pydantic · davidhewitt · Aug 15, 2023 · Jul 28, 2023 · Aug 8, 2023 · Aug 10, 2023
diff --git a/generate_self_schema.py b/generate_self_schema.py
@@ -133,6 +133,17 @@ def type_dict_schema(typed_dict) -> dict[str, Any]:  # noqa: C901
                     schema = {'type': &#
8000
39;dict', 'keys_schema': {'type': 'str'}, 'values_schema': schema_ref_validator}
                 elif fr_arg == 'Dict[Hashable, CoreSchema]':
                     schema = {'type': 'dict', 'keys_schema': {'type': 'any'}, 'values_schema': schema_ref_validator}
+                elif fr_arg == 'List[Union[CoreSchema, Tuple[CoreSchema, str]]]':
+                    schema = {
+                        'type': 'list',
+                        'items_schema': {
+                            'type': 'union',
+                            'choices': [
+                                schema_ref_validator,
+                                {'type': 'tuple-positional', 'items_schema': [schema_ref_validator, {'type': 'str'}]},
+                            ],
+                        },
+                    }
                 else:
                     raise ValueError(f'Unknown Schema forward ref: {fr_arg}')
             else:

diff --git a/python/pydantic_core/core_schema.py b/python/pydantic_core/core_schema.py
@@ -9,7 +9,7 @@
 from collections.abc import Mapping
 from datetime import date, datetime, time, timedelta
 from decimal import Decimal
-from typing import TYPE_CHECKING, Any, Callable, Dict, Hashable, List, Set, Type, Union
+from typing import TYPE_CHECKING, Any, Callable, Dict, Hashable, List, Set, Tuple, Type, Union
 
 if sys.version_info < (3, 11):
     from typing_extensions import Protocol, Required, TypeAlias
@@ -2454,7 +2454,7 @@ def nullable_schema(
 
 class UnionSchema(TypedDict, total=False):
     type: Required[Literal['union']]
-    choices: Required[List[CoreSchema]]
+    choices: Required[List[Union[CoreSchema, Tuple[CoreSchema, str]]]]
     # default true, whether to automatically collapse unions with one element to the inner validator
     auto_collapse: bool
     custom_error_type: str
@@ -2467,7 +2467,7 @@ class UnionSchema(TypedDict, total=False):
 
 
 def union_schema(
-    choices: list[CoreSchema],
+    choices: list[CoreSchema | tuple[CoreSchema, str]],
     *,
     auto_collapse: bool | None = None,
     custom_error_type: str | None = None,
@@ -2491,7 +2491,7 @@ def union_schema(
     ```
 
     Args:
-        choices: The schemas to match
+        choices: The schemas to match. If a tuple, the second item is used as the label for the case.
         auto_collapse: whether to automatically collapse unions with one element to the inner validator, default true
         custom_error_type: The custom error type to use if the validation fails
         custom_error_message: The custom error message to use if the validation fails

diff --git a/src/serializers/type_serializers/union.rs b/src/serializers/type_serializers/union.rs
@@ -1,6 +1,6 @@
 use pyo3::intern;
 use pyo3::prelude::*;
-use pyo3::types::{PyDict, PyList};
+use pyo3::types::{PyDict, PyList, PyTuple};
 use std::borrow::Cow;
 
 use crate::build_tools::py_schema_err;
@@ -31,7 +31,13 @@ impl BuildSerializer for UnionSerializer {
         let choices: Vec<CombinedSerializer> = schema
             .get_as_req::<&PyList>(intern!(py, "choices"))?
             .iter()
-            .map(|choice| CombinedSerializer::build(choice.downcast()?, config, definitions))
+            .map(|choice| {
+                let choice: &PyAny = match choice.downcast::<PyTuple>() {
+                    Ok(py_tuple) => py_tuple.get_item(0)?,
+                    Err(_) => choice,
+                };
+                CombinedSerializer::build(choice.downcast()?, config, definitions)
+            })
             .collect::<PyResult<Vec<CombinedSerializer>>>()?;
 
         Self::from_choices(choices)

diff --git a/src/validators/union.rs b/src/validators/union.rs
@@ -1,7 +1,7 @@
 use std::fmt::Write;
 
 use pyo3::prelude::*;
-use pyo3::types::{PyDict, PyList, PyString};
+use pyo3::types::{PyDict, PyList, PyString, PyTuple};
 use pyo3::{intern, PyTraverseError, PyVisit};
 
 use crate::build_tools::py_schema_err;
@@ -19,7 +19,7 @@ use super::{build_validator, BuildValidator, CombinedValidator, Definitions, Def
 
 #[derive(Debug, Clone)]
 pub struct UnionValidator {
-    choices: Vec<CombinedValidator>,
+    choices: Vec<(CombinedValidator, Option<String>)>,
     custom_error: Option<CustomError>,
     strict: bool,
     name: String,
@@ -36,18 +36,33 @@ impl BuildValidator for UnionValidator {
         definitions: &mut DefinitionsBuilder<CombinedValidator>,
     ) -> PyResult<CombinedValidator> {
         let py = schema.py();
-        let choices: Vec<CombinedValidator> = schema
+        let choices: Vec<(CombinedValidator, Option<String>)> = schema
             .get_as_req::<&PyList>(intern!(py, "choices"))?
             .iter()
-            .map(|choice| build_validator(choice, config, definitions))
-            .collect::<PyResult<Vec<CombinedValidator>>>()?;
+            .map(|choice| {
+                let mut label: Option<String> = None;
+                let choice: &PyAny = match choice.downcast::<PyTuple>() {
+                    Ok(py_tuple) => {
+                        let choice = py_tuple.get_item(0)?;
+                        label = Some(py_tuple.get_item(1)?.to_string());
+                        choice
+                    }
+                    Err(_) => choice,
+                };
+                Ok((build_validator(choice, config, definitions)?, label))
+            })
+            .collect::<PyResult<Vec<(CombinedValidator, Option<String>)>>>()?;
 
         let auto_collapse = || schema.get_as_req(intern!(py, "auto_collapse")).unwrap_or(true);
         match choices.len() {
             0 => py_schema_err!("One or more union choices required"),
-            1 if auto_collapse() => Ok(choices.into_iter().next().unwrap()),
+            1 if auto_collapse() => Ok(choices.into_iter().next().unwrap().0),
             _ => {
-                let descr = choices.iter().map(Validator::get_name).collect::<Vec<_>>().join(",");
+                let descr = choices
+                    .iter()
+                    .map(|(choice, label)| label.as_deref().unwrap_or(choice.get_name()))
+                    .collect::<Vec<_>>()
+                    .join(",");
 
                 Ok(Self {
                     choices,
@@ -77,7 +92,12 @@ impl UnionValidator {
     }
 }
 
-impl_py_gc_traverse!(UnionValidator { choices });
+impl PyGcTraverse for UnionValidator {
+    fn py_gc_traverse(&self, visit: &PyVisit<'_>) -> Result<(), PyTraverseError> {
+        self.choices.iter().try_for_each(|(v, _)| v.py_gc_traverse(visit))?;
+        Ok(())
+    }
+}
 
 impl Validator for UnionValidator {
     fn validate<'s, 'data>(
@@ -94,7 +114,9 @@ impl Validator for UnionValidator {
             if let Some(res) = self
                 .choices
                 .iter()
-                .map(|validator| validator.validate(py, input, &ultra_strict_extra, definitions, recursion_guard))
+                .map(|(validator, _label)| {
+                    validator.validate(py, input, &ultra_strict_extra, definitions, recursion_guard)
+                })
                 .find(ValResult::is_ok)
             {
                 return res;
@@ -108,18 +130,17 @@ impl Validator for UnionValidator {
             };
             let strict_extra = extra.as_strict(false);
 
-            for validator in &self.choices {
+            for (validator, label) in &self.choices {
                 let line_errors = match validator.validate(py, input, &strict_extra, definitions, recursion_guard) {
                     Err(ValError::LineErrors(line_errors)) => line_errors,
                     otherwise => return otherwise,
                 };
 
                 if let Some(ref mut errors) = errors {
-                    errors.extend(
-                        line_errors
-                            .into_iter()
-                            .map(|err| err.with_outer_location(validator.get_name().into())),
-                    );
+                    errors.extend(line_errors.into_iter().map(|err| {
+                        let case_label = label.as_deref().unwrap_or(validator.get_name());
+                        err.with_outer_location(case_label.into())
+                    }));
                 }
             }
 
@@ -132,7 +153,9 @@ impl Validator for UnionValidator {
                 if let Some(res) = self
                     .choices
                     .iter()
-                    .map(|validator| validator.validate(py, input, &strict_extra, definitions, recursion_guard))
+                    .map(|(validator, _label)| {
+                        validator.validate(py, input, &strict_extra, definitions, recursion_guard)
+                    })
                     .find(ValResult::is_ok)
                 {
                     return res;
@@ -145,18 +168,17 @@ impl Validator for UnionValidator {
             };
 
             // 2nd pass: check if the value can be coerced into one of the Union types, e.g. use validate
-            for validator in &self.choices {
+            for (validator, label) in &self.choices {
                 let line_errors = match validator.validate(py, input, extra, definitions, recursion_guard) {
                     Err(ValError::LineErrors(line_errors)) => line_errors,
                     success => return success,
                 };
 
                 if let Some(ref mut errors) = errors {
-                    errors.extend(
-                        line_errors
-                            .into_iter()
-                            .map(|err| err.with_outer_location(validator.get_name().into())),
-                    );
+                    errors.extend(line_errors.into_iter().map(|err| {
+                        let case_label = label.as_deref().unwrap_or(validator.get_name());
+                        err.with_outer_location(case_label.into())
+                    }));
                 }
             }
 
@@ -171,15 +193,15 @@ impl Validator for UnionValidator {
     ) -> bool {
         self.choices
             .iter()
-            .any(|v| v.different_strict_behavior(definitions, ultra_strict))
+            .any(|(v, _)| v.different_strict_behavior(definitions, ultra_strict))
     }
 
     fn get_name(&self) -> &str {
         &self.name
     }
 
     fn complete(&mut self, definitions: &DefinitionsBuilder<CombinedValidator>) -> PyResult<()> {
-        self.choices.iter_mut().try_for_each(|v| v.complete(definitions))?;
+        self.choices.iter_mut().try_for_each(|(v, _)| v.complete(definitions))?;
         self.strict_required = self.different_strict_behavior(Some(definitions), false);
         self.ultra_strict_required = self.different_strict_behavior(Some(definitions), true);
         Ok(())

diff --git a/tests/serializers/test_union.py b/tests/serializers/test_union.py
@@ -15,9 +15,14 @@ def __init__(self, **kwargs) -> None:
             setattr(self, name, value)
 
 
+@pytest.mark.parametrize('bool_case_label', [False, True])
+@pytest.mark.parametrize('int_case_label', [False, True])
 @pytest.mark.parametrize('input_value,expected_value', [(True, True), (False, False), (1, 1), (123, 123), (-42, -42)])
-def test_union_bool_int(input_value, expected_value):
-    s = SchemaSerializer(core_schema.union_schema([core_schema.bool_schema(), core_schema.int_schema()]))
+def test_union_bool_int(input_value, expected_value, bool_case_label, int_case_label):
+    bool_case = core_schema.bool_schema() if not bool_case_label else (core_schema.bool_schema(), 'my_bool_label')
+    int_case = core_schema.int_schema() if not int_case_label else (core_schema.int_schema(), 'my_int_label')
+    s = SchemaSerializer(core_schema.union_schema([bool_case, int_case]))
+
     assert s.to_python(input_value) == expected_value
     assert s.to_python(input_value, mode='json') == expected_value
     assert s.to_json(input_value) == json.dumps(expected_value).encode()

diff --git a/tests/validators/test_union.py b/tests/validators/test_union.py
@@ -411,3 +411,23 @@ def test_strict_reference():
 
     assert repr(v.validate_python((1, 2))) == '(1.0, 2)'
     assert repr(v.validate_python((1.0, (2.0, 3)))) == '(1.0, (2.0, 3))'
+
+
+def test_case_labels():
+    v = SchemaValidator(
+        {'type': 'union', 'choices': [{'type': 'none'}, ({'type': 'int'}, 'my_label'), {'type': 'str'}]}
+    )
+    assert v.validate_python(None) is None
+    assert v.validate_python(1) == 1
+    with pytest.raises(ValidationError, match=r'3 validation errors for union\[none,my_label,str]') as exc_info:
+        v.validate_python(1.5)
+    assert exc_info.value.errors(include_url=False) == [
+        {'input': 1.5, 'loc': ('none',), 'msg': 'Input should be None', 'type': 'none_required'},
+        {
+            'input': 1.5,
+            'loc': ('my_label',),
+            'msg': 'Input should be a valid integer, got a number with a fractional part',
+            'type': 'int_from_float',
+        },
+        {'input': 1.5, 'loc': ('str',), 'msg': 'Input should be a valid string', 'type': 'string_type'},
+    ]