Improve handling of **kwargs in FromParams (#4629)

* Improve handling of **kwargs in FromParams

* remove duplicated line

* improved handling of *args and *kwargs in FromParams

* changelog

* **kwargs is needed

* Revert "**kwargs is needed"

This reverts commit c74a94c.

* revert FromParams changes

* ignore *args

* change optimizers' inheritance order

* add bare constructor to ForParams

* Revert "add bare constructor to ForParams"

This reverts commit 4afe072.

* Revert "change optimizers' inheritance order"

This reverts commit d9fe036.

* remove comment

Author: eladsegal

CHANGELOG.md

@@ -7,6 +7,10 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
 
 ## Unreleased
 
+### Fixed
+
+- Ignore *args when constructing classes with `FromParams`.
+
 ## [v1.1.0](https://github.com/allenai/allennlp/releases/tag/v1.1.0) - 2020-09-08
 
 ### Fixed

allennlp/common/from_params.py

@@ -113,18 +113,22 @@ def remove_optional(annotation: type):
 
 
 def infer_params(cls: Type[T], constructor: Callable[..., T] = None) -> Dict[str, Any]:
-    if cls == FromParams:
-        return {}
     if constructor is None:
         constructor = cls.__init__
 
     signature = inspect.signature(constructor)
     parameters = dict(signature.parameters)
 
     has_kwargs = False
+    var_positional_key = None
     for param in parameters.values():
         if param.kind == param.VAR_KEYWORD:
             has_kwargs = True
+        elif param.kind == param.VAR_POSITIONAL:
+            var_positional_key = param.name
+
+    if var_positional_key:
+        del parameters[var_positional_key]
 
     if not has_kwargs:
         return parameters

tests/common/from_params_test.py

@@ -961,3 +961,22 @@ def __init__(self, a: int, b: str = None, **kwargs) -> None:
         assert foo.a == 2
         assert foo.b == "hi"
         assert foo.c == {"2": "3"}
+
+    def test_from_params_child_has_kwargs_base_implicit_constructor(self):
+        class Foo(FromParams):
+            pass
+
+        class Bar(Foo):
+            def __init__(self, a: int, **kwargs) -> None:
+                self.a = a
+
+        bar = Bar.from_params(Params({"a": 2}))
+        assert bar.a == 2
+
+    def test_from_params_has_args(self):
+        class Foo(FromParams):
+            def __init__(self, a: int, *args) -> None:
+                self.a = a
+
+        foo = Foo.from_params(Params({"a": 2}))
+        assert foo.a == 2