python - 结构打包/拆包类设计

Question

我正在实现二进制文件格式(~15 种不同的结构)读取器/写入器，但我有点进退两难，应该使用哪种设计模式。

1. 每个结构体都包含打包/解包的方法:

   class Struct1:  
       def pack():
           struct.pack(self.foo)
           ...
       def unpack():  
           self.foo = struct.unpack()
           ...
   class Struct2:  
       def pack():
           struct.pack(self.foo)
           ...
       def unpack():  
           self.foo = struct.unpack()
           ...
   ...
   

   对比

2. 每个结构都有一个具有读/写方法的 Reader/Writer 类:

   class Reader:
       def read_struct1():
           s = Struct1()
           s.foo = struct.unpack()
           ...
           return s
       def read_struct2():
           s = Struct2()
           s.foo = struct.unpack()
           ...
           return s
       ...
   class Writer:
       def write_struct1(s):
           struct.pack(s.foo)
           ...
       def write_struct2(s):
           struct.pack(s.foo)
           ...
       ...
   

Answer 1

编写单独的 Reader/Writer 类的目的是支持不同的输出处理 - 写入控制台、写入字节、写入 JSON 字符串等等。重要的是每个类 Struct1、Struct2 等都知道哪些属性对于保存其状态很重要。这就是 pickle 模块所使用的哲学。

class Struct1(object):
    fields = ('a','b','c')
    ...

class Struct2(object):
    fields = ('foo', 'bar', 'baz')
    ...

现在可以使用此元数据编写各种编写器类，而无需编写特定于类的代码:

class StructWriter(object):
    packTypeMap = {int:'i', float:'f', str:'s'}

    def write(self, obj):
        fields = obj.fields
        packstring = ''.join(packTypeMap[type(f)] for f in fields)
        packargs = (getattr(obj,f) for f in fields)
        return struct.pack(packstring, *packargs)

class DictWriter(object):
    def write(self, obj):
        return dict((f, getattr(obj,f)) for f in obj.fields)

class JSONWriter(object):
    jsonTypeMap = {str:lambda s:"'"+s+"'"}
    defaultJsonFunc = lambda x:str(x)
    def write(self, obj):
        # not really recommended to roll your own strings, but for illustration...
        fields = obj.fields
        outargs = (getattr(obj,f) for f in fields)
        outvals = (jsonTypeMap.get(type(arg),defaultJsonFunc)(arg) 
                       for arg in outargs)
        return ('{' +
            ','.join("'%s':%s" % field_val for field_val in zip(fields, outvals))
            '}')

class ZipJSONWriter(JSONWriter):
    def write(self, obj):
        import zlib
        return zlib.compress(super(ZipJSONWriter,self).write(obj))        

class HTMLTableWriter(object):
    def write(self, obj):
        out = "<table>"
        for field in obj.fields:
            out += "<tr><td>%s</td><td>%s</td></tr>" % (field, getattr(obj,field))
        out += "</table>"
        return out