Fix empty list default for GraphQL list fields

src/datamodel_code_generator/model/pydantic/base_model.py

@@ -126,9 +126,23 @@ 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 the case where self.data_type.is_list is True directly (e.g., GraphQL)
+        if self.data_type.is_list and len(self.data_type.data_types) == 1:
+            data_type_child = self.data_type.data_types[0]
+            if (
+                data_type_child.reference
+                and isinstance(data_type_child.reference.source, BaseModelBase)
+                and isinstance(self.default, list)
+            ):
+                if not self.default:
+                    return STANDARD_LIST
+                return (  # pragma: no cover
+                    f"lambda :[{data_type_child.alias or data_type_child.reference.source.class_name}."
+                    f"{self._PARSE_METHOD}(v) for v in {self.default!r}]"
+                )
         for data_type in self.data_type.data_types or (self.data_type,):
             # TODO: Check nested data_types
             if data_type.is_dict:

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

@@ -0,0 +1,32 @@
+# 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`.
+"""
+
+
+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 TagInput(BaseModel):
+    name: String
+    value: String
+    typename__: Literal['TagInput'] | None = Field('TagInput', alias='__typename')
+
+
+class ModelInput(BaseModel):
+    tags: list[TagInput] | None = Field(default_factory=list)
+    typename__: Literal['ModelInput'] | None = Field('ModelInput', alias='__typename')

tests/data/graphql/empty_list_default.graphql

@@ -0,0 +1,8 @@
+input TagInput {
+  name: String!
+  value: String!
+}
+
+input ModelInput {
+  tags: [TagInput!] = []
+}

tests/main/graphql/test_main_graphql.py

@@ -890,3 +890,22 @@ def test_main_graphql_no_typename(output_file: Path) -> None:
         expected_file="no_typename.py",
         extra_args=["--graphql-no-typename"],
     )
+
+
+def test_main_graphql_empty_list_default(output_file: Path) -> None:
+    """Test that empty list default generates default_factory=list, not model_validate([]).
+
+    When a GraphQL field has a list type with an empty list default (e.g., tags: [TagInput!] = []),
+    the generator should produce `Field(default_factory=list)` instead of
+    `Field(default_factory=lambda: TagInput.model_validate([]))`.
+
+    This test verifies fix for issue #2947.
+    """
+    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="empty_list_default.py",
+        extra_args=["--output-model-type", "pydantic_v2.BaseModel"],
+    )