Add support for using keyword arguments with a named struct in an RPC

capnp/lib/capnp.pyx

@@ -2050,7 +2050,7 @@ cdef class _DynamicCapabilityClient:
         return _find_field_order(params.struct)
 
     cdef _set_fields(self, Request * request, name, args, kwargs):
-        if args is not None:
+        if args is not None and len(args) > 0:
             arg_names = self._find_method_args(name)
             if len(args) > len(arg_names):
                 raise KjException('Too many arguments passed to `%s`. Expected %d and got %d' % (name, len(arg_names), len(args)))

test/test_capability.capnp

@@ -81,3 +81,11 @@ interface TestTailCaller {
 interface TestPassedCap {
   foo @0 (cap :TestInterface) -> (x: Text);
 }
+
+interface TestStructArg {
+  bar @0 BarParams -> (c: Text);
+}
+struct BarParams {
+  a @0 :Text;
+  b @1 :Int32;
+}

test/test_capability.py

@@ -358,3 +358,15 @@ def test_null_cap():
 
     with pytest.raises(capnp.KjException):
         client.foo().wait()
+
+
+class TestStructArg(capability.TestStructArg.Server):
+    def bar(self, a, b, **kwargs):
+        return a + str(b)
+
+
+def test_struct_args():
+    client = capability.TestStructArg._new_client(TestStructArg())
+    assert client.bar(a='test', b=1).wait().c == 'test1'
+    with pytest.raises(capnp.KjException):
+        assert client.bar('test', 1).wait().c == 'test1'