访问子字符串

import struct

def fields(baseformat, theline, lastfield=False):
    # theline超出的长度也由这个base-format确定
    # （通过struct.calcsize计算确切的长度）
    numremain = len(theline) - struct.calcsize(baseformat)
    # 用合适的s或x字段完成格式，然后unpack
    format = "%s %d%s" % (baseformat, numremain, lastfield and "s" or "x")
    return struct.unpack(format, theline)  

import struct

def fields(baseformat, theline, lastfield=False, _cache={}):
# 生成键并尝试获得缓存的格式字符串
key = baseformat, len(theline), lastfield
format = _cache.get(key)
if format is None:
# 没有缓存的格式字符串，创建并缓存之
numremain = len(theline) - struct.calcsize(baseformat)
_cache[key] = format = "%s %d%s" % (baseformat, numremain, lastfield and "s" or "x")
return struct.unpack(format, theline)