分析dnspython TLSA 代码

自己刚开始用了好几天的dnspython ，因为没有用过python ，所以感觉比较吃力，想记录下来自己对于dnspython的 TLSA的浅显的理解。理解存在错误在所难免

import structimport dns.rdataimport dns.rdatatypeclass TLSA(dns.rdata.Rdata):    """TLSA record    @ivar usage: The certificate usage    @type usage: int    @ivar selector: The selector field    @type selector: int    @ivar mtype: The 'matching type' field    @type mtype: int    @ivar cert: The 'Certificate Association Data' field    @type cert: string    @see: RFC 6698"""    __slots__ = ['usage', 'selector', 'mtype', 'cert']   #TLSA对象的四个属性    def __init__(self, rdclass, rdtype, usage, selector,                 mtype, cert):        super(TLSA, self).__init__(rdclass, rdtype)        self.usage = usage        self.selector = selector        self.mtype = mtype        self.cert = cert    def to_text(self, origin=None, relativize=True, **kw): #打印TLSA对象        return '%d %d %d %s' % (self.usage,                                self.selector,                                self.mtype,                                dns.rdata._hexify(self.cert,                                               chunksize=128))    def from_text(cls, rdclass, rdtype, tok, origin = None, relativize = True):#从传入的参数值构建TLSA对象        usage = tok.get_uint8() #usage 及另外两个属性都是1字节 的无符号整数        selector = tok.get_uint8()        mtype = tok.get_uint8()        cert_chunks = []        while 1:            t = tok.get().unescape()            if t.is_eol_or_eof():                break            if not t.is_identifier():                raise dns.exception.SyntaxError            cert_chunks.append(t.value)        cert = ''.join(cert_chunks)        cert = cert.decode('hex_codec')        return cls(rdclass, rdtype, usage, selector, mtype, cert)    from_text = classmethod(from_text)    def to_wire(self, file, compress = None, origin = None):#to_wire函数是将对象转换成2进制的串以便于tcp传输        header = struct.pack("!BBB", self.usage, self.selector, self.mtype)#使用struct pack命令，！表示网络字节序（大端），B对应的是1字节的无符号整数，BBB亦即说明除cert外的三个属性都会以相应的形式被转成2进制

        file.write(header)        file.write(self.cert)#写入三个属性和cert值后，tlsa文件就转化成了2进制的表示，是不是类似java的序列化    def from_wire(cls, rdclass, rdtype, wire, current, rdlen, origin = None):#将2进制形式转化成TLSA对象，与to_wire相对应        header = struct.unpack("!BBB", wire[current : current + 3])#3 指的是字节数        current += 3        rdlen -= 3        cert = wire[current : current + rdlen].unwrap()        return cls(rdclass, rdtype, header[0], header[1], header[2], cert)    from_wire = classmethod(from_wire)    def _cmp(self, other): #比较TLSA对象        hs = struct.pack("!BBB", self.usage, self.selector, self.mtype)        ho = struct.pack("!BBB", other.usage, other.selector, other.mtype)        v = cmp(hs, ho)        if v == 0:            v = cmp(self.cert, other.cert)        return v