fix(union_serializer): do not raise warnings in nested unions

In case unions of unions are used, this will bubble-up the errors rather
than warning immediately. If no solution is found among all serializers
by the top-level union, it will warn as before.

Signed-off-by: Luka Peschke <luka.peschke@toucantoco.com>

src/serializers/type_serializers/union.rs

@@ -65,10 +65,61 @@ impl UnionSerializer {
             }
         }
     }
+
+    fn _to_python(
+        &self,
+        value: &Bound<'_, PyAny>,
+        include: Option<&Bound<'_, PyAny>>,
+        exclude: Option<&Bound<'_, PyAny>>,
+        extra: &Extra,
+    ) -> ToPythonExtractorResult {
+        to_python_extractor(value, include, exclude, extra, &self.choices)
+    }
 }
 
 impl_py_gc_traverse!(UnionSerializer { choices });
 
+#[derive(Debug)]
+enum ToPythonExtractorResult {
+    Success(PyObject),
+    Errors(SmallVec<[PyErr; SMALL_UNION_THRESHOLD]>),
+}
+
+fn to_python_extractor(
+    value: &Bound<'_, PyAny>,
+    include: Option<&Bound<'_, PyAny>>,
+    exclude: Option<&Bound<'_, PyAny>>,
+    extra: &Extra,
+    choices: &[CombinedSerializer],
+) -> ToPythonExtractorResult {
+    let mut errors: SmallVec<[PyErr; SMALL_UNION_THRESHOLD]> = SmallVec::new();
+
+    for comb_serializer in choices {
+        match comb_serializer {
+            CombinedSerializer::Union(union_serializer) => {
+                match union_serializer._to_python(value, include, exclude, extra) {
+                    ToPythonExtractorResult::Errors(errs) => errors.extend(errs),
+                    ToPythonExtractorResult::Success(success) => return ToPythonExtractorResult::Success(success),
+                }
+            }
+            CombinedSerializer::TaggedUnion(tagged_union_serializer) => {
+                match tagged_union_serializer._to_python(value, include, exclude, extra) {
+                    ToPythonExtractorResult::Errors(errs) => errors.extend(errs),
+                    ToPythonExtractorResult::Success(success) => return ToPythonExtractorResult::Success(success),
+                }
+            }
+            _ => {
+                match comb_serializer.to_python(value, include, exclude, extra) {
+                    Ok(v) => return ToPythonExtractorResult::Success(v),
+                    Err(err) => errors.push(err),
+                };
+            }
+        }
+    }
+
+    ToPythonExtractorResult::Errors(errors)
+}
+
 fn to_python(
     value: &Bound<'_, PyAny>,
     include: Option<&Bound<'_, PyAny>>,
@@ -80,14 +131,13 @@ fn to_python(
     // try the serializers in left to right order with error_on fallback=true
     let mut new_extra = extra.clone();
     new_extra.check = SerCheck::Strict;
-    let mut errors: SmallVec<[PyErr; SMALL_UNION_THRESHOLD]> = SmallVec::new();
 
-    for comb_serializer in choices {
-        match comb_serializer.to_python(value, include, exclude, &new_extra) {
-            Ok(v) => return Ok(v),
-            Err(err) => errors.push(err),
-        }
-    }
+    let res = to_python_extractor(value, include, exclude, &new_extra, choices);
+
+    let errors = match res {
+        ToPythonExtractorResult::Success(obj) => return Ok(obj),
+        ToPythonExtractorResult::Errors(errs) => errs,
+    };
 
     if retry_with_lax_check {
         new_extra.check = SerCheck::Lax;
@@ -392,6 +442,38 @@ impl TypeSerializer for TaggedUnionSerializer {
 }
 
 impl TaggedUnionSerializer {
+    fn _to_python(
+        &self,
+        value: &Bound<'_, PyAny>,
+        include: Option<&Bound<'_, PyAny>>,
+        exclude: Option<&Bound<'_, PyAny>>,
+        extra: &Extra,
+    ) -> ToPythonExtractorResult {
+        let mut new_extra = extra.clone();
+        new_extra.check = SerCheck::Strict;
+
+        if let Some(tag) = self.get_discriminator_value(value, extra) {
+            let tag_str = tag.to_string();
+            if let Some(&serializer_index) = self.lookup.get(&tag_str) {
+                let serializer = &self.choices[serializer_index];
+
+                match serializer.to_python(value, include, exclude, &new_extra) {
+                    Ok(v) => return ToPythonExtractorResult::Success(v),
+                    Err(_) => {
+                        if self.retry_with_lax_check() {
+                            new_extra.check = SerCheck::Lax;
+                            if let Ok(v) = serializer.to_python(value, include, exclude, &new_extra) {
+                                return ToPythonExtractorResult::Success(v);
+                            }
+                        }
+                    }
+                }
+            }
+        }
+
+        to_python_extractor(value, include, exclude, extra, &self.choices)
+    }
+
     fn get_discriminator_value(&self, value: &Bound<'_, PyAny>, extra: &Extra) -> Option<Py<PyAny>> {
         let py = value.py();
         let discriminator_value = match &self.discriminator {

tests/serializers/test_union.py

@@ -778,3 +778,69 @@ class ModelB:
     model_b = ModelB(field=1)
     assert s.to_python(model_a) == {'field': 1, 'TAG': 'a'}
     assert s.to_python(model_b) == {'field': 1, 'TAG': 'b'}
+
+
+class ModelDog:
+    def __init__(self, type_: Literal['dog']) -> None:
+        self.type_ = 'dog'
+
+
+class ModelCat:
+    def __init__(self, type_: Literal['cat']) -> None:
+        self.type_ = 'cat'
+
+
+def test_union_of_unions_of_models() -> None:
+    s = SchemaSerializer(
+        core_schema.union_schema(
+            [
+                core_schema.union_schema(
+                    [
+                        core_schema.model_schema(
+                            cls=ModelA,
+                            schema=core_schema.model_fields_schema(
+                                fields={
+                                    'a': core_schema.model_field(core_schema.str_schema()),
+                                    'b': core_schema.model_field(core_schema.str_schema()),
+                                },
+                            ),
+                        ),
+                        core_schema.model_schema(
+                            cls=ModelB,
+                            schema=core_schema.model_fields_schema(
+                                fields={
+                                    'c': core_schema.model_field(core_schema.str_schema()),
+                                    'd': core_schema.model_field(core_schema.str_schema()),
+                                },
+                            ),
+                        ),
+                    ]
+                ),
+                core_schema.union_schema(
+                    [
+                        core_schema.model_schema(
+                            cls=ModelCat,
+                            schema=core_schema.model_fields_schema(
+                                fields={
+                                    'type_': core_schema.model_field(core_schema.literal_schema(['cat'])),
+                                },
+                            ),
+                        ),
+                        core_schema.model_schema(
+                            cls=ModelDog,
+                            schema=core_schema.model_fields_schema(
+                                fields={
+                                    'type_': core_schema.model_field(core_schema.literal_schema(['dog'])),
+                                },
+                            ),
+                        ),
+                    ]
+                ),
+            ]
+        )
+    )
+
+    assert s.to_python(ModelA(a='a', b='b'), warnings='error') == {'a': 'a', 'b': 'b'}
+    assert s.to_python(ModelB(c='c', d='d'), warnings='error') == {'c': 'c', 'd': 'd'}
+    assert s.to_python(ModelCat(type_='cat'), warnings='error') == {'type_': 'cat'}
+    assert s.to_python(ModelDog(type_='dog'), warnings='error') == {'type_': 'dog'}