Fix invalid Union syntax when using --collapse-root-models (#2594)

* Add support for collapsing root models with empty unions to generate Any type

* Refactor import handling to correctly manage optional types in type hints

* Refactor type imports for improved clarity and organization in type hints

Author: koxudaxi

src/datamodel_code_generator/model/base.py

@@ -28,6 +28,7 @@
 from datamodel_code_generator.types import (
     ANY,
     NONE,
+    OPTIONAL_PREFIX,
     UNION_PREFIX,
     DataType,
     Nullable,
@@ -188,16 +189,16 @@ def imports(self) -> tuple[Import, ...]:
         """Get all imports required for this field's type hint."""
         type_hint = self.type_hint
         has_union = not self.data_type.use_union_operator and UNION_PREFIX in type_hint
+        has_optional = OPTIONAL_PREFIX in type_hint
         imports: list[tuple[Import] | Iterator[Import]] = [
-            iter(i for i in self.data_type.all_imports if not (not has_union and i == IMPORT_UNION))
+            iter(
+                i
+                for i in self.data_type.all_imports
+                if not ((not has_union and i == IMPORT_UNION) or (not has_optional and i == IMPORT_OPTIONAL))
+            )
         ]
 
-        if self.fall_back_to_nullable:
-            if (
-                self.nullable or (self.nullable is None and not self.required)
-            ) and not self.data_type.use_union_operator:
-                imports.append((IMPORT_OPTIONAL,))
-        elif self.nullable and not self.data_type.use_union_operator:  # pragma: no cover
+        if has_optional:
             imports.append((IMPORT_OPTIONAL,))
         if self.use_annotated and self.annotated:
             imports.append((IMPORT_ANNOTATED,))

src/datamodel_code_generator/parser/base.py

@@ -38,6 +38,7 @@
 from datamodel_code_generator.imports import (
     IMPORT_ANNOTATIONS,
     IMPORT_LITERAL,
+    IMPORT_OPTIONAL,
     Import,
     Imports,
 )
@@ -1284,7 +1285,7 @@ def __collapse_root_models(  # noqa: PLR0912
                     original_field = get_most_of_parent(data_type, DataModelFieldBase)
                     if original_field:  # pragma: no cover
                         # TODO: Improve detection of reference type
-                        imports.append(original_field.imports)
+                        imports.append([i for i in original_field.imports if i != IMPORT_OPTIONAL])
 
                     data_type.remove_reference()
 

src/datamodel_code_generator/types.py

@@ -38,6 +38,7 @@
     IMPORT_ABC_MAPPING,
     IMPORT_ABC_SEQUENCE,
     IMPORT_ABC_SET,
+    IMPORT_ANY,
     IMPORT_DICT,
     IMPORT_FROZEN_SET,
     IMPORT_LIST,
@@ -52,16 +53,6 @@
 from datamodel_code_generator.reference import Reference, _BaseModel
 from datamodel_code_generator.util import PYDANTIC_V2, ConfigDict
 
-if TYPE_CHECKING:
-    import builtins
-    from collections.abc import Iterable, Iterator, Sequence
-
-    from datamodel_code_generator.model.base import DataModelFieldBase
-
-if PYDANTIC_V2:
-    from pydantic import GetCoreSchemaHandler
-    from pydantic_core import core_schema
-
 T = TypeVar("T")
 
 OPTIONAL = "Optional"
@@ -90,6 +81,16 @@
 NOT_REQUIRED = "NotRequired"
 NOT_REQUIRED_PREFIX = f"{NOT_REQUIRED}["
 
+if TYPE_CHECKING:
+    import builtins
+    from collections.abc import Iterable, Iterator, Sequence
+
+    from datamodel_code_generator.model.base import DataModelFieldBase
+
+if PYDANTIC_V2:
+    from pydantic import GetCoreSchemaHandler
+    from pydantic_core import core_schema
+
 
 class StrictTypes(Enum):
     """Strict type options for generated models."""
@@ -486,7 +487,7 @@ def type_hint(self) -> str:  # noqa: PLR0912, PLR0915
                 data_types: list[str] = []
                 for data_type in self.data_types:
                     data_type_type = data_type.type_hint
-                    if data_type_type in data_types:  # pragma: no cover
+                    if not data_type_type or data_type_type in data_types:
                         continue
 
                     if data_type_type == NONE:
@@ -501,7 +502,10 @@ def type_hint(self) -> str:  # noqa: PLR0912, PLR0915
                         self.is_optional = True
 
                     data_types.append(non_optional_data_type_type)
-                if len(data_types) == 1:
+                if not data_types:
+                    type_ = ANY
+                    self.import_ = self.import_ or IMPORT_ANY
+                elif len(data_types) == 1:
                     type_ = data_types[0]
                 elif self.use_union_operator:
                     type_ = UNION_OPERATOR_DELIMITER.join(data_types)

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

@@ -0,0 +1,13 @@
+# generated by datamodel-codegen:
+#   filename:  collapse_root_models_empty_union.json
+#   timestamp: 2019-07-26T00:00:00+00:00
+
+from __future__ import annotations
+
+from typing import Any
+
+from pydantic import BaseModel
+
+
+class Model(BaseModel):
+    field: Any = None

tests/data/jsonschema/collapse_root_models_empty_union.json

@@ -0,0 +1,20 @@
+{
+  "$schema": "https://json-schema.org/draft/2020-12/schema",
+  "type": "object",
+  "properties": {
+    "field": {
+      "anyOf": [
+        {"$ref": "#/$defs/NullType1"},
+        {"$ref": "#/$defs/NullType2"}
+      ]
+    }
+  },
+  "$defs": {
+    "NullType1": {
+      "type": "null"
+    },
+    "NullType2": {
+      "type": "null"
+    }
+  }
+}

tests/main/jsonschema/test_main_jsonschema.py

@@ -2975,3 +2975,18 @@ def test_main_jsonschema_empty_items_array(output_file: Path) -> None:
         input_file_type="jsonschema",
         assert_func=assert_file_content,
     )
+
+
+def test_main_jsonschema_collapse_root_models_empty_union(output_file: Path) -> None:
+    """Test that collapse-root-models with empty union fallback generates Any instead of invalid Union syntax.
+
+    This test covers the fix for issue #2161 where --collapse-root-models could generate
+    invalid Python syntax like Union[, str] when all union members are filtered out.
+    """
+    run_main_and_assert(
+        input_path=JSON_SCHEMA_DATA_PATH / "collapse_root_models_empty_union.json",
+        output_path=output_file,
+        input_file_type="jsonschema",
+        assert_func=assert_file_content,
+        extra_args=["--collapse-root-models"],
+    )