Add support for deprecated fields

docs/concepts/fields.md

@@ -692,6 +692,46 @@ print(user.model_dump())  # (1)!
 
 See the [Serialization] section for more details.
 
+## Deprecated fields
+
+The `deprecated` parameter can be used to mark a field as being deprecated. Doing so will result in:
+
+* a runtime deprecation warning emitted when accessing the field.
+* `"deprecated": true` being set in the generated JSON schema.
+
+```py
+from typing_extensions import Annotated
+
+from pydantic import BaseModel, Field
+
+
+class Model(BaseModel):
+    deprecated_field: Annotated[int, Field(deprecated='This is deprecated')]
+
+
+print(Model.model_json_schema()['properties']['deprecated_field'])
+#> {'deprecated': True, 'title': 'Deprecated Field', 'type': 'integer'}
+```
+
+Alternatively, the `warnings.deprecated` decorator (and the `typing_extensions` backport) can be used:
+
+```py test="skip"
+from typing_extensions import Annotated, deprecated
+
+from pydantic import BaseModel, Field
+
+
+class Model(BaseModel):
+    deprecated_field: Annotated[int, deprecated('This is deprecated')]
+
+    # Or explicitly using `Field`:
+    alt_form: Annotated[int, Field(deprecated=deprecated('This is deprecated'))]
+```
+
+!!! note "Support for `category` and `stacklevel`"
+    The current implementation of this feature does not take into account the `category` and `stacklevel`
+    arguments to the `deprecated` decorator. This might land in a future version of Pydantic.
+
 ## Customizing JSON Schema
 
 Some field parameters are used exclusively to customize the generated JSON schema. The parameters in question are:
@@ -733,6 +773,25 @@ print(b.model_dump())
 #> {'width': 1.0, 'height': 2.0, 'depth': 3.0, 'volume': 6.0}
 ```
 
+As with regular fields, computed fields can be marked as being deprecated:
+
+```py
+from typing_extensions import deprecated
+
+from pydantic import BaseModel, computed_field
+
+
+class Box(BaseModel):
+    width: float
+    height: float
+    depth: float
+
+    @computed_field
+    @deprecated("'volume' is deprecated")
+    def volume(self) -> float:
+        return self.width * self.height * self.depth
+```
+
 
 [JSON Schema Draft 2020-12]: https://json-schema.org/understanding-json-schema/reference/numeric.html#numeric-types
 [Discriminated Unions]: ../concepts/unions.md#discriminated-unions

pydantic/_internal/_generate_schema.py

@@ -80,7 +80,7 @@
 from ._schema_generation_shared import (
     CallbackGetCoreSchemaHandler,
 )
-from ._typing_extra import is_finalvar
+from ._typing_extra import DEPRECATED_TYPES, is_finalvar
 from ._utils import lenient_issubclass
 
 if TYPE_CHECKING:
@@ -1135,6 +1135,7 @@ def set_discriminator(schema: CoreSchema) -> CoreSchema:
         json_schema_updates = {
             'title': field_info.title,
             'description': field_info.description,
+            'deprecated': isinstance(field_info.deprecated, (DEPRECATED_TYPES, str)) or None,
             'examples': to_jsonable_python(field_info.examples),
         }
         json_schema_updates = {k: v for k, v in json_schema_updates.items() if v is not None}
@@ -1731,6 +1732,9 @@ def set_computed_field_metadata(schema: CoreSchemaOrField, handler: GetJsonSchem
             if description is not None:
                 json_schema['description'] = description
 
+            if isinstance(d.info.deprecated, (DEPRECATED_TYPES, str)):
+                json_schema['deprecated'] = True
+
             examples = d.info.examples
             if examples is not None:
                 json_schema['examples'] = to_jsonable_python(examples)

pydantic/_internal/_model_construction.py

@@ -1,14 +1,15 @@
 """Private logic for creating models."""
 from __future__ import annotations as _annotations
 
+import builtins
 import operator
 import typing
 import warnings
 import weakref
 from abc import ABCMeta
 from functools import partial
 from types import FunctionType
-from typing import Any, Callable, Generic
+from typing import Any, Callable, Generic, NoReturn
 
 import typing_extensions
 from pydantic_core import PydanticUndefined, SchemaSerializer
@@ -211,6 +212,8 @@ def wrapped_model_post_init(self: BaseModel, __context: Any) -> None:
             # the generic computed fields return type is set to PydanticUndefined
             cls.model_computed_fields = {k: v.info for k, v in cls.__pydantic_decorators__.computed_fields.items()}
 
+            set_deprecated_descriptors(cls)
+
             # using super(cls, cls) on the next line ensures we only call the parent class's __pydantic_init_subclass__
             # I believe the `type: ignore` is only necessary because mypy doesn't realize that this code branch is
             # only hit for _proper_ subclasses of BaseModel
@@ -571,6 +574,59 @@ def complete_model_class(
     return True
 
 
+def set_deprecated_descriptors(cls: type[BaseModel]) -> None:
+    """Set data descriptors on the class for deprecated fields."""
+    for field, field_info in cls.model_fields.items():
+        if (msg := field_info.deprecation_message) is not None:
+            desc = _DeprecatedFieldDescriptor(msg)
+            desc.__set_name__(cls, field)
+            setattr(cls, field, desc)
+
+    for field, computed_field_info in cls.model_computed_fields.items():
+        if (
+            not computed_field_info.from_deprecated_decorator  # Avoid having two warnings emitted
+            and (msg := computed_field_info.deprecation_message) is not None
+        ):
+            desc = _DeprecatedFieldDescriptor(msg, computed_field_info.wrapped_property)
+            desc.__set_name__(cls, field)
+            setattr(cls, field, desc)
+
+
+class _DeprecatedFieldDescriptor:
+    """Data descriptor used to emit a runtime deprecation warning before accessing a deprecated field.
+
+    Attributes:
+        msg: The deprecation message to be emitted.
+        wrapped_property: The property instance if the deprecated field is a computed field, or `None`.
+        field_name: The name of the field being deprecated.
+    """
+
+    field_name: str
+
+    def __init__(self, msg: str, wrapped_property: property | None = None) -> None:
+        self.msg = msg
+        self.wrapped_property = wrapped_property
+
+    def __set_name__(self, cls: type[BaseModel], name: str) -> None:
+        self.field_name = name
+
+    def __get__(self, obj: BaseModel | None, obj_type: type[BaseModel] | None = None) -> Any:
+        if obj is None:
+            raise AttributeError(self.field_name)
+
+        warnings.warn(self.msg, builtins.DeprecationWarning, stacklevel=2)
+
+        if self.wrapped_property is not None:
+            return self.wrapped_property.__get__(obj, obj_type)
+        return obj.__dict__[self.field_name]
+
+    # Defined to take precedence over the instance's dictionary
+    # Note that it will not be called when setting a value on a model instance
+    # as `BaseModel.__setattr__` is defined and takes priority.
+    def __set__(self, obj: Any, value: Any) -> NoReturn:
+        raise AttributeError(self.field_name)
+
+
 class _PydanticWeakRef:
     """Wrapper for `weakref.ref` that enables `pickle` serialization.
 

pydantic/_internal/_typing_extra.py

@@ -5,12 +5,13 @@
 import sys
 import types
 import typing
+import warnings
 from collections.abc import Callable
 from functools import partial
 from types import GetSetDescriptorType
 from typing import TYPE_CHECKING, Any, Final
 
-from typing_extensions import Annotated, Literal, TypeAliasType, TypeGuard, get_args, get_origin
+from typing_extensions import Annotated, Literal, TypeAliasType, TypeGuard, deprecated, get_args, get_origin
 
 if TYPE_CHECKING:
     from ._dataclasses import StandardDataclass
@@ -62,6 +63,11 @@ def origin_is_union(tp: type[Any] | None) -> bool:
 if hasattr(typing, 'Literal'):
     LITERAL_TYPES.add(typing.Literal)  # type: ignore
 
+# Check if `deprecated` is a type to prevent errors when using typing_extensions < 4.9.0
+DEPRECATED_TYPES: tuple[Any, ...] = (deprecated,) if isinstance(deprecated, type) else ()
+if hasattr(warnings, 'deprecated'):
+    DEPRECATED_TYPES = (*DEPRECATED_TYPES, warnings.deprecated)  # type: ignore
+
 NONE_TYPES: tuple[Any, ...] = (None, NoneType, *(tp[None] for tp in LITERAL_TYPES))
 
 
@@ -80,6 +86,10 @@ def is_literal_type(type_: type[Any]) -> bool:
     return Literal is not None and get_origin(type_) in LITERAL_TYPES
 
 
+def is_deprecated_instance(instance: Any) -> TypeGuard[deprecated]:
+    return isinstance(instance, DEPRECATED_TYPES)
+
+
 def literal_values(type_: type[Any]) -> tuple[Any, ...]:
     return get_args(type_)