FromConfig not handling correctly required parameters

CHANGELOG.rst

@@ -20,6 +20,8 @@ Fixed
 - Union types with str and default comment-like string incorrectly parsed as a
   stringified exception of an other subtype (`#812
   <https://github.com/omni-us/jsonargparse/pull/812>`__).
+- ``FromConfig`` not handling correctly required parameters (`#813
+  <https://github.com/omni-us/jsonargparse/pull/813>`__).
 
 Changed
 ^^^^^^^

jsonargparse/_from_config.py

@@ -9,6 +9,7 @@
 __all__ = ["FromConfigMixin"]
 
 T = TypeVar("T")
+OVERRIDE_KINDS = {inspect.Parameter.KEYWORD_ONLY, inspect.Parameter.POSITIONAL_OR_KEYWORD}
 
 
 class FromConfigMixin:
@@ -55,6 +56,10 @@ def _parse_class_kwargs_from_config(cls: Type[T], config: Union[str, PathLike, d
     """Parse the init kwargs for ``cls`` from a config file or dict."""
     parser = ArgumentParser(exit_on_error=False, **kwargs)
     parser.add_class_arguments(cls)
+    for required in parser.required_args:
+        action = next((a for a in parser._actions if a.dest == required), None)
+        action._required = False  # type: ignore[union-attr]
+    parser.required_args.clear()
     if isinstance(config, dict):
         cfg = parser.parse_object(config, defaults=False)
     else:
@@ -79,12 +84,15 @@ def _override_init_defaults_this_class(cls: Type[T], defaults: dict) -> None:
     params = inspect.signature(cls.__init__).parameters
     for name, default in defaults.copy().items():
         param = params.get(name)
-        if param and param.kind in {inspect.Parameter.KEYWORD_ONLY, inspect.Parameter.POSITIONAL_OR_KEYWORD}:
+        if param and param.kind in OVERRIDE_KINDS:
+            if param.default == inspect._empty:
+                raise TypeError(f"Overriding of required parameters not allowed: '{param.name}'")
             defaults.pop(name)
             if param.kind == inspect.Parameter.KEYWORD_ONLY:
                 cls.__init__.__kwdefaults__[name] = default  # type: ignore[index]
             else:
-                index = list(params).index(name) - 1
+                required = [p for p in params.values() if p.kind in OVERRIDE_KINDS and p.default == inspect._empty]
+                index = list(params).index(name) - len(required)
                 aux = cls.__init__.__defaults__ or ()
                 cls.__init__.__defaults__ = aux[:index] + (default,) + aux[index + 1 :]
 

jsonargparse_tests/test_from_config.py

@@ -80,6 +80,38 @@
     assert instance.child2 == 2
 
 
+def test_init_defaults_override_preserve_required(tmp_cwd):
+    config_path = tmp_cwd / "config.yaml"
+    config_path.write_text(json_or_yaml_dump({"param2": 2}))
+
+    class DefaultsOverrideRequiredParameters(FromConfigMixin):
+        __from_config_init_defaults__ = config_path
+
+        def __init__(self, param1: str, param2: int = 1):
+            self.param1 = param1
+            self.param2 = param2
+
+    with pytest.raises(TypeError, match="missing 1 required positional argument: 'param1'"):
+        DefaultsOverrideRequiredParameters()
+
+    instance = DefaultsOverrideRequiredParameters(param1="required")
+    assert instance.param1 == "required"
+    assert instance.param2 == 2
+
+
+def test_init_defaults_override_required_not_allowed(tmp_cwd):
+    config_path = tmp_cwd / "config.yaml"
+    config_path.write_text(json_or_yaml_dump({"param1": 2}))
+
+    with pytest.raises(TypeError, match="Overriding of required parameters not allowed: 'param1'"):
+
+        class DefaultsOverrideRequiredNotAllowed(FromConfigMixin):
+            __from_config_init_defaults__ = config_path
+
+            def __init__(self, param1: int):
+                self.param1 = param1
+
+
 def test_init_defaults_override_class_with_init_subclass(tmp_cwd):
     config_path = tmp_cwd / "config.yaml"
     config_path.write_text(json_or_yaml_dump({"parent": "overridden_parent", "child": "overridden_child"}))