Improve literal dataclass field conversion to argparse argument (#17391)

Signed-off-by: Harry Mellor <19981378+hmellor@users.noreply.github.com>

tests/engine/test_arg_utils.py

@@ -11,7 +11,8 @@ import pytest
 from vllm.config import PoolerConfig, config
 from vllm.engine.arg_utils import (EngineArgs, contains_type, get_kwargs,
                                    get_type, is_not_builtin, is_type,
-                                   nullable_kvs, optional_type)
+                                   literal_to_kwargs, nullable_kvs,
+                                   optional_type)
 from vllm.utils import FlexibleArgumentParser
 
 
@@ -71,6 +72,21 @@ def test_get_type(type_hints, type, expected):
     assert get_type(type_hints, type) == expected
 
 
+@pytest.mark.parametrize(("type_hints", "expected"), [
+    ({Literal[1, 2]}, {
+        "type": int,
+        "choices": [1, 2]
+    }),
+    ({Literal[1, "a"]}, Exception),
+])
+def test_literal_to_kwargs(type_hints, expected):
+    context = nullcontext()
+    if expected is Exception:
+        context = pytest.raises(expected)
+    with context:
+        assert literal_to_kwargs(type_hints) == expected
+
+
 @config
 @dataclass
 class DummyConfigClass:
@@ -81,11 +97,15 @@ class DummyConfigClass:
     optional_literal: Optional[Literal["x", "y"]] = None
     """Optional literal with default None"""
     tuple_n: tuple[int, ...] = field(default_factory=lambda: (1, 2, 3))
-    """Tuple with default (1, 2, 3)"""
+    """Tuple with variable length"""
     tuple_2: tuple[int, int] = field(default_factory=lambda: (1, 2))
-    """Tuple with default (1, 2)"""
+    """Tuple with fixed length"""
     list_n: list[int] = field(default_factory=lambda: [1, 2, 3])
-    """List with default [1, 2, 3]"""
+    """List with variable length"""
+    list_literal: list[Literal[1, 2]] = field(default_factory=list)
+    """List with literal choices"""
+    literal_literal: Literal[Literal[1], Literal[2]] = 1
+    """Literal of literals with default 1"""
 
 
 @pytest.mark.parametrize(("type_hint", "expected"), [
@@ -111,6 +131,12 @@ def test_get_kwargs():
     # lists should work
     assert kwargs["list_n"]["type"] is int
     assert kwargs["list_n"]["nargs"] == "+"
+    # lists with literals should have the correct choices
+    assert kwargs["list_literal"]["type"] is int
+    assert kwargs["list_literal"]["nargs"] == "+"
+    assert kwargs["list_literal"]["choices"] == [1, 2]
+    # literals of literals should have merged choices
+    assert kwargs["literal_literal"]["choices"] == [1, 2]
 
 
 @pytest.mark.parametrize(("arg", "expected"), [