Fall back on __len__ if __nonzero__ doesnt exist

src/runtime/objmodel.cpp

@@ -1689,6 +1689,8 @@ extern "C" bool nonzero(Box* obj) {
 
     // go through descriptor logic
     Box* func = getclsattr_internal(obj, "__nonzero__", NULL);
+    if (!func)
+        func = getclsattr_internal(obj, "__len__", NULL);
 
     if (func == NULL) {
         ASSERT(isUserDefined(obj->cls) || obj->cls == classobj_cls, "%s.__nonzero__",
@@ -1697,6 +1699,7 @@ extern "C" bool nonzero(Box* obj) {
     }
 
     Box* r = runtimeCall0(func, ArgPassSpec(0));
+    // I believe this behavior is handled by the slot wrappers in CPython:
     if (r->cls == bool_cls) {
         BoxedBool* b = static_cast<BoxedBool*>(r);
         bool rtn = b->n;