Fix --use-default making required fields nullable

--use-default previously skipped setting field.required=True, which made
fields non-required and therefore nullable (e.g. str | None = 'foo').

Now fields stay required=True with a new use_default_with_required flag
that allows defaults to render without changing the type. Produces
`status: str = 'foo'` instead of `status: str | None = 'foo'`.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

Author: butvinm

src/datamodel_code_generator/model/base.py

@@ -173,6 +173,7 @@ class DataModelFieldBase(_BaseModel):
     use_frozen_field: bool = False
     use_serialization_alias: bool = False
     use_default_factory_for_optional_nested_models: bool = False
+    use_default_with_required: bool = False
 
     if not TYPE_CHECKING:  # pragma: no branch
 

src/datamodel_code_generator/model/dataclass.py

@@ -172,7 +172,7 @@ def __str__(self) -> str:
         if self.default != UNDEFINED and self.default is not None:
             data["default"] = self.default
 
-        if self.required:
+        if self.required and not self.use_default_with_required:
             data = {
                 k: v
                 for k, v in data.items()

src/datamodel_code_generator/model/msgspec.py

@@ -311,7 +311,7 @@ def __str__(self) -> str:  # noqa: PLR0912
         elif self._not_required and "default_factory" not in data:
             data["default"] = None if self.nullable else UNSET
 
-        if self.required:
+        if self.required and not self.use_default_with_required:
             data = {
                 k: v
                 for k, v in data.items()

src/datamodel_code_generator/model/pydantic_base.py

@@ -211,7 +211,12 @@ def __str__(self) -> str:  # noqa: PLR0912
 
         if self.use_annotated:
             field_arguments = self._process_annotated_field_arguments(field_arguments)
-        elif self.required and not default_factory and not self.extras.get("validate_default"):
+        elif (
+            self.required
+            and not self.use_default_with_required
+            and not default_factory
+            and not self.extras.get("validate_default")
+        ):
             field_arguments = ["...", *field_arguments]
         elif not default_factory:
             default_repr = repr_set_sorted(self.default) if isinstance(self.default, set) else repr(self.default)

src/datamodel_code_generator/model/template/dataclass.jinja2

@@ -30,7 +30,7 @@ class {{ class_name }}:
     {{ field.name }}: {{ field.type_hint }} = {{ field.field }}
     {%- else %}
     {{ field.name }}: {{ field.type_hint }}
-    {%- if not (field.required or (field.represented_default == 'None' and field.strip_default_none))
+    {%- if not ((field.required and not field.use_default_with_required) or (field.represented_default == 'None' and field.strip_default_none))
             %} = {{ field.represented_default }}
     {%- endif -%}
     {%- endif %}

src/datamodel_code_generator/model/template/msgspec.jinja2

@@ -29,7 +29,7 @@ class {{ class_name }}:
     {%- else %}
     {{ field.name }}: {{ field.type_hint }}
     {%- endif %}
-    {%- if not field.field and (not field.required or field.data_type.is_optional or field.nullable)
+    {%- if not field.field and (not field.required or field.use_default_with_required or field.data_type.is_optional or field.nullable)
             %} = {{ field.represented_default }}
     {%- endif -%}
     {%- endif %}

src/datamodel_code_generator/model/template/pydantic_v2/BaseModel.jinja2

@@ -27,7 +27,7 @@ class {{ class_name }}({{ base_class }}):{% if comment is defined %}  # {{ comme
     {%- else %}
     {{ field.name }}: {{ field.type_hint }}
     {%- endif %}
-    {%- if not field.has_default_factory_in_field and not field.required and (field.represented_default != 'None' or not field.strip_default_none or field.data_type.is_optional)
+    {%- if not field.has_default_factory_in_field and (not field.required or field.use_default_with_required) and (field.represented_default != 'None' or not field.strip_default_none or field.data_type.is_optional)
             %} = {{ field.represented_default }}
     {%- endif -%}
     {%- endif %}

src/datamodel_code_generator/model/template/pydantic_v2/dataclass.jinja2

@@ -41,7 +41,7 @@ class {{ class_name }}:
     {%- else %}
     {{ field.name }}: {{ field.type_hint }}
     {%- endif %}
-    {%- if not field.has_default_factory_in_field and not field.required and (field.represented_default != 'None' or not field.strip_default_none or field.data_type.is_optional)
+    {%- if not field.has_default_factory_in_field and (not field.required or field.use_default_with_required) and (field.represented_default != 'None' or not field.strip_default_none or field.data_type.is_optional)
             %} = {{ field.represented_default }}
     {%- endif -%}
     {%- endif %}

src/datamodel_code_generator/parser/graphql.py

@@ -372,8 +372,7 @@ def parse_field(
             class_name=class_name,
         )
 
-        if self.apply_default_values_for_required_fields and effective_has_default:
-            required = False
+        use_default_with_required = required and self.apply_default_values_for_required_fields and effective_has_default
 
         extras = {} if self.default_field_extras is None else self.default_field_extras.copy()
 
@@ -387,7 +386,7 @@ def parse_field(
             validation_aliases = alias
         else:
             single_alias = alias
-        return self.data_model_field_type(
+        new_field = self.data_model_field_type(
             name=field_name,
             default=effective_default,
             data_type=final_data_type,
@@ -406,6 +405,8 @@ def parse_field(
             has_default=effective_has_default,
             use_serialization_alias=self.use_serialization_alias,
         )
+        new_field.use_default_with_required = use_default_with_required
+        return new_field
 
     def parse_object_like(
         self,

src/datamodel_code_generator/parser/jsonschema.py

@@ -2477,22 +2477,22 @@ def _parse_object_common_part(  # noqa: PLR0912, PLR0913, PLR0915
                 return self.data_type(reference=base_classes[0])
         if required:
             for field in fields:
-                if self.force_optional_for_required_fields or (  # pragma: no cover
-                    self.apply_default_values_for_required_fields and field.has_default
-                ):
+                if self.force_optional_for_required_fields:  # pragma: no cover
                     continue  # pragma: no cover
                 if (field.original_name or field.name) in required:
                     field.required = True
+                    if self.apply_default_values_for_required_fields and field.has_default:
+                        field.use_default_with_required = True
         if obj.required:
             field_name_to_field = {f.original_name or f.name: f for f in fields}
             for required_ in obj.required:
                 if required_ in field_name_to_field:
                     field = field_name_to_field[required_]
-                    if self.force_optional_for_required_fields or (
-                        self.apply_default_values_for_required_fields and field.has_default
-                    ):
+                    if self.force_optional_for_required_fields:
                         continue
                     field.required = True
+                    if self.apply_default_values_for_required_fields and field.has_default:
+                        field.use_default_with_required = True
                 else:
                     fields.append(
                         self.data_model_field_type(required=True, original_name=required_, data_type=DataType())
@@ -2828,24 +2828,23 @@ def parse_object_fields(
                 class_name=class_name,
             )
 
-            if self.force_optional_for_required_fields or (
-                self.apply_default_values_for_required_fields and effective_has_default
-            ):
+            if self.force_optional_for_required_fields:
                 required: bool = False
             else:
                 required = original_field_name in requires
-            fields.append(
-                self.get_object_field(
-                    field_name=field_name,
-                    field=field,
-                    required=required,
-                    field_type=field_type,
-                    alias=alias,
-                    original_field_name=original_field_name,
-                    effective_default=effective_default,
-                    effective_has_default=effective_has_default,
-                )
+            new_field = self.get_object_field(
+                field_name=field_name,
+                field=field,
+                required=required,
+                field_type=field_type,
+                alias=alias,
+                original_field_name=original_field_name,
+                effective_default=effective_default,
+                effective_has_default=effective_has_default,
             )
+            if required and self.apply_default_values_for_required_fields and effective_has_default:
+                new_field.use_default_with_required = True
+            fields.append(new_field)
         return fields
 
     def parse_object(