struct pack unpack详解

额。。。。最近被python的数据类型折腾得有点够呛了。。。struct的pack和unpack可以解决很多和C兼容的数据类型问题，但是有很多trick...

pack可以把一个C语言类似struct的结构转换成一个python的string数据结构，有了str结构，就可以进行write操作什么的了。
from python help

    The optional first format char indicates byte order, size and alignment:
     @: native order, size & alignment (default)
     =: native order, std. size & alignment
     <: little-endian, std. size & alignment
     >: big-endian, std. size & alignment
     !: same as >
   
    The remaining chars indicate types of args and must match exactly;
    these can be preceded by a decimal repeat count:
     x: pad byte (no data); c:char; b:signed byte; B:unsigned byte;
     h:short; H:unsigned short; i:int; I:unsigned int;
     l:long; L:unsigned long; f:float; d:double.
    Special cases (preceding decimal count indicates length):
     s:string (array of char); p: pascal string (with count byte).
    Special case (only available in native format):
     P:an integer type that is wide enough to hold a pointer.
    Special case (not in native mode unless 'long long' in platform C):
     q:long long; Q:unsigned long long
    Whitespace between formats is ignored.

用法就是 string = struct.pack(format, data) ,format就是上面的usage
例：
struct.pack('
>>> struct.pack(''xV4\x12

>>> struct.pack(''xV4\x12k'
struct
{
  unsigned int a,
  char         b
};

>>> struct.pack(''xV4\x12k\x01\x00'
2B代表说是2个字节，2代表连续2个B。

>>> struct.pack('__main__:1: DeprecationWarning: 'B' format requires 0 <= number <= 255
'xV4\x12k\x01\n'

>>> struct.calcsize('cBi')
8
>>> struct.calcsize('i')
4
>>> struct.calcsize('c')
1
>>> struct.calcsize('B')
1
>>> struct.calcsize('cBi')
8
>>> struct.calcsize('=cBi')
6
不带=号，会进行对齐操作，加等号则优化。


>>> struct.pack(''xV4\x12k\x01\xe94\x12'
>>> struct.unpack('(305419896, 'k', 1, 233, 4660)
unpack注意！这里返回的是一个tuple！