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：
1 f = open("file","rb")
2 #读取个整型
3 data = f.read(4)
4 #读取完毕，关了文件
5 f.close()
6 #转换
7 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。
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