Support ClassVar for Pydantic v2 (#2920)

* Support ClassVar for Pydantic v2

* Implement ClassVar feature in code without changing template

* Remove commented-out debug code

* Fix unused Field import for ClassVar fields

* Add UNDEFINED check for ClassVar default and additional tests

* Fix import order

---------

Co-authored-by: Koudai Aono <koxudaxi@gmail.com>

Author: ubaumann

src/datamodel_code_generator/model/pydantic/base_model.py

@@ -19,7 +19,7 @@
     DataModelFieldBase,
 )
 from datamodel_code_generator.model._types import WrappedDefault
-from datamodel_code_generator.model.base import UNDEFINED
+from datamodel_code_generator.model.base import UNDEFINED, repr_set_sorted
 from datamodel_code_generator.model.pydantic.imports import (
     IMPORT_ANYURL,
     IMPORT_EXTRA,
@@ -91,6 +91,8 @@ def validator(self) -> str | None:
     @property
     def field(self) -> str | None:
         """For backwards compatibility."""
+        if self.is_class_var:
+            return None
         result = str(self)
         if (
             self.use_default_kwarg
@@ -250,17 +252,32 @@ def __str__(self) -> str:  # noqa: PLR0912
         elif self.required:
             field_arguments = ["...", *field_arguments]
         elif not default_factory:
-            from datamodel_code_generator.model.base import repr_set_sorted  # noqa: PLC0415
-
             default_repr = repr_set_sorted(self.default) if isinstance(self.default, set) else repr(self.default)
             field_arguments = [default_repr, *field_arguments]
 
+        if self.is_class_var:
+            if self.default is UNDEFINED:  # pragma: no cover
+                return ""
+            return repr_set_sorted(self.default) if isinstance(self.default, set) else repr(self.default)
+
         return f"Field({', '.join(field_arguments)})"
 
+    @property
+    def is_class_var(self) -> bool:
+        """Check if this field is a ClassVar."""
+        return self.extras.get("x-is-classvar") is True
+
+    @property
+    def type_hint(self) -> str:
+        """Get the type hint including ClassVar if applicable."""
+        if self.is_class_var:
+            return f"ClassVar[{super().type_hint}]"
+        return super().type_hint
+
     @property
     def annotated(self) -> str | None:
         """Get the Annotated type hint if use_annotated is enabled."""
-        if not self.use_annotated or not str(self):
+        if not self.use_annotated or not str(self) or self.is_class_var:
             return None
         return f"Annotated[{self.type_hint}, {self!s}]"
 

src/datamodel_code_generator/model/pydantic_v2/base_model.py

@@ -14,6 +14,7 @@
 
 from datamodel_code_generator.imports import IMPORT_ANY, Import
 from datamodel_code_generator.model.base import ALL_MODEL, UNDEFINED, BaseClassDataType, DataModelFieldBase
+from datamodel_code_generator.model.imports import IMPORT_CLASSVAR
 from datamodel_code_generator.model.pydantic.base_model import (
     BaseModelBase,
 )
@@ -192,6 +193,8 @@ def imports(self) -> tuple[Import, ...]:
         """Get all required imports including AliasChoices and Field for discriminator."""
         base_imports = super().imports
         extra_imports: list[Import] = []
+        if self.is_class_var:
+            extra_imports.append(IMPORT_CLASSVAR)
         if self.validation_aliases:
             from datamodel_code_generator.model.pydantic_v2.imports import IMPORT_ALIAS_CHOICES  # noqa: PLC0415
 

tests/data/expected/main/jsonschema/has_classvar_extra.py

@@ -0,0 +1,13 @@
+# generated by datamodel-codegen:
+#   filename:  has_classvar_extra.json
+#   timestamp: 2019-07-26T00:00:00+00:00
+
+from __future__ import annotations
+
+from typing import ClassVar
+
+from pydantic import BaseModel
+
+
+class Model(BaseModel):
+    namespace: ClassVar[str | None] = 'test'

tests/data/expected/main/jsonschema/has_classvar_extra_annotated.py

@@ -0,0 +1,13 @@
+# generated by datamodel-codegen:
+#   filename:  has_classvar_extra_annotated.json
+#   timestamp: 2019-07-26T00:00:00+00:00
+
+from __future__ import annotations
+
+from typing import ClassVar
+
+from pydantic import BaseModel
+
+
+class Model(BaseModel):
+    namespace: ClassVar[str | None] = 'test'

tests/data/expected/main/jsonschema/has_classvar_extra_set.py

@@ -0,0 +1,13 @@
+# generated by datamodel-codegen:
+#   filename:  has_classvar_extra_set.json
+#   timestamp: 2019-07-26T00:00:00+00:00
+
+from __future__ import annotations
+
+from typing import ClassVar
+
+from pydantic import BaseModel
+
+
+class Model(BaseModel):
+    tags: ClassVar[set[str] | None] = {'a', 'b'}

tests/data/jsonschema/has_classvar_extra.json

@@ -0,0 +1,10 @@
+{
+    "type": "object",
+    "properties": {
+        "namespace": {
+            "type": "string",
+            "x-is-classvar": true,
+            "default": "test"
+        }
+    }
+}

tests/data/jsonschema/has_classvar_extra_annotated.json

@@ -0,0 +1,11 @@
+{
+    "type": "object",
+    "properties": {
+        "namespace": {
+            "type": "string",
+            "x-is-classvar": true,
+            "default": "test",
+            "minLength": 1
+        }
+    }
+}

tests/data/jsonschema/has_classvar_extra_set.json

@@ -0,0 +1,14 @@
+{
+    "type": "object",
+    "properties": {
+        "tags": {
+            "type": "array",
+            "x-is-classvar": true,
+            "default": ["a", "b"],
+            "uniqueItems": true,
+            "items": {
+                "type": "string"
+            }
+        }
+    }
+}

tests/main/jsonschema/test_main_jsonschema.py

@@ -8001,6 +8001,52 @@ def test_validators_requires_pydantic_v2(output_file: Path, tmp_path: Path, caps
     )
 
 
+def test_jsonschema_classvar_extra_pydantic_v2(output_file: Path) -> None:
+    """Test default value handling."""
+    run_main_and_assert(
+        input_path=JSON_SCHEMA_DATA_PATH / "has_classvar_extra.json",
+        output_path=output_file,
+        input_file_type="jsonschema",
+        assert_func=assert_file_content,
+        expected_file="has_classvar_extra.py",
+        extra_args=["--output-model-type", "pydantic_v2.BaseModel", "--field-include-all-keys"],
+    )
+
+
+def test_jsonschema_classvar_extra_set_pydantic_v2(output_file: Path) -> None:
+    """Test ClassVar with set default value."""
+    run_main_and_assert(
+        input_path=JSON_SCHEMA_DATA_PATH / "has_classvar_extra_set.json",
+        output_path=output_file,
+        input_file_type="jsonschema",
+        assert_func=assert_file_content,
+        expected_file="has_classvar_extra_set.py",
+        extra_args=[
+            "--output-model-type",
+            "pydantic_v2.BaseModel",
+            "--field-include-all-keys",
+            "--use-unique-items-as-set",
+        ],
+    )
+
+
+def test_jsonschema_classvar_extra_annotated_pydantic_v2(output_file: Path) -> None:
+    """Test ClassVar with use_annotated option."""
+    run_main_and_assert(
+        input_path=JSON_SCHEMA_DATA_PATH / "has_classvar_extra_annotated.json",
+        output_path=output_file,
+        input_file_type="jsonschema",
+        assert_func=assert_file_content,
+        expected_file="has_classvar_extra_annotated.py",
+        extra_args=[
+            "--output-model-type",
+            "pydantic_v2.BaseModel",
+            "--field-include-all-keys",
+            "--use-annotated",
+        ],
+    )
+
+
 @PYDANTIC_V2_SKIP
 def test_unique_items_enum_set(output_file: Path) -> None:
     """Test set with enum items does not add __hash__ to enum (already hashable)."""