Added validation_error_cause to config. (#7626)

pydantic/_internal/_config.py

@@ -81,6 +81,7 @@ class ConfigWrapper:
     json_schema_mode_override: Literal['validation', 'serialization', None]
     coerce_numbers_to_str: bool
     regex_engine: Literal['rust-regex', 'python-re']
+    validation_error_cause: bool
 
     def __init__(self, config: ConfigDict | dict[str, Any] | type[Any] | None, *, check: bool = True):
         if check:
@@ -178,6 +179,7 @@ def dict_not_none(**kwargs: Any) -> Any:
                 hide_input_in_errors=self.config_dict.get('hide_input_in_errors'),
                 coerce_numbers_to_str=self.config_dict.get('coerce_numbers_to_str'),
                 regex_engine=self.config_dict.get('regex_engine'),
+                validation_error_cause=self.config_dict.get('validation_error_cause'),
             )
         )
         return core_config
@@ -251,6 +253,7 @@ def _context_manager() -> Iterator[None]:
     json_schema_mode_override=None,
     coerce_numbers_to_str=False,
     regex_engine='rust-regex',
+    validation_error_cause=False,
 )
 
 

pydantic/config.py

@@ -857,5 +857,16 @@ class Model(BaseModel):
     ```
     """
 
+    validation_error_cause: bool
+    """
+    If `True`, python exceptions that were part of a validation failure will be shown as an exception group as a cause. Can be useful for debugging. Defaults to `False`.
+
+    Note:
+        Python 3.10 and older don't support exception groups natively. <=3.10, backport must be installed: `pip install exceptiongroup`.
+
+    Note:
+        The structure of validation errors are likely to change in future pydantic versions. Pydantic offers no guarantees about the structure of validation errors. Should be used for visual traceback debugging only.
+    """
+
 
 __getattr__ = getattr_migration(__name__)

tests/test_config.py

@@ -19,6 +19,7 @@
     PydanticSchemaGenerationError,
     ValidationError,
     create_model,
+    field_validator,
     validate_call,
 )
 from pydantic._internal._config import ConfigWrapper, config_defaults
@@ -538,6 +539,34 @@ def test_config_wrapper_match():
     ), 'ConfigDict and ConfigWrapper must have the same annotations (except ConfigWrapper.config_dict)'
 
 
+@pytest.mark.skipif(sys.version_info < (3, 11), reason='requires backport pre 3.11, fully tested in pydantic core')
+def test_config_validation_error_cause():
+    class Foo(BaseModel):
+        foo: int
+
+        @field_validator('foo')
+        def check_foo(cls, v):
+            assert v > 5, 'Must be greater than 5'
+
+    # Should be disabled by default:
+    with pytest.raises(ValidationError) as exc_info:
+        Foo(foo=4)
+    assert exc_info.value.__cause__ is None
+
+    Foo.model_config = ConfigDict(validation_error_cause=True)
+    Foo.model_rebuild(force=True)
+    with pytest.raises(ValidationError) as exc_info:
+        Foo(foo=4)
+    # Confirm python error attached as a cause, and error location specified in a note:
+    assert exc_info.value.__cause__ is not None
+    assert isinstance(exc_info.value.__cause__, ExceptionGroup)
+    assert len(exc_info.value.__cause__.exceptions) == 1
+    src_exc = exc_info.value.__cause__.exceptions[0]
+    assert repr(src_exc) == "AssertionError('Must be greater than 5\\nassert 4 > 5')"
+    assert len(src_exc.__notes__) == 1
+    assert src_exc.__notes__[0] == '\nPydantic: cause of loc: foo'
+
+
 @pytest.mark.skipif(sys.version_info < (3, 10), reason='different on older versions')
 def test_config_defaults_match():
     localns = {'_GenerateSchema': GenerateSchema, 'GenerateSchema': GenerateSchema}