Allow str as argument to Discriminator (#8047)

docs/concepts/fields.md

@@ -597,8 +597,8 @@ print(user)
 ## Discriminator
 
 The parameter `discriminator` can be used to control the field that will be used to discriminate between different
-models in a union. It takes either the name of a field or a `CallableDiscriminator` instance. The `CallableDiscriminator`
-approach can be useful when the discriminator fields aren't the same for all of the models in the `Union`.
+models in a union. It takes either the name of a field or a `Discriminator` instance. The `Discriminator`
+approach can be useful when the discriminator fields aren't the same for all the models in the `Union`.
 
 The following example shows how to use `discriminator` with a field name:
 
@@ -628,14 +628,14 @@ print(Model.model_validate({'pet': {'pet_type': 'cat', 'age': 12}}))  # (1)!
 
 1. See more about [Helper Functions] in the [Models] page.
 
-The following example shows how to use `discriminator` with a `CallableDiscriminator` instance:
+The following example shows how to use the `discriminator` keyword argument with a `Discriminator` instance:
 
 ```py requires="3.8"
 from typing import Literal, Union
 
 from typing_extensions import Annotated
 
-from pydantic import BaseModel, CallableDiscriminator, Field, Tag
+from pydantic import BaseModel, Discriminator, Field, Tag
 
 
 class Cat(BaseModel):
@@ -656,7 +656,7 @@ def pet_discriminator(v):
 
 class Model(BaseModel):
     pet: Union[Annotated[Cat, Tag('cat')], Annotated[Dog, Tag('dog')]] = Field(
-        discriminator=CallableDiscriminator(pet_discriminator)
+        discriminator=Discriminator(pet_discriminator)
     )
 
 

docs/concepts/unions.md

@@ -184,17 +184,18 @@ except ValidationError as e:
     """
 ```
 
-### Discriminated Unions with `CallableDiscriminator` discriminators
+### Discriminated Unions with callable `Discriminator`s
 
 In the case of a `Union` with multiple models, sometimes there isn't a single uniform field
-across all models that you can use as a discriminator. This is the perfect use case for the `CallableDiscriminator` approach.
+across all models that you can use as a discriminator.
+This is the perfect use case for a callable `Discriminator`.
 
 ```py requires="3.8"
 from typing import Any, Literal, Union
 
 from typing_extensions import Annotated
 
-from pydantic import BaseModel, CallableDiscriminator, Tag
+from pydantic import BaseModel, Discriminator, Tag
 
 
 class Pie(BaseModel):
@@ -222,7 +223,7 @@ class ThanksgivingDinner(BaseModel):
             Annotated[ApplePie, Tag('apple')],
             Annotated[PumpkinPie, Tag('pumpkin')],
         ],
-        CallableDiscriminator(get_discriminator_value),
+        Discriminator(get_discriminator_value),
     ]
 
 
@@ -249,7 +250,7 @@ ThanksgivingDinner(dessert=PumpkinPie(time_to_cook=40, num_ingredients=6, fillin
 """
 ```
 
-`CallableDiscriminators` can also be used to validate `Union` types with combinations of models and primitive types.
+`Discriminator`s can also be used to validate `Union` types with combinations of models and primitive types.
 
 For example:
 
@@ -258,7 +259,7 @@ from typing import Any, Union
 
 from typing_extensions import Annotated
 
-from pydantic import BaseModel, CallableDiscriminator, Tag
+from pydantic import BaseModel, Discriminator, Tag
 
 
 def model_x_discriminator(v: Any) -> str:
@@ -274,7 +275,7 @@ class DiscriminatedModel(BaseModel):
             Annotated[str, Tag('str')],
             Annotated['DiscriminatedModel', Tag('model')],
         ],
-        CallableDiscriminator(
+        Discriminator(
             model_x_discriminator,
             custom_error_type='invalid_union_member',
             custom_error_message='Invalid union member',
@@ -303,11 +304,11 @@ assert m.model_dump() == data
     some_field: Annotated[Union[...], Field(discriminator='my_discriminator')]
     ```
 
-    For `CallableDiscriminator` discriminators:
+    For callable `Discriminator`s:
     ```
-    some_field: Union[...] = Field(discriminator=CallableDiscriminator(...))
-    some_field: Annotated[Union[...], CallableDiscriminator(...)]
-    some_field: Annotated[Union[...], Field(discriminator=CallableDiscriminator(...))]
+    some_field: Union[...] = Field(discriminator=Discriminator(...))
+    some_field: Annotated[Union[...], Discriminator(...)]
+    some_field: Annotated[Union[...], Field(discriminator=Discriminator(...))]
     ```
 
 !!! warning
@@ -390,7 +391,7 @@ from typing import Union
 
 from typing_extensions import Annotated
 
-from pydantic import BaseModel, CallableDiscriminator, Tag, ValidationError
+from pydantic import BaseModel, Discriminator, Tag, ValidationError
 
 
 # Errors are quite verbose with a normal Union:
@@ -474,7 +475,7 @@ class DiscriminatedModel(BaseModel):
             Annotated[str, Tag('str')],
             Annotated['DiscriminatedModel', Tag('model')],
         ],
-        CallableDiscriminator(
+        Discriminator(
             model_x_discriminator,
             custom_error_type='invalid_union_member',
             custom_error_message='Invalid union member',

docs/errors/usage_errors.md

@@ -367,14 +367,14 @@ assert Model(pet={'pet_type': 'kitten'}).pet.pet_type == 'cat'
 
 ## Callable discriminator case with no tag {#callable-discriminator-no-tag}
 
-This error is raised when a `Union` that uses a `CallableDiscriminator` doesn't have `Tag` annotations for all cases.
+This error is raised when a `Union` that uses a callable `Discriminator` doesn't have `Tag` annotations for all cases.
 
 ```py
 from typing import Union
 
 from typing_extensions import Annotated
 
-from pydantic import BaseModel, CallableDiscriminator, PydanticUserError, Tag
+from pydantic import BaseModel, Discriminator, PydanticUserError, Tag
 
 
 def model_x_discriminator(v):
@@ -390,7 +390,7 @@ try:
     class DiscriminatedModel(BaseModel):
         x: Annotated[
             Union[str, 'DiscriminatedModel'],
-            CallableDiscriminator(model_x_discriminator),
+            Discriminator(model_x_discriminator),
         ]
 
 except PydanticUserError as exc_info:
@@ -402,7 +402,7 @@ try:
     class DiscriminatedModel(BaseModel):
         x: Annotated[
             Union[Annotated[str, Tag('str')], 'DiscriminatedModel'],
-            CallableDiscriminator(model_x_discriminator),
+            Discriminator(model_x_discriminator),
         ]
 
 except PydanticUserError as exc_info:
@@ -414,7 +414,7 @@ try:
     class DiscriminatedModel(BaseModel):
         x: Annotated[
             Union[str, Annotated['DiscriminatedModel', Tag('model')]],
-            CallableDiscriminator(model_x_discriminator),
+            Discriminator(model_x_discriminator),
         ]
 
 except PydanticUserError as exc_info:

pydantic/__init__.py

@@ -182,7 +182,7 @@
     'Base64UrlStr',
     'GetPydanticSchema',
     'Tag',
-    'CallableDiscriminator',
+    'Discriminator',
     'JsonValue',
     # type_adapter
     'TypeAdapter',
@@ -324,7 +324,7 @@
     'Base64UrlStr': (__package__, '.types'),
     'GetPydanticSchema': (__package__, '.types'),
     'Tag': (__package__, '.types'),
-    'CallableDiscriminator': (__package__, '.types'),
+    'Discriminator': (__package__, '.types'),
     'JsonValue': (__package__, '.types'),
     # type_adapter
     'TypeAdapter': (__package__, '.type_adapter'),

pydantic/_internal/_discriminated_union.py

@@ -14,7 +14,7 @@
 )
 
 if TYPE_CHECKING:
-    from ..types import CallableDiscriminator
+    from ..types import Discriminator
 
 CORE_SCHEMA_METADATA_DISCRIMINATOR_PLACEHOLDER_KEY = 'pydantic.internal.union_discriminator'
 
@@ -62,7 +62,7 @@ def inner(s: core_schema.CoreSchema, recurse: _core_utils.Recurse) -> core_schem
 
 def apply_discriminator(
     schema: core_schema.CoreSchema,
-    discriminator: str | CallableDiscriminator,
+    discriminator: str | Discriminator,
     definitions: dict[str, core_schema.CoreSchema] | None = None,
 ) -> core_schema.CoreSchema:
     """Applies the discriminator and returns a new core schema.
@@ -88,10 +88,13 @@ def apply_discriminator(
             - If discriminator fields have different aliases.
             - If discriminator field not of type `Literal`.
     """
-    from ..types import CallableDiscriminator
+    from ..types import Discriminator
 
-    if isinstance(discriminator, CallableDiscriminator):
-        return discriminator._convert_schema(schema)
+    if isinstance(discriminator, Discriminator):
+        if isinstance(discriminator.discriminator, str):
+            discriminator = discriminator.discriminator
+        else:
+            return discriminator._convert_schema(schema)
 
     return _ApplyInferredDiscriminator(discriminator, definitions or {}).apply(schema)
 

pydantic/_internal/_generate_schema.py

@@ -82,7 +82,7 @@
 if TYPE_CHECKING:
     from ..fields import ComputedFieldInfo, FieldInfo
     from ..main import BaseModel
-    from ..types import CallableDiscriminator
+    from ..types import Discriminator
     from ..validators import FieldValidatorModes
     from ._dataclasses import StandardDataclass
     from ._schema_generation_shared import GetJsonSchemaFunction
@@ -374,7 +374,7 @@ def _unknown_type_schema(self, obj: Any) -> CoreSchema:
         )
 
     def _apply_discriminator_to_union(
-        self, schema: CoreSchema, discriminator: str | CallableDiscriminator | None
+        self, schema: CoreSchema, discriminator: str | Discriminator | None
     ) -> CoreSchema:
         if discriminator is None:
             return schema

pydantic/fields.py

@@ -64,7 +64,7 @@ class _FromFieldInfoInputs(typing_extensions.TypedDict, total=False):
     max_digits: int | None
     decimal_places: int | None
     union_mode: Literal['smart', 'left_to_right'] | None
-    discriminator: str | types.CallableDiscriminator | None
+    discriminator: str | types.Discriminator | None
     json_schema_extra: JsonDict | typing.Callable[[JsonDict], None] | None
     frozen: bool | None
     validate_default: bool | None
@@ -101,7 +101,7 @@ class FieldInfo(_repr.Representation):
         description: The description of the field.
         examples: List of examples of the field.
         exclude: Whether to exclude the field from the model serialization.
-        discriminator: Field name or CallableDiscriminator for discriminating the type in a tagged union.
+        discriminator: Field name or Discriminator for discriminating the type in a tagged union.
         json_schema_extra: Dictionary of extra JSON schema properties.
         frozen: Whether the field is frozen.
         validate_default: Whether to validate the default value of the field.
@@ -122,7 +122,7 @@ class FieldInfo(_repr.Representation):
     description: str | None
     examples: list[Any] | None
     exclude: bool | None
-    discriminator: str | types.CallableDiscriminator | None
+    discriminator: str | types.Discriminator | None
     json_schema_extra: JsonDict | typing.Callable[[JsonDict], None] | None
     frozen: bool | None
     validate_default: bool | None
@@ -682,7 +682,7 @@ def Field(  # noqa: C901
     description: str | None = _Unset,
     examples: list[Any] | None = _Unset,
     exclude: bool | None = _Unset,
-    discriminator: str | types.CallableDiscriminator | None = _Unset,
+    discriminator: str | types.Discriminator | None = _Unset,
     json_schema_extra: JsonDict | typing.Callable[[JsonDict], None] | None = _Unset,
     frozen: bool | None = _Unset,
     validate_default: bool | None = _Unset,
@@ -727,7 +727,7 @@ def Field(  # noqa: C901
         description: Human-readable description.
         examples: Example values for this field.
         exclude: Whether to exclude the field from the model serialization.
-        discriminator: Field name or CallableDiscriminator for discriminating the type in a tagged union.
+        discriminator: Field name or Discriminator for discriminating the type in a tagged union.
         json_schema_extra: Any additional JSON schema data for the schema property.
         frozen: Whether the field is frozen.
         validate_default: Run validation that isn't only checking existence of defaults. This can be set to `True` or `False`. If not set, it defaults to `None`.