Introduction to the usage of pack and unpack in Python

Sometimes you need to use python to process binary data, for example, when accessing files and socket operations. At this time, you can use python's struct module to complete it. You can use struct to process structures in the C language.

The three most important functions in the struct module are pack(), unpack(), calcsize()

pack(fmt, v1, v2, ...) Follow the instructions given A given format (fmt), encapsulate the data into a string (actually a byte stream similar to a c structure)

unpack(fmt, string) Parse the bytes according to the given format (fmt) Stream string, return the parsed tuple

calcsize(fmt) Calculate how many bytes of memory the given format (fmt) occupies

The formats supported in the struct are as follows Table:

Format C Type Python Number of bytes

x pad byte no value 1

c char string of length 1 1

b signed char integer 1

B unsigned char integer 1

? _Bool bool 1

h short integer 2

H unsigned short integer 2

i int integer 4

I unsigned int integer or long 4

l long integer 4

L unsigned long long 4

q long long long 8

Q unsigned long long long 8

f float float 4

d double float 8

s char[] string 1

p char [] string 1

P void * long

Note 1.q and Q are only interesting when the machine supports 64-bit operations

Note 2. Each format can be preceded by A number, representing the number

Note 3. The s format represents a string of a certain length, 4s represents a string of length 4, but p represents a pascal string

Note 4. P is used to convert a pointer, its length is related to the machine word length

Note 5. The last one can be used to represent the pointer type, occupying 4 bytes

In order to be the same as the structure in c When exchanging data by body, you should also consider that some c or c++ compilers use byte alignment, which is usually a 32-bit system with 4 bytes as the unit. Therefore, the struct is converted according to the local machine byte order. You can use the byte order in the format. One character to change the alignment. The definition is as follows:

Character Byte order Size and alignment

@ native native Make up enough 4 bytes

= native standard Press the original word Number of sections

> big-endian standard based on the original number of bytes

! network (= big-endian)

standard Based on the original number of bytes

The method of use is to put it at the first position of fmt, just like '@5s6sif'

Example 1:

The structure is as follows:

struct Header
{
    unsigned short id;
    char[4] tag;
    unsigned int version;
    unsigned int count;
}

The above structure data is received through socket.recv, which is stored in the string s. Now it needs to be parsed out. You can use the unpack() function:

import struct
id, tag, version, count = struct.unpack("!H4s2I", s)

In the above format string, ! means that we need to use network byte order for parsing, because our data is received from the network, and when it is transmitted over the network, it is in network byte order. The following H means An unsigned short id, 4s represents a 4-byte long string, and 2I represents two unsigned int type data.

Just pass an unpack, and now the id, tag, version, and count have been saved. Our information.

Similarly, it is also very convenient to pack local data into struct format:

ss = struct.pack("!H4s2I", id, tag, version, count);

The pack function converts id, tag, version, count according to the specified format Becoming a structure Header, ss is now a string (actually a byte stream similar to a c structure), and this string can be sent out through socket.send(ss).

Example 2:

import struct
a=12.34
#将a变为二进制
bytes=struct.pack('i',a)

At this time bytes is a string string, and the string is the same as the binary storage content of a in bytes.

Then perform the reverse operation, and convert the existing binary data bytes (actually a string) into the python data type:

#Note that unpack returns a tuple! !

a,=struct.unpack('i',bytes)

If it is composed of multiple data, it can be like this:

a='hello'
b='world!'
c=2
d=45.123
bytes=struct.pack('5s6sif',a,b,c,d)

The bytes at this time are data in binary form, and can be written directly to a file such as binfile.write(bytes)

Then, when we need it, we can read it out, bytes=binfile.read()

and then decode it into a python variable through struct.unpack():

a,b,c,d=struct.unpack('5s6sif',bytes)

' 5s6sif' is called fmt, which is a formatted string, consisting of numbers and characters. 5s means a string of 5 characters, 2i means 2 integers, etc. The following are the available characters and types. ctype means that it can be used with python The types in have one-to-one correspondence.

Note: Problems encountered when processing binary files

When we process binary files, we need to use the following method:

binfile=open(filepath,'rb')    
#读二进制文件
binfile=open(filepath,'wb')   
#写二进制文件

Then binfile=open(filepath, What is the difference between the results of 'r')?

There are two differences:

First, if you encounter '0x1A' when using 'r', it will be regarded as the end of the file, which is EOF. Using 'rb' does not have this problem. That is, if you write in binary and read out in text, only part of the file will be read out if '0X1A' is present. When using 'rb', it will read to the end of the file.

Second, for the string x=’abc\ndef’, we can use len(x) to get its length to be 7. \n is called a newline character, which is actually ‘0X0A’. When we write in 'w', which is text mode, '0X0A' will be automatically changed into two characters '0X0D', '0X0A' on the Windows platform, that is, the file length actually becomes 8. When reading in 'r' text mode, it is automatically converted to the original newline character. If you change to 'wb' binary mode to write, one character will remain unchanged, and it will be read as it is when reading. So if you write in text mode and read in binary mode, you have to consider this extra byte. '0X0D' is also called the carriage return character. It will not change under Linux. Because linux only uses '0X0A' to represent line breaks.

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