Fix TypeError building key error message for a tuple key (#253)

schema/__init__.py

@@ -494,7 +494,7 @@ def validate(self, data: Any, **kwargs: Dict[str, Any]) -> Any:
                                         svalue, error=e, ignore_extra_keys=i
                                     ).validate(value, **kwargs)
                                 except SchemaError as x:
-                                    k = "Key '%s' error:" % nkey
+                                    k = "Key '%s' error:" % (nkey,)
                                     message = self._prepend_schema_name(k)
                                     raise SchemaError(
                                         [message] + x.autos,

test_schema.py

@@ -2059,3 +2059,13 @@ def test_callable_error():
     except SchemaError as ex:
         e = ex
     assert e.errors == ["This is the error message"]
+
+
+def test_tuple_key_error_message_does_not_crash():
+    # Regression test for #253: a tuple dict key (including the empty tuple) must not raise
+    # "TypeError: not enough arguments for format string" when building the key error message.
+    assert Schema({(): [(str,)]}).is_valid({(): ["foo", ("bar",)]}) is False
+
+    with raises(SchemaError) as exc_info:
+        Schema({("a", "b"): int}).validate({("a", "b"): "not-an-int"})
+    assert "Key '('a', 'b')' error:" in exc_info.value.code