Fix IndexError when using --reuse-scope=tree with single file output

src/datamodel_code_generator/parser/base.py

@@ -1752,7 +1752,7 @@ def __validate_shared_module_name(
     ) -> None:
         """Validate that the shared module name doesn't conflict with existing modules."""
         shared_module = self.shared_module_name
-        existing_module_names = {module[0] for module, _ in module_models}
+        existing_module_names = {module[0] for module, _ in module_models if module}
         if shared_module in existing_module_names:
             msg = (
                 f"Schema file or directory '{shared_module}' conflicts with the shared module name. "

tests/data/openapi/issue_2953.yaml

@@ -0,0 +1,65 @@
+openapi: "3.0.0"
+info:
+    title: Issue 2953 Test
+    version: "1.0.0"
+components:
+    schemas:
+        Collection-Wrapper:
+            description: Generic response wrapper containing a collection of items and pagination metadata.
+            type: object
+            required:
+                - data
+                - pagination
+            properties:
+                data:
+                    description: Array of items in the collection.
+                    type: array
+                    items:
+                        type: object
+                pagination:
+                    description: Pagination metadata for the collection.
+                    $ref: "#/components/schemas/Metadata"
+        DataType:
+            type: object
+            required:
+                - id
+            properties:
+                property_1:
+                    type: string
+                    format: uuid
+                    readOnly: true
+                    description: Unique identifier for the installation.
+                property_2:
+                    type: string
+                    description: Description of the installation.
+                property_3:
+                    type: string
+                    format: date-time
+                    readOnly: true
+                    description: Timestamp when the installation was created.
+
+        PaginatedDataTypeList:
+            allOf:
+                - properties:
+                      data:
+                          type: array
+                          items:
+                              $ref: "#/components/schemas/DataType"
+                - $ref: "#/components/schemas/Collection-Wrapper"
+        Metadata:
+            type: object
+            required:
+                - limit
+                - page
+            properties:
+                limit:
+                    type: integer
+                    minimum: 1
+                    maximum: 100
+                    description: Number of data types returned in this response
+                    example: 20
+                page:
+                    type: integer
+                    minimum: 1
+                    description: The page number to retrieve
+                    example: 2

tests/main/openapi/test_main_openapi.py

@@ -4939,3 +4939,28 @@ def test_main_openapi_deprecated_field(output_file: Path) -> None:
         expected_file="deprecated_field.py",
         extra_args=["--output-model-type", "pydantic_v2.BaseModel"],
     )
+
+
+@SKIP_PYDANTIC_V1
+def test_main_openapi_reuse_scope_tree_single_file_error(capsys: pytest.CaptureFixture[str], output_file: Path) -> None:
+    """Test --reuse-scope=tree with single file output raises proper error (#2953).
+
+    When using reuse-scope=tree with single file output and duplicate models,
+    it should raise a proper error about needing a directory output instead of
+    crashing with IndexError.
+    """
+    run_main_and_assert(
+        input_path=OPEN_API_DATA_PATH / "issue_2953.yaml",
+        output_path=output_file,
+        input_file_type="openapi",
+        expected_exit=Exit.ERROR,
+        capsys=capsys,
+        expected_stderr_contains="Modular references require an output directory, not a file",
+        extra_args=[
+            "--output-model-type",
+            "pydantic_v2.BaseModel",
+            "--reuse-model",
+            "--reuse-scope",
+            "tree",
+        ],
+    )