Union handles lists

simple_parsing/wrappers/field_wrapper.py

@@ -330,6 +330,8 @@ def get_arg_options(self) -> dict[str, Any]:
         elif self.is_union:
             logger.debug("Parsing a Union type!")
             _arg_options["type"] = get_parsing_fn(self.type)
+            if any(utils.is_list(o) for o in utils.get_args(self.type)):
+                _arg_options["nargs"] = "*"
 
         elif self.is_enum:
             logger.debug(f"Adding an Enum attribute '{self.name}'")
@@ -501,6 +503,13 @@ def postprocess(self, raw_parsed_value: Any) -> Any:
             else:
                 return raw_parsed_value
 
+        elif self.is_union:
+            list_in = [utils.is_list(o) for o in utils.get_args(self.type)]
+            # if type is like Union[str, list[str]] and only a single value was passed,
+            if any(list_in) and (not all(list_in)) and (len(raw_parsed_value) == 1):
+                raw_parsed_value = raw_parsed_value[0]
+            return raw_parsed_value
+
         elif self.is_subparser:
             return raw_parsed_value
 

test/test_union.py

@@ -32,3 +32,25 @@ class Foo2(TestSetup):
 
     foo = Foo2.setup("--x 2")
     assert foo.x == 2 and type(foo.x) is int
+
+
+def test_union_type_with_list():
+    @dataclass
+    class Foo(TestSetup):
+        x: Union[str, list[str]]
+
+    foo = Foo.setup("--x bob")
+    assert foo.x == "bob"
+
+    foo = Foo.setup("--x bob alice")
+    assert foo.x == ["bob", "alice"]
+
+    @dataclass
+    class Foo(TestSetup):
+        x: Union[list[int], list[str]]
+
+    foo = Foo.setup("--x bob alice")
+    assert foo.x == ["bob", "alice"]
+
+    foo = Foo.setup("--x 1 2")
+    assert foo.x == [1, 2]