Make ValidationError constructor a non-init method (#601)

Author: adriangb

pydantic_core/_pydantic_core.pyi

@@ -171,9 +171,16 @@ class SchemaError(Exception):
     def errors(self) -> 'list[ErrorDetails]': ...
 
 class ValidationError(ValueError):
-    def __init__(
-        self, title: str, errors: 'list[InitErrorDetails]', error_mode: Literal['python', 'json'] = 'python'
-    ) -> None: ...
+    @staticmethod
+    def from_exception_data(
+        title: str, errors: 'list[InitErrorDetails]', error_mode: Literal['python', 'json'] = 'python'
+    ) -> ValidationError:
+        """
+        Provisory constructor for a Validation Error.
+        This API will probably change and be deprecated in the the future; we will make it easier and more
+        powerful to construct and use ValidationErrors, but we cannot do that before our initial Pydantic V2 release.
+        So if you use this method please be aware that it may change or be removed before Pydantic V3.
+        """
     @property
     def title(self) -> str: ...
     def error_count(self) -> int: ...

src/errors/validation_exception.rs

@@ -24,7 +24,7 @@ use super::types::{ErrorMode, ErrorType};
 use super::value_exception::PydanticCustomError;
 use super::ValError;
 
-#[pyclass(subclass, extends=PyValueError, module="pydantic_core._pydantic_core")]
+#[pyclass(extends=PyValueError, module="pydantic_core._pydantic_core")]
 #[derive(Clone)]
 #[cfg_attr(debug_assertions, derive(Debug))]
 pub struct ValidationError {
@@ -123,13 +123,21 @@ impl<'a> IntoPy<ValError<'a>> for ValidationError {
 
 #[pymethods]
 impl ValidationError {
-    #[new]
-    fn py_new(title: PyObject, line_errors: &PyList, error_mode: Option<&str>) -> PyResult<Self> {
-        Ok(Self {
-            line_errors: line_errors.iter().map(PyLineError::try_from).collect::<PyResult<_>>()?,
-            title,
-            error_mode: ErrorMode::try_from(error_mode)?,
-        })
+    #[staticmethod]
+    fn from_exception_data(
+        py: Python,
+        title: PyObject,
+        line_errors: &PyList,
+        error_mode: Option<&str>,
+    ) -> PyResult<Py<Self>> {
+        Py::new(
+            py,
+            Self {
+                line_errors: line_errors.iter().map(PyLineError::try_from).collect::<PyResult<_>>()?,
+                title,
+                error_mode: ErrorMode::try_from(error_mode)?,
+            },
+        )
     }
 
     #[getter]

tests/test_errors.py

@@ -553,27 +553,31 @@ def test_error_json_loc():
 
 def test_raise_validation_error():
     with pytest.raises(ValidationError, match='1 validation error for Foobar\n') as exc_info:
-        raise ValidationError('Foobar', [{'type': 'greater_than', 'loc': ('a', 2), 'input': 4, 'ctx': {'gt': 5}}])
+        raise ValidationError.from_exception_data(
+            'Foobar', [{'type': 'greater_than', 'loc': ('a', 2), 'input': 4, 'ctx': {'gt': 5}}]
+        )
 
     # insert_assert(exc_info.value.errors(include_url=False))
     assert exc_info.value.errors(include_url=False) == [
         {'type': 'greater_than', 'loc': (2, 'a'), 'msg': 'Input should be greater than 5', 'input': 4, 'ctx': {'gt': 5}}
     ]
     with pytest.raises(TypeError, match='GreaterThan requires context: {gt: Number}'):
-        raise ValidationError('Foobar', [{'type': 'greater_than', 'loc': ('a', 2), 'input': 4}])
+        raise ValidationError.from_exception_data('Foobar', [{'type': 'greater_than', 'loc': ('a', 2), 'input': 4}])
 
 
 def test_raise_validation_error_json():
     with pytest.raises(ValidationError) as exc_info:
-        raise ValidationError('Foobar', [{'type': 'none_required', 'loc': [-42], 'input': 'x'}])
+        raise ValidationError.from_exception_data('Foobar', [{'type': 'none_required', 'loc': [-42], 'input': 'x'}])
 
     # insert_assert(exc_info.value.errors(include_url=False))
     assert exc_info.value.errors(include_url=False) == [
         {'type': 'none_required', 'loc': (-42,), 'msg': 'Input should be None', 'input': 'x'}
     ]
 
     with pytest.raises(ValidationError) as exc_info:
-        raise ValidationError('Foobar', [{'type': 'none_required', 'loc': (), 'input': 'x'}], 'json')
+        raise ValidationError.from_exception_data(
+            'Foobar', [{'type': 'none_required', 'loc': (), 'input': 'x'}], 'json'
+        )
 
     # insert_assert(exc_info.value.errors(include_url=False))
     assert exc_info.value.errors(include_url=False) == [
@@ -586,7 +590,7 @@ def test_raise_validation_error_custom():
         'my_error', 'this is a custom error {missed} {foo} {bar}', {'foo': 'X', 'bar': 42}
     )
     with pytest.raises(ValidationError) as exc_info:
-        raise ValidationError('Foobar', [{'type': custom_error, 'input': 'x'}])
+        raise ValidationError.from_exception_data('Foobar', [{'type': custom_error, 'input': 'x'}])
 
     # insert_assert(exc_info.value.errors(include_url=False))
     assert exc_info.value.errors(include_url=False) == [
@@ -631,14 +635,3 @@ def test_loc_with_dots():
         "[type=int_parsing, input_value='x', input_type=str]\n"
         f'    For further information visit https://errors.pydantic.dev/{__version__}/v/int_parsing'
     )
-
-
-def subclass_validation_error():
-    class MyValidationError(ValidationError):
-        def __init__(self, title: str, errors, error_mode='python'):
-            self.body = 123
-            super().__init__(title, errors, error_mode)
-
-    e = MyValidationError('testing', [])
-    assert e.body == 123
-    assert e.title == 'testing'