fix(cli): import yaml lazily so the CLI works without the yaml extra

litestar/cli/commands/schema.py

@@ -10,13 +10,12 @@
 except ImportError:
     import click  # type: ignore[no-redef]
 
-from yaml import dump as dump_yaml
-
 from litestar import Litestar
 from litestar._openapi.typescript_converter.converter import (
     convert_openapi_to_typescript,
 )
 from litestar.cli._utils import JSBEAUTIFIER_INSTALLED, LitestarCLIException, LitestarGroup
+from litestar.exceptions import MissingDependencyException
 from litestar.serialization import encode_json, get_serializer
 
 __all__ = ("generate_openapi_schema", "generate_typescript_specs", "schema_group")
@@ -31,6 +30,13 @@ def _generate_openapi_schema(app: Litestar, output: Path) -> None:
     """Generate an OpenAPI Schema."""
     serializer = get_serializer(app.type_encoders)
     if output.suffix in (".yml", ".yaml"):
+        try:
+            # Import lazily: pyyaml is an optional dependency, and importing it at
+            # module level made every CLI invocation fail when it is not installed.
+            # https://github.com/litestar-org/litestar/issues/4449
+            from yaml import dump as dump_yaml
+        except ImportError as e:
+            raise MissingDependencyException("pyyaml", extra="yaml") from e
         content = dump_yaml(
             msgspec.to_builtins(app.openapi_schema.to_schema(), enc_hook=serializer),
             default_flow_style=False,

tests/unit/test_cli/test_schema_commands.py

@@ -103,3 +103,22 @@ def test_openapi_typescript_command_without_jsbeautifier(
     result = runner.invoke(cli_command, command)
     assert result.exit_code == 0
     assert mock_path_write_text.called
+
+
+def test_cli_importable_without_pyyaml(monkeypatch: pytest.MonkeyPatch) -> None:
+    """The CLI must not require pyyaml at import time.
+
+    An eager module-level yaml import made every CLI invocation fail with
+    ``ModuleNotFoundError: No module named 'yaml'`` when litestar was installed
+    without the ``yaml`` extra. https://github.com/litestar-org/litestar/issues/4449
+    """
+    import importlib
+    import sys
+
+    # Simulate pyyaml not being installed: a None entry makes `import yaml`
+    # raise ImportError.
+    monkeypatch.setitem(sys.modules, "yaml", None)
+    monkeypatch.delitem(sys.modules, "litestar.cli.commands.schema", raising=False)
+
+    module = importlib.import_module("litestar.cli.commands.schema")
+    assert module.schema_group is not None