Add warning when components/schemas is empty but paths exist (#2622)

* Add warning for empty schemas when paths are defined

* [pre-commit.ci] auto fixes from pre-commit.com hooks

for more information, see https://pre-commit.ci

* Simplify warning handling for empty schemas in test cases

---------

Co-authored-by: pre-commit-ci[bot] <66853113+pre-commit-ci[bot]@users.noreply.github.com>

Author: koxudaxi

pyproject.toml

@@ -208,6 +208,7 @@ filterwarnings = [
   "ignore:^.*black doesn't support `experimental-string-processing` option for wrapping string literal in .*",
   "ignore:^.*jsonschema.exceptions.RefResolutionError is deprecated as of version 4.18.0. If you wish to catch potential reference resolution errors, directly catch referencing.exceptions.Unresolvable..*",
   "ignore:^.*`experimental string processing` has been included in `preview` and deprecated. Use `preview` instead..*",
+  "ignore:^.*No schemas found in components/schemas.*",
 ]
 norecursedirs = "tests/data/*"
 verbosity_assertions = 2

src/datamodel_code_generator/parser/openapi.py

@@ -719,7 +719,15 @@ def parse_raw(self) -> None:  # noqa: PLR0912, PLR0914, PLR0915
             self.raw_obj = specification
             self._collect_discriminator_schemas()
             schemas: dict[str, Any] = specification.get("components", {}).get("schemas", {})
+            paths: dict[str, Any] = specification.get("paths", {})
             security: list[dict[str, list[str]]] | None = specification.get("security")
+            # Warn if schemas is empty but paths exist and only Schemas scope is used
+            if not schemas and self.open_api_scopes == [OpenAPIScope.Schemas] and paths:
+                warn(
+                    "No schemas found in components/schemas. If your schemas are defined in "
+                    "external files referenced from paths, consider using --openapi-scopes paths",
+                    stacklevel=2,
+                )
             if OpenAPIScope.Schemas in self.open_api_scopes:
                 for obj_name, raw_obj in schemas.items():
                     self.parse_raw_obj(
@@ -728,7 +736,6 @@ def parse_raw(self) -> None:  # noqa: PLR0912, PLR0914, PLR0915
                         [*path_parts, "#/components", "schemas", obj_name],
                     )
             if OpenAPIScope.Paths in self.open_api_scopes:
-                paths: dict[str, Any] = specification.get("paths", {})
                 parameters: list[dict[str, Any]] = [
                     self._get_ref_body(p["$ref"]) if "$ref" in p else p
                     for p in paths.get("parameters", [])

tests/main/openapi/test_main_openapi.py

@@ -2,6 +2,7 @@
 
 from __future__ import annotations
 
+import contextlib
 import json
 import platform
 import warnings
@@ -3242,3 +3243,23 @@ def test_main_openapi_circular_imports_mixed_prefixes(output_dir: Path) -> None:
             expected_directory=EXPECTED_OPENAPI_PATH / "circular_imports_mixed_prefixes",
             input_file_type="openapi",
         )
+
+
+def test_warning_empty_schemas_with_paths(tmp_path: Path) -> None:
+    """Test warning when components/schemas is empty but paths exist."""
+    openapi_file = tmp_path / "openapi.yaml"
+    openapi_file.write_text("""
+openapi: 3.1.0
+info:
+  title: Test
+  version: '1'
+paths:
+  /test:
+    get:
+      responses:
+        200:
+          description: OK
+""")
+
+    with pytest.warns(UserWarning, match=r"No schemas found.*--openapi-scopes paths"), contextlib.suppress(Exception):
+        generate(openapi_file)