5.4. Combining Pydantic and semver

According to its homepage, Pydantic “enforces type hints at runtime, and provides user friendly errors when data is invalid.”

To work with Pydantic>2.0, use the following steps:

1. Derive a new class from Version first and add the magic methods __get_pydantic_core_schema__() and __get_pydantic_json_schema__() like this:

   from typing import Annotated, Any, Callable
   from pydantic import GetJsonSchemaHandler
   from pydantic_core import core_schema
   from pydantic.json_schema import JsonSchemaValue
   from semver import Version
   
   
   class _VersionPydanticAnnotation:
       @classmethod
       def __get_pydantic_core_schema__(
           cls,
           _source_type: Any,
           _handler: Callable[[Any], core_schema.CoreSchema],
       ) -> core_schema.CoreSchema:
           def validate_from_str(value: str) -> Version:
               return Version.parse(value)
   
           from_str_schema = core_schema.chain_schema(
               [
                   core_schema.str_schema(),
                   core_schema.no_info_plain_validator_function(validate_from_str),
               ]
           )
   
           return core_schema.json_or_python_schema(
               json_schema=from_str_schema,
               python_schema=core_schema.union_schema(
                   [
                       core_schema.is_instance_schema(Version),
                       from_str_schema,
                   ]
               ),
               serialization=core_schema.plain_serializer_function_ser_schema(
                   lambda instance: instance.x
               ),
           )
   
       @classmethod
       def __get_pydantic_json_schema__(
           cls, _core_schema: core_schema.CoreSchema, handler: GetJsonSchemaHandler
       ) -> JsonSchemaValue:
           return handler(core_schema.str_schema())
   
   ManifestVersion = Annotated[Version, _VersionPydanticAnnotation]
   

2. Create a new model (in this example MyModel) and derive it from pydantic.BaseModel:

   import pydantic
   
   class MyModel(pydantic.BaseModel):
       version: _VersionPydanticAnnotation
   

3. Use your model like this:

   model = MyModel.parse_obj({"version": "1.2.3"})
   

   The attribute model.version will be an instance of Version. If the version is invalid, the construction will raise a pydantic.ValidationError.