Propagate conversion hints for predicates, too

And while we're at it, stabilize the Converter API. Rather than calling
the predicate and converter functions directly, we now introduce stable
supports and convert methods that always accept hints kwargs, regardless
of whether the linked predicate and/or converter functions do.

Author: ctrueden

src/scyjava/_convert.py

@@ -28,26 +28,37 @@ class Priority:
     LAST = -1e300
 
 
+def _has_kwargs(f):
+    return not isjava(f) and any(
+        p.kind == inspect.Parameter.VAR_KEYWORD
+        for p in inspect.signature(f).parameters.values()
+    )
+
+
 class Converter(NamedTuple):
     predicate: Callable[[Any], bool]
     converter: Callable[[Any], Any]
     priority: float = Priority.NORMAL
 
+    def supports(self, obj: Any, **hints: dict) -> bool:
+        return (
+            self.predicate(obj, **hints)
+            if _has_kwargs(self.predicate)
+            else self.predicate(obj)
+        )
+
+    def convert(self, obj: Any, **hints: dict) -> Any:
+        return (
+            self.converter(obj, **hints)
+            if _has_kwargs(self.converter)
+            else self.converter(obj)
+        )
+
 
 def _convert(obj: Any, converters: List[Converter], **hints: dict) -> Any:
-    suitable_converters = filter(lambda c: c.predicate(obj), converters)
+    suitable_converters = [c for c in converters if c.supports(obj, **hints)]
     prioritized = max(suitable_converters, key=lambda c: c.priority)
-
-    # check if selected converter supports hints
-    uses_hints = not isjava(prioritized.converter) and any(
-        p.kind == inspect.Parameter.VAR_KEYWORD
-        for p in inspect.signature(prioritized.converter).parameters.values()
-    )
-    return (
-        prioritized.converter(obj, **hints)
-        if uses_hints
-        else prioritized.converter(obj)
-    )
+    return prioritized.convert(obj, **hints)
 
 
 # -- Python to Java --