@staticmethod
def cache_walker(inst, start_time, end_time, freq, column_names):
    "\n        If the expressions of one instrument haven't been calculated before,\n        calculate it and write it into expression cache.\n\n        "
    for field in column_names:
        ExpressionD.expression(inst, field, start_time, end_time, freq)
If the expressions of one instrument haven't been calculated before,
calculate it and write it into expression cache.
@staticmethod
def cache_walker(inst, start_time, end_time, freq, column_names):
    "\n        If the expressions of one instrument haven't been calculated before,\n        calculate it and write it into expression cache.\n\n        "
    for field in column_names:
        ExpressionD.expression(inst, field, start_time, end_time, freq)