Add missing RequestValidationError.__str__

`PYDANTIC_V2` moved from `_compat` to `exceptions` to avoid duplication
(`exceptions` can't import `_compat` due to circular imports).

Fixes #12125.

fastapi/_compat.py

@@ -19,17 +19,20 @@ from typing import (
     cast,
 )
 
-from fastapi.exceptions import RequestErrorModel
+from fastapi.exceptions import (
+    PYDANTIC_V2 as PYDANTIC_V2,
+)
+from fastapi.exceptions import (
+    PYDANTIC_VERSION_MINOR_TUPLE as PYDANTIC_VERSION_MINOR_TUPLE,
+)
+from fastapi.exceptions import (
+    RequestErrorModel,
+)
 from fastapi.types import IncEx, ModelNameMap, UnionType
 from pydantic import BaseModel, create_model
-from pydantic.version import VERSION as PYDANTIC_VERSION
 from starlette.datastructures import UploadFile
 from typing_extensions import Annotated, Literal, get_args, get_origin
 
-PYDANTIC_VERSION_MINOR_TUPLE = tuple(int(x) for x in PYDANTIC_VERSION.split(".")[:2])
-PYDANTIC_V2 = PYDANTIC_VERSION_MINOR_TUPLE[0] == 2
-
-
 sequence_annotation_to_type = {
     Sequence: list,
     List: list,

fastapi/exceptions.py

@@ -1,10 +1,27 @@
-from typing import Any, Dict, Optional, Sequence, Type, Union
+from typing import TYPE_CHECKING, Any, Dict, List, Optional, Sequence, Type, Union, cast
 
 from pydantic import BaseModel, create_model
+from pydantic.version import VERSION as PYDANTIC_VERSION
 from starlette.exceptions import HTTPException as StarletteHTTPException
 from starlette.exceptions import WebSocketException as StarletteWebSocketException
 from typing_extensions import Annotated, Doc
 
+PYDANTIC_VERSION_MINOR_TUPLE = tuple(int(x) for x in PYDANTIC_VERSION.split(".")[:2])
+PYDANTIC_V2 = PYDANTIC_VERSION_MINOR_TUPLE[0] == 2
+
+if PYDANTIC_V2:
+    from pydantic.v1.error_wrappers import display_errors
+
+    if TYPE_CHECKING:  # pragma: nocover
+        from pydantic.v1.error_wrappers import ErrorDict
+else:
+    from pydantic.error_wrappers import (  # type: ignore[no-redef]
+        display_errors,
+    )
+
+    if TYPE_CHECKING:  # pragma: nocover
+        from pydantic.error_wrappers import ErrorDict  # type: ignore[no-redef]
+
 
 class HTTPException(StarletteHTTPException):
     """
@@ -159,6 +176,14 @@ class RequestValidationError(ValidationException):
         super().__init__(errors)
         self.body = body
 
+    def __str__(self) -> str:
+        errors = cast(List["ErrorDict"], self.errors())
+        no_errors = len(errors)
+        return (
+            f"{no_errors} validation error{'' if no_errors == 1 else 's'}\n"
+            f"{display_errors(errors)}"
+        )
+
 
 class WebSocketRequestValidationError(ValidationException):
     pass

tests/test_tutorial/test_handling_errors/test_tutorial004.py

@@ -1,3 +1,4 @@
+from fastapi._compat import PYDANTIC_VERSION_MINOR_TUPLE
 from fastapi.testclient import TestClient
 
 from docs_src.handling_errors.tutorial004 import app
@@ -8,18 +9,19 @@ client = TestClient(app)
 def test_get_validation_error():
     response = client.get("/items/foo")
     assert response.status_code == 400, response.text
-    # TODO: remove when deprecating Pydantic v1
-    assert (
-        # TODO: remove when deprecating Pydantic v1
-        "path -> item_id" in response.text
-        or "'loc': ('path', 'item_id')" in response.text
-    )
-    assert (
-        # TODO: remove when deprecating Pydantic v1
-        "value is not a valid integer" in response.text
-        or "Input should be a valid integer, unable to parse string as an integer"
-        in response.text
-    )
+    if PYDANTIC_VERSION_MINOR_TUPLE < (2, 0):
+        assert response.text == (
+            "1 validation error\n"
+            "path -> item_id\n"
+            "  value is not a valid integer (type=type_error.integer)"
+        )
+    else:
+        assert response.text == (
+            "1 validation error\n"
+            "path -> item_id\n"
+            "  Input should be a valid integer, unable to parse string as an integer "
+            "(type=int_parsing)"
+        )
 
 
 def test_get_http_error():