Change Summary

Add support fortyping.Never.

Validation

The validator implementation might look a bit weird. This is because ofRootModel. Even though it doesn't really make sense, I assume that the following should work:

fromtypingimportNeverfrompydanticimportRootModelv=RootModel[Never]()

and becauseRootModel passesPydanticUndefined to the validator when no data is provided, the validator needs to work withPydanticUndefined. If we don't expectRootModel[Never]() to ever work, we can simplify the implementation and simply reject all values, but there might be another issue: users might wonder how the following works whileRootModel does not:

fromtypingimportNeverfrompydanticimportBaseModelclassModel(BaseModel):a:Never

Also, simply rejecting everything in the validator means that the validated model instance will not contain any value for theNever fields. When users try to access those fields, they will get an error, but the error is going to be something like

AttributeError: 'Model' object has no attribute 'a'

This kind of makes sense because indeed theNever fields never hold any value, but at the same time they are defined in the model, so it kind of makes sense as well to say that the instances do have those attributes. We should provide some better error messages, but before that gets handled in pydantic, I think in core we can returnPydanticUndefined as a placeholder.

Serialisation

Serialising the never fieldsthemselves will trigger an error. For instance,

v=RootModel[Never]()v.model_dump()# TypeError: type `never` cannot be serialized

On the other hand, serialising models containing never fields should work and the never fields should be omitted:

classModel(BaseModel):a:NeverassertModel().model_dump()== {}

These are covered by the new test cases included in this PR.

Manual integration test

Tested locally with the counterpart implementation for pydantic and it worked.

Related issue number

Part ofpydantic/pydantic#9731

Checklist

• Unit tests for the changes exist
• Documentation reflects the changes where applicable
• Pydantic tests pass with thispydantic-core (except for expected changes)
• My PR is ready to review,please add a comment including the phrase "please review" to assign reviewers

Selected Reviewer:@sydney-runkle