Add missing RequestValidationError.__str__

Fixes #12125.

fastapi/exceptions.py

@@ -153,6 +153,24 @@ class ValidationException(Exception):
     def errors(self) -> Sequence[Any]:
         return self._errors
 
+    def __str__(self) -> str:
+        # Adapted from https://github.com/pydantic/pydantic/blob/c326748b/pydantic/v1/error_wrappers.py#L70-L76.
+        def display_errors(errors: Sequence[Any]) -> str:
+            return "\n".join(
+                f'{_display_error_loc(e)}\n  {e["msg"]} (type={e["type"]})'
+                for e in errors
+            )
+
+        def _display_error_loc(error: Any) -> str:
+            return " -> ".join(str(e) for e in error["loc"])
+
+        errors = self.errors()
+        no_errors = len(errors)
+        return (
+            f'{no_errors} validation error{"" if no_errors == 1 else "s"}\n'
+            f'{display_errors(errors)}'
+        )
+
 
 class RequestValidationError(ValidationException):
     def __init__(self, errors: Sequence[Any], *, body: Any = None) -> None:
@@ -168,9 +186,3 @@ class ResponseValidationError(ValidationException):
     def __init__(self, errors: Sequence[Any], *, body: Any = None) -> None:
         super().__init__(errors)
         self.body = body
-
-    def __str__(self) -> str:
-        message = f"{len(self._errors)} validation errors:\n"
-        for err in self._errors:
-            message += f"  {err}\n"
-        return message

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 +9,20 @@ 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
+    if PYDANTIC_VERSION_MINOR_TUPLE < (2, 0):
-    assert (
+        assert (
-        # TODO: remove when deprecating Pydantic v1
+            response.text
-        "path -> item_id" in response.text
+            == """1 validation error
-        or "'loc': ('path', 'item_id')" in response.text
+path -> item_id
-    )
+  value is not a valid integer (type=type_error.integer)"""
-    assert (
+        )
-        # TODO: remove when deprecating Pydantic v1
+    else:
-        "value is not a valid integer" in response.text
+        assert (
-        or "Input should be a valid integer, unable to parse string as an integer"
+            response.text
-        in response.text
+            == """1 validation error
-    )
+path -> item_id
+  Input should be a valid integer, unable to parse string as an integer (type=int_parsing)"""
+        )
 
 
 def test_get_http_error():