def _iswritable(self):

        return self.mode in ('w', 'a', 'r+')
ValueRef fields are grammar-checked
dataset/ETHPy150Open PyTables/PyTables/tables/file.py/File._iswritable--crossover with--dataset/ETHPy150Open wrobstory/vincent/tests/test_vega.py/TestPropertySet.test_validation_checking