Deduce type from default value

auto_cli/parsing.py

@@ -46,9 +46,14 @@ def create_parser(command: Command) -> ArgumentParser:
     param_types = command.parameter_types or {}
     for param_name, parameter in parameters.items():
         annotation = param_types.get(param_name, parameter.annotation)
+        has_default_value = _has_default(parameter)
+        if annotation == inspect.Parameter.empty and has_default_value:
+            # Deduce the type from the default value
+            annotation = type(parameter.default)
+
         kwargs = {
-            "required": not _has_default(parameter),
-            "default": parameter.default if _has_default(parameter) else None,
+            "required": not has_default_value,
+            "default": parameter.default if has_default_value else None,
             # The params above might be overwritten by the function below
             **_get_type_params(annotation, param_name, function),
         }

auto_cli/tests/test_parsing.py

@@ -59,6 +59,15 @@ def func_to_test(a: Tuple[int, int], b: bool) -> int:
     assert args == {"a": nums, "b": False}
 
 
+def test_type_from_default() -> None:
+    def func_to_test(a=4) -> int:
+        return a
+
+    parser = create_parser(Command.from_func(func_to_test))
+    args = parser.parse(["--a", "4"])
+    assert args == {"a": 4}
+
+
 def test_override_param_type() -> None:
     def func_to_test(a: int, b):
         return a + b