Fix required field default rendering and --use-default nullable types (#3054)

* Fix required fields inconsistently rendering defaults (#3048)

__set_validate_default_on_fields set validate_default=True on fields
with model references without checking field.required, causing required
model-ref fields to render defaults while required scalar fields didn't.

Skip required fields (unless use_default_with_required) so all required
fields consistently have no defaults rendered.

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

* 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>

* Add tests for allOf --use-default and --force-optional coverage

Cover the previously uncovered code paths:
- allOf with $ref + sub-schema required fields + --use-default
- allOf with outer required fields + --force-optional

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

* Pass use_default_with_required via constructor instead of patching

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

* Propagate use_default_with_required to missed field-assignment sites

Three regressions where the new use_default_with_required flag wasn't
propagated to all the rendering sites that decide whether a field has
an assignment. Each produces broken output on this branch but the
correct output on baseline (92bdc27).

1. pydantic_base.py:205 — early-return for required+nullable fields
   with no other Field args returned Field(...) without checking
   use_default_with_required, dropping the user's default.
   Repro: --use-default --strict-nullable, required nullable str with
   default. Got: x: str | None = Field(...). Want: x: str | None = 'hello'.

2. dataclass.py:has_field_assignment — sort-key helper didn't account
   for use_default_with_required, so a required field with default null
   (use_default_with_required=True, field.field empty) was sorted as
   no-assignment alongside truly required fields. Schema order was then
   preserved, placing the defaulted field before the non-defaulted one.
   Repro: --use-default --output-model-type dataclasses.dataclass, required
   nullable a (default null) before required b. Got: a before b
   (TypeError on import). Want: b before a. Same fix covers
   pydantic_v2.dataclass since it shares the helper.

3. msgspec.py:_has_field_assignment — same class of bug. Helper checked
   neither bool(field.field) nor use_default_with_required, breaking
   ordering for both string-defaulted and null-defaulted required fields.
   Repro: --use-default --output-model-type msgspec.Struct on the existing
   allof_required_use_default.json fixture. Got: name='unnamed' before tag
   (TypeError on import). Want: tag before name='unnamed'.

Adds three regression tests in tests/main/jsonschema/test_main_jsonschema.py
covering the three repros above.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

* Fix required default handling

---------

Co-authored-by: Claude Opus 4.6 (1M context) <noreply@anthropic.com>
Co-authored-by: Koudai Aono <koxudaxi@gmail.com>

Author: 3 people

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

@@ -39,7 +39,8 @@
 def has_field_assignment(field: DataModelFieldBase) -> bool:
     """Check if a dataclass field has a default value or field() assignment."""
     return bool(field.field) or not (
-        field.required or (field.represented_default == "None" and field.strip_default_none)
+        (field.required and not field.use_default_with_required)
+        or (field.represented_default == "None" and field.strip_default_none)
     )
 
 
@@ -172,7 +173,7 @@ 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

@@ -70,7 +70,11 @@ def __str__(self) -> str:
 
 
 def _has_field_assignment(field: DataModelFieldBase) -> bool:
-    return not (field.required or (field.represented_default == "None" and field.strip_default_none))
+    return (
+        bool(field.field)
+        or field.use_default_with_required
+        or not (field.required or (field.represented_default == "None" and field.strip_default_none))
+    )
 
 
 DataModelFieldBaseT = TypeVar("DataModelFieldBaseT", bound=DataModelFieldBase)
@@ -311,7 +315,7 @@ 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

@@ -202,7 +202,7 @@ def __str__(self) -> str:  # noqa: PLR0912
         field_arguments = sorted(f"{k}={v!r}" for k, v in data.items() if v is not None)
 
         if not field_arguments and not default_factory:
-            if self.nullable and self.required:
+            if self.nullable and self.required and not self.use_default_with_required:
                 return "Field(...)"  # Field() is for mypy
             return ""
 
@@ -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/base.py

@@ -2180,6 +2180,8 @@ def __set_validate_default_on_fields(  # noqa: PLR6301
             if isinstance(model, Enum):
                 continue
             for model_field in model.fields:
+                if model_field.required and not model_field.use_default_with_required:
+                    continue
                 if model_field.default is None or model_field.default is UNDEFINED:
                     continue
                 if isinstance(model_field.default, Member):
@@ -2227,6 +2229,8 @@ def __override_required_field(
                 copied_original_field.data_type = data_type
                 copied_original_field.parent = model
                 copied_original_field.required = True
+                if self.apply_default_values_for_required_fields and copied_original_field.has_default:
+                    copied_original_field.use_default_with_required = True
                 model.fields.insert(index, copied_original_field)
                 model.fields.remove(model_field)
 

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()
 
@@ -405,6 +404,7 @@ def parse_field(
             original_name=field_name,
             has_default=effective_has_default,
             use_serialization_alias=self.use_serialization_alias,
+            use_default_with_required=use_default_with_required,
         )
 
     def parse_object_like(