit https://errors.pydantic.dev/2.5/v/literal_error
  Input should be None [type=none_required, input_value=2, input_type=int]
    For further information visit https://errors.pydantic.dev/latest/v/none_required

[Andrew-S-Rosen]

This works, although it is a bit clunky that there are two separate "input should be" statements.

[christianzzz]

For a union type, there are more separate statements for validation errors. As Optional is essentially a union type, maybe we can just add the none-requierd error back.