fix: ignore an empty object default instead of dropping the schema

.changeset/ignore_empty_object_default_on_models.md

@@ -0,0 +1,9 @@
+---
+default: patch
+---
+
+# Do not drop a schema over an empty object default
+
+A property whose type is a model schema could not carry `default: {}`. The parser rejected it with `ModelProperty cannot have a default value`, or with `Value {} is not valid, only None is allowed` when the model sat in a union. The generator warned, dropped the enclosing schema and every endpoint that referenced it, and still exited 0.
+
+An empty object carries no information for a model default, so it is now ignored and the property generates with no default. This is how an inline `default: {}` on a property and a bare `$ref` with a sibling `default` already behave. A non-empty default is still rejected.

end_to_end_tests/functional_tests/generated_code_execution/test_empty_object_defaults.py

@@ -0,0 +1,55 @@
+from end_to_end_tests.functional_tests.helpers import (
+    with_generated_client_fixture,
+    with_generated_code_imports,
+)
+
+
+@with_generated_client_fixture(
+    """
+components:
+  schemas:
+    Holder:
+      type: object
+      properties:
+        extras:
+          anyOf:
+            - type: object
+              additionalProperties: true
+            - type: "null"
+          default: {}
+"""
+)
+@with_generated_code_imports(".models.Holder", ".types.UNSET")
+class TestEmptyObjectDefaultOnAUnionMember:
+    """An empty object default on a union member leaves the enclosing schema in place. The property generates
+    with no default."""
+
+    def test_model_generates_without_the_default(self, Holder, UNSET):
+        assert Holder().extras is UNSET
+
+    def test_explicit_value_is_kept(self, Holder):
+        assert Holder.from_dict({"extras": {"a": 1}}).to_dict() == {"extras": {"a": 1}}
+
+
+@with_generated_client_fixture(
+    """
+components:
+  schemas:
+    Free:
+      type: object
+      additionalProperties: true
+    Holder:
+      type: object
+      properties:
+        extras:
+          allOf:
+            - $ref: "#/components/schemas/Free"
+          default: {}
+"""
+)
+@with_generated_code_imports(".models.Holder", ".types.UNSET")
+class TestEmptyObjectDefaultOnAReferencedModel:
+    """The same holds when the model carrying the default is referenced through an ``allOf``."""
+
+    def test_model_generates_without_the_default(self, Holder, UNSET):
+        assert Holder().extras is UNSET

openapi_python_client/parser/properties/model_property.py

@@ -128,8 +128,11 @@ def build(
 
     @classmethod
     def convert_value(cls, value: Any) -> Value | PropertyError | None:
+        if isinstance(value, dict) and not value:
+            # An empty object adds nothing to the default, and rejecting it drops the whole schema.
+            return None
         if value is not None:
-            return PropertyError(detail="ModelProperty cannot have a default value")  # pragma: no cover
+            return PropertyError(detail="ModelProperty cannot have a default value")
         return None
 
     def __attrs_post_init__(self) -> None:

tests/test_parser/test_properties/test_model_property.py

@@ -54,6 +54,15 @@ def test_get_base_type_string(self, model_property_factory):
         m = model_property_factory()
         assert m.get_base_type_string() == PythonCode("MyClass")
 
+    def test_convert_value_passes_none_through(self, model_property_factory):
+        assert model_property_factory().convert_value(None) is None
+
+    def test_convert_value_ignores_an_empty_object_default(self, model_property_factory):
+        assert model_property_factory().convert_value({}) is None
+
+    def test_convert_value_rejects_a_non_empty_default(self, model_property_factory):
+        assert isinstance(model_property_factory().convert_value({"a": 1}), PropertyError)
+
 
 class TestBuild:
     @pytest.mark.parametrize(