python中使用struct模块处理二进制数据

假设这样个场景：

你有个文件，里面全是二进制方式存储的整型，你需要读取出来。于是你随手写出了这样的代码：

f = open("file","rb")
#读取个整型
data = f.read(4)
#读取完毕，关了文件
f.close()
#转换
num = int(data) 然后就会报错：

Traceback (most recent call last):
File "<stdin>", line 1, in <module>
ValueError: null byte in argument for int()

你查看下data：

data = '8\x00:\x00'

神马！！竟然是十六进制！！这货怎么转换？

你翻开《CookBook》 里面扯了一堆不着边用不到的东西，就是没有这个问题。绝望之时，你在文档里看到了struct模块，文档里这么说：

This module performs conversions between Python values and C structs represented as Python strings. It uses format strings (explained below) as compact descriptions of the lay-out of the C structs and the intended conversion to/from Python values. This can be used in handling binary data stored in files or from network connections, among other sources.

没错，找的就是这货！

大概看了下，主要用到的是pack 函数和unpack函数。

pack:

struct.pack(fmt, v1, v2, ...)
Return a string containing the values v1, v2, ... packed according to the given format. The arguments must match the values required by the format exactly. unpack:

struct.unpack(fmt, string)
Unpack the string (presumably packed by pack(fmt, ...)) according to the given format. The result is a tuple even if it contains exactly one item. The string must contain exactly the amount of data required by the format (len(string) must equal calcsize(fmt)).

下面还有个对应C语言中数据类型的表格：

Format	C Type	Python	Notes
x	pad byte	no value
c	char	string of length 1
b	signed char	integer
B	unsigned char	integer
?	_Bool	bool	(1)
h	short	integer
H	unsigned short	integer
i	int	integer
I	unsigned int	integer or long
l	long	integer
L	unsigned long	long
q	long long	long	(2)
Q	unsigned long long	long	(2)
f	float	float
d	double	float
s	char[]	string
p	char[]	string
P	void *	long

怎么样？大概看明白了吧？你把代码改成了：

f = open("file","rb")
data = f.read(4)
#转换格式
num = struct.unpack("i",data)
f.close()
print(num)

这样就正常输出了吧？

嘿嘿，可以早点下班了。

如果还不明白，就接着看文档去，还有可以参考下这篇文章：
http://www.cnblogs.com/tonychopper/archive/2010/07/23/1783501.html
再次赞颂简洁万能的Python。