Allow v1.BaseModels to be used when pydantic v2 is installed

fastapi/_compat.py

@@ -19,14 +19,13 @@ from typing import (
 
 from fastapi.exceptions import RequestErrorModel
 from fastapi.types import IncEx, ModelNameMap, UnionType
-from pydantic import BaseModel, create_model
+from pydantic import BaseModel, PydanticDeprecatedSince20, create_model, v1
 from pydantic.version import VERSION as PYDANTIC_VERSION
 from starlette.datastructures import UploadFile
 from typing_extensions import Annotated, Literal, get_args, get_origin
 
 PYDANTIC_V2 = PYDANTIC_VERSION.startswith("2.")
 
-
 sequence_annotation_to_type = {
     Sequence: list,
     List: list,
@@ -98,9 +97,12 @@ if PYDANTIC_V2:
             return self.field_info.annotation
 
         def __post_init__(self) -> None:
-            self._type_adapter: TypeAdapter[Any] = TypeAdapter(
-                Annotated[self.field_info.annotation, self.field_info]
-            )
+            try:
+                self._type_adapter: TypeAdapter[Any] = TypeAdapter(
+                    Annotated[self.field_info.annotation, self.field_info]
+                )
+            except PydanticDeprecatedSince20:
+                pass
 
         def get_default(self) -> Any:
             if self.field_info.is_required():
@@ -123,6 +125,14 @@ if PYDANTIC_V2:
                 return None, _regenerate_error_with_loc(
                     errors=exc.errors(), loc_prefix=loc
                 )
+            except AttributeError:
+                # pydantic v1
+                try:
+                    return v1.parse_obj_as(self.type_, value), None
+                except v1.ValidationError as exc:
+                    return None, _regenerate_error_with_loc(
+                        errors=exc.errors(), loc_prefix=loc
+                    )
 
         def serialize(
             self,
@@ -136,18 +146,42 @@ if PYDANTIC_V2:
             exclude_defaults: bool = False,
             exclude_none: bool = False,
         ) -> Any:
-            # What calls this code passes a value that already called
-            # self._type_adapter.validate_python(value)
-            return self._type_adapter.dump_python(
-                value,
-                mode=mode,
-                include=include,
-                exclude=exclude,
-                by_alias=by_alias,
-                exclude_unset=exclude_unset,
-                exclude_defaults=exclude_defaults,
-                exclude_none=exclude_none,
-            )
+            try:
+                # What calls this code passes a value that already called
+                # self._type_adapter.validate_python(value)
+                return self._type_adapter.dump_python(
+                    value,
+                    mode=mode,
+                    include=include,
+                    exclude=exclude,
+                    by_alias=by_alias,
+                    exclude_unset=exclude_unset,
+                    exclude_defaults=exclude_defaults,
+                    exclude_none=exclude_none,
+                )
+            except AttributeError:
+                # pydantic v1
+                try:
+                    return value.dict(
+                        include=include,
+                        exclude=exclude,
+                        by_alias=by_alias,
+                        exclude_unset=exclude_unset,
+                        exclude_defaults=exclude_defaults,
+                        exclude_none=exclude_none,
+                    )
+                except AttributeError:
+                    return [
+                        item.dict(
+                            include=include,
+                            exclude=exclude,
+                            by_alias=by_alias,
+                            exclude_unset=exclude_unset,
+                            exclude_defaults=exclude_defaults,
+                            exclude_none=exclude_none,
+                        )
+                        for item in value
+                    ]
 
         def __hash__(self) -> int:
             # Each ModelField is unique for our purposes, to allow making a dict from

tests/test_pydantic_v1_models.py

@@ -0,0 +1,80 @@
+from typing import Annotated
+
+import pytest
+from fastapi import Body, FastAPI
+from fastapi.exceptions import ResponseValidationError
+from fastapi.testclient import TestClient
+from pydantic import v1
+
+
+class Item(v1.BaseModel):
+    name: str
+    description: str | None = None
+    price: float
+    tax: float | None = None
+    tags: list = []
+
+
+class Model(v1.BaseModel):
+    name: str
+
+
+app = FastAPI()
+
+
+@app.post("/request_body")
+async def request_body(body: Annotated[Item, Body()]):
+    return body
+
+
+@app.get("/response_model", response_model=Model)
+async def response_model():
+    return Model(name="valid_model")
+
+
+@app.get("/response_model__invalid", response_model=Model)
+async def response_model__invalid():
+    return 1
+
+
+@app.get("/response_model_list", response_model=list[Model])
+async def response_model_list():
+    return [Model(name="valid_model")]
+
+
+@app.get("/response_model_list__invalid", response_model=list[Model])
+async def response_model_list__invalid():
+    return [1]
+
+
+client = TestClient(app)
+
+
+class TestResponseModel:
+    def test_simple__valid(self):
+        response = client.get("/response_model")
+        assert response.status_code == 200
+        assert response.json() == {"name": "valid_model"}
+
+    def test_simple__invalid(self):
+        with pytest.raises(ResponseValidationError):
+            client.get("/response_model__invalid")
+
+    def test_list__valid(self):
+        response = client.get("/response_model_list")
+        assert response.status_code == 200
+        assert response.json() == [{"name": "valid_model"}]
+
+    def test_list__invalid(self):
+        with pytest.raises(ResponseValidationError):
+            client.get("/response_model_list__invalid")
+
+
+class TestRequestBody:
+    def test_model__valid(self):
+        response = client.post("/request_body", json={"name": "myname", "price": 1.0})
+        assert response.status_code == 200, response.text
+
+    def test_model__invalid(self):
+        response = client.post("/request_body", json={"name": "myname"})
+        assert response.status_code == 422, response.text

tests/test_response_model_v1.py

@@ -1,25 +0,0 @@
-from typing import List
-
-from fastapi import FastAPI
-from fastapi.testclient import TestClient
-from pydantic import BaseModel, v1
-
-
-class Model(v1.BaseModel):
-    name: str
-
-
-app = FastAPI()
-
-
-@app.get("/valid", response_model=Model)
-def valid1():
-    pass
-
-
-client = TestClient(app)
-
-
-def test_path_operations():
-    response = client.get("/valid")
-    assert response.status_code == 200, response.text