Model construction fails on non-union types annotated with discriminator. #11661
Description
Initial Checks
- I confirm that I'm using Pydantic V2
Description
Annotating a type with a discriminator that is not a union fails with the following error:
File /path/to/venv/lib/python3.13/site-packages/pydantic/_internal/_model_construction.py:237, in ModelMetaclass.__new__(mcs, cls_name, bases, namespace, __pydantic_generic_metadata__, __pydantic_reset_parent_namespace__, _create_model_module, **kwargs)
233 set_model_mocks(cls)
234 else:
235 # Any operation that requires accessing the field infos instances should be put inside
236 # `complete_model_class()`:
--> 237 complete_model_class(
238 cls,
239 config_wrapper,
240 raise_errors=False,
241 ns_resolver=ns_resolver,
242 create_model_module=_create_model_module,
243 )
245 if config_wrapper.frozen and '__hash__' not in namespace:
246 set_default_hash_func(cls, bases)
File /path/to/venv/lib/python3.13/site-packages/pydantic/_internal/_model_construction.py:620, in complete_model_class(cls, config_wrapper, raise_errors, ns_resolver, create_model_module)
616 set_deprecated_descriptors(cls)
618 cls.__pydantic_core_schema__ = schema
--> 620 cls.__pydantic_validator__ = create_schema_validator(
621 schema,
622 cls,
623 create_model_module or cls.__module__,
624 cls.__qualname__,
625 'create_model' if create_model_module else 'BaseModel',
626 core_config,
627 config_wrapper.plugin_settings,
628 )
629 cls.__pydantic_serializer__ = SchemaSerializer(schema, core_config)
630 cls.__pydantic_complete__ = True
File /path/to/venv/lib/python3.13/site-packages/pydantic/plugin/_schema_validator.py:51, in create_schema_validator(schema, schema_type, schema_type_module, schema_type_name, schema_kind, config, plugin_settings)
41 return PluggableSchemaValidator(
42 schema,
43 schema_type,
(...)
48 plugin_settings or {},
49 )
50 else:
---> 51 return SchemaValidator(schema, config)
SchemaError: Definitions error: definition `__main__.Foo:4365581472` was never filled
This only happens when FooWithDiscriminator is declared as a PEP-695-style type alias.
Notes:
- This only started failing with Pydantic 2.11.1. The provided code works in 2.11.0
- This might be a mis-use of discriminator, given that it should typically discriminate between different union cases. However, I would still consider this as a bug in Pydantic given that the exception does not clarify what is wrong with the declared model.
Example Code
from pydantic import BaseModel, Field
from typing import Annotated, Literal
class Foo(BaseModel):
type: Literal["foo"] = "foo"
type FooWithDiscriminator = Annotated[Foo, Field(discriminator="type")]
class Container(BaseModel):
f: FooWithDiscriminatorPython, Pydantic & OS Version
pydantic version: 2.11.1
pydantic-core version: 2.33.0
pydantic-core build: profile=release pgo=false
install path: /path/to/venv/lib/python3.13/site-packages/pydantic
python version: 3.13.1 (main, Dec 3 2024, 17:59:52) [Clang 19.1.5 ]
platform: macOS-15.4-arm64-arm-64bit-Mach-O
related packages: mypy-1.15.0 pydantic-settings-2.8.1 fastapi-0.115.12 typing_extensions-4.13.0
commit: unknown