Fix empty list default for GraphQL list fields

When GraphQL input fields have empty list defaults (e.g., `requiredList: [String!]! = []`),
the generated code was incorrectly producing:
- `Field(...)` for required lists (ignoring the default)
- `Field([])` for nullable lists (using mutable default)

This fix ensures all list fields with empty defaults generate `Field(default_factory=list)`,
which is the correct way to handle mutable defaults in Pydantic.

Changes:
- Add early check in `_get_default_as_pydantic_model()` for any list type with empty default
- Update `__str__()` to not skip default handling when `required=True` but `has_default=True`
- Update field argument generation to not add `...` when there's already a `default_factory`

Fixes issue where GraphQL schemas with default empty arrays would generate incorrect Pydantic code.

Co-Authored-By: Claude Opus 4.5 <[email protected]>

Author: koxudaxi

src/datamodel_code_generator/model/pydantic/base_model.py

@@ -126,9 +126,12 @@ def _get_strict_field_constraint_value(self, constraint: str, value: Any) -> Any
             return value
         return int(value)
 
-    def _get_default_as_pydantic_model(self) -> str | None:
+    def _get_default_as_pydantic_model(self) -> str | None:  # noqa: PLR0911, PLR0912
         if isinstance(self.default, WrappedDefault):
             return f"lambda :{self.default!r}"
+        # Handle empty list defaults for any list type (including primitive types like String)
+        if self.data_type.is_list and isinstance(self.default, list) and not self.default:
+            return STANDARD_LIST
         for data_type in self.data_type.data_types or (self.data_type,):
             # TODO: Check nested data_types
             if data_type.is_dict:
@@ -220,7 +223,7 @@ def __str__(self) -> str:  # noqa: PLR0912
             elif isinstance(discriminator, dict):  # pragma: no cover
                 data["discriminator"] = discriminator["propertyName"]
 
-        if self.required:
+        if self.required and not self.has_default:
             default_factory = None
         elif self.default is not UNDEFINED and self.default is not None and "default_factory" not in data:
             default_factory = self._get_default_as_pydantic_model()
@@ -249,7 +252,7 @@ def __str__(self) -> str:  # noqa: PLR0912
 
         if self.use_annotated:
             field_arguments = self._process_annotated_field_arguments(field_arguments)
-        elif self.required:
+        elif self.required and not default_factory:
             field_arguments = ["...", *field_arguments]
         elif not default_factory:
             default_repr = repr_set_sorted(self.default) if isinstance(self.default, set) else repr(self.default)

tests/data/expected/main/graphql/empty_list_default.py

@@ -0,0 +1,44 @@
+# generated by datamodel-codegen:
+#   filename:  empty_list_default.graphql
+#   timestamp: 2019-07-26T00:00:00+00:00
+
+from __future__ import annotations
+
+from typing import Literal, TypeAlias
+
+from pydantic import BaseModel, Field
+
+Boolean: TypeAlias = bool
+"""
+The `Boolean` scalar type represents `true` or `false`.
+"""
+
+
+ID: TypeAlias = str
+"""
+The `ID` scalar type represents a unique identifier, often used to refetch an object or as key for a cache. The ID type appears in a JSON response as a String; however, it is not intended to be human-readable. When expected as an input type, any string (such as `"4"`) or integer (such as `4`) input value will be accepted as an ID.
+"""
+
+
+String: TypeAlias = str
+"""
+The `String` scalar type represents textual data, represented as UTF-8 character sequences. The String type is most often used by GraphQL to represent free-form human-readable text.
+"""
+
+
+class ItemInput(BaseModel):
+    id: ID
+    typename__: Literal['ItemInput'] | None = Field('ItemInput', alias='__typename')
+
+
+class TestInput(BaseModel):
+    nullableList: list[String] | None = Field(
+        default_factory=list, description='Nullable list with empty default'
+    )
+    requiredItems: list[ItemInput] = Field(
+        default_factory=list, description='Required list of items with empty default'
+    )
+    requiredList: list[String] = Field(
+        default_factory=list, description='Required list with empty default'
+    )
+    typename__: Literal['TestInput'] | None = Field('TestInput', alias='__typename')

tests/data/expected/main/graphql/pydantic_v2_empty_list_default.py

@@ -0,0 +1,45 @@
+# generated by datamodel-codegen:
+#   filename:  empty_list_default.graphql
+#   timestamp: 2019-07-26T00:00:00+00:00
+
+from __future__ import annotations
+
+from typing import Literal
+
+from pydantic import BaseModel, Field
+from typing_extensions import TypeAliasType
+
+Boolean = TypeAliasType("Boolean", bool)
+"""
+The `Boolean` scalar type represents `true` or `false`.
+"""
+
+
+ID = TypeAliasType("ID", str)
+"""
+The `ID` scalar type represents a unique identifier, often used to refetch an object or as key for a cache. The ID type appears in a JSON response as a String; however, it is not intended to be human-readable. When expected as an input type, any string (such as `"4"`) or integer (such as `4`) input value will be accepted as an ID.
+"""
+
+
+String = TypeAliasType("String", str)
+"""
+The `String` scalar type represents textual data, represented as UTF-8 character sequences. The String type is most often used by GraphQL to represent free-form human-readable text.
+"""
+
+
+class ItemInput(BaseModel):
+    id: ID
+    typename__: Literal['ItemInput'] | None = Field('ItemInput', alias='__typename')
+
+
+class TestInput(BaseModel):
+    nullableList: list[String] | None = Field(
+        default_factory=list, description='Nullable list with empty default'
+    )
+    requiredItems: list[ItemInput] = Field(
+        default_factory=list, description='Required list of items with empty default'
+    )
+    requiredList: list[String] = Field(
+        default_factory=list, description='Required list with empty default'
+    )
+    typename__: Literal['TestInput'] | None = Field('TestInput', alias='__typename')

tests/data/graphql/empty_list_default.graphql

@@ -0,0 +1,18 @@
+type Query {
+    test: String
+}
+
+input ItemInput {
+    id: ID!
+}
+
+input TestInput {
+    "Required list with empty default"
+    requiredList: [String!]! = []
+
+    "Nullable list with empty default"
+    nullableList: [String!] = []
+
+    "Required list of items with empty default"
+    requiredItems: [ItemInput!]! = []
+}

tests/main/graphql/test_main_graphql.py

@@ -890,3 +890,32 @@ def test_main_graphql_no_typename(output_file: Path) -> None:
         expected_file="no_typename.py",
         extra_args=["--graphql-no-typename"],
     )
+
+
+@pytest.mark.parametrize(
+    ("output_model", "expected_output"),
+    [
+        (
+            "pydantic.BaseModel",
+            "empty_list_default.py",
+        ),
+        (
+            "pydantic_v2.BaseModel",
+            "pydantic_v2_empty_list_default.py",
+        ),
+    ],
+)
+def test_main_graphql_empty_list_default(output_model: str, expected_output: str, output_file: Path) -> None:
+    """Test that empty list defaults in GraphQL input types generate default_factory=list.
+
+    This test verifies that fields like `requiredList: [String!]! = []` correctly
+    generate `Field(default_factory=list)` instead of `Field(...)` or `Field([])`.
+    """
+    run_main_and_assert(
+        input_path=GRAPHQL_DATA_PATH / "empty_list_default.graphql",
+        output_path=output_file,
+        input_file_type="graphql",
+        assert_func=assert_file_content,
+        expected_file=expected_output,
+        extra_args=["--output-model-type", output_model],
+    )