fix(cli): import yaml lazily so the CLI works without the yaml extra (#4929)

Import yaml lazily in the schema CLI command

The module-level 'from yaml import dump' in litestar/cli/commands/schema.py
is imported eagerly via litestar.cli.main, so every CLI invocation failed
with ModuleNotFoundError when litestar was installed without the yaml extra
(pyyaml only comes in transitively with the standard extras via uvicorn).

Import yaml inside the YAML export branch and raise
MissingDependencyException('pyyaml', extra='yaml') if it is missing, matching
how other optional dependencies are handled.

Fixes #4449

Co-authored-by: uttam12331 <uttam12331@users.noreply.github.com>

Author: uttam12331

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