在很多应用层的网络传输协议中，为了提高效率，会把很多个数据元素以二进制的方式拼凑在一起，比如socks的协议：

Client to SOCKS server:

• field 1: SOCKS version number, 1 byte, must be 0x04 for this version
• field 2: command code, 1 byte:
  • 0x01 = establish a TCP/IP stream connection
  • 0x02 = establish a TCP/IP port binding
• field 3: network byte order port number, 2 bytes
• field 4: deliberate invalid IP address, 4 bytes, first three must be 0x00 and the last one must not be 0x00
• field 5: the user ID string, variable length, terminated with a null (0x00)
• field 6: the domain name of the host we want to contact, variable length, terminated with a null (0x00)

Server to SOCKS client:

• field 1: null byte
• field 2: status, 1 byte:
  • 0x5a = request granted
  • 0x5b = request rejected or failed
  • 0x5c = request failed because client is not running identd (or not reachable from the server)
  • 0x5d = request failed because client's identd could not confirm the user ID string in the request
• field 3: network byte order port number, 2 bytes
• field 4: network byte order IP address, 4 bytes

要解析这样的数据，我们可以使用Python的Slice方法，例如：

data[x:y]表示端口号，data[m:n]表示IP地址，等等。

为了使程序更加清晰易懂，我们可以将上述Slice定义为Naming Slice，使之意义更加明确（而不是一堆Magic Number），这样可以大大方便程序的阅读者理解数据段的含义。

Naming Slice的使用方法示例如下：

items=[0,1,2,3,4,5,6,7,8,9]
x=slice(2,4)
print(items[x])
items[x]=[1,2,3]
print(items)
del items[x]
print(items)
print(items[x])

参考资料：

http://docs.python.org/3/library/functions.html#slice