Detailed introduction to the struct.pack() function and struct.unpack() function in python

The struct in python is mainly used to process C structure data. When reading, it is first converted to Python's string type, and then converted to Python's structured type, such as tuple. Generally, the input channels come from files or network binary streams.

　 1.struct.pack() and struct.unpack()

During the conversion process, a format strings is mainly used. Use To specify the conversion method and format.

Let’s talk about the main methods:

1.1 struct.pack(fmt,v1,v2,...)

Put v1, v2 and other parameters The value is wrapped in one layer, and the wrapping method is specified by fmt. The wrapped parameters must strictly conform to fmt. Finally returns a wrapped string .

　1.2 struct.unpack(fmt,string)

　As the name suggests, unpack. For example, pack is packaged, and then unpacked can be used to unpack. Returns a tuple (tuple) obtained by unpacking data (string), even if there is only one data, it will be unpacked into a tuple. Among them, len(string) must be equal to calcsize(fmt), which involves a calcsize function. struct.calcsize(fmt): This is used to calculate the size of the structure described in the fmt format.

The format string consists of one or more format characters. For the description of these format characters, refer to the Python manual as follows

　2. Code example

import struct 

# native byteorder 
buffer = struct.pack("ihb", 1, 2, 3) 
print repr(buffer) 
print struct.unpack("ihb", buffer) 

# data from a sequence, network byteorder 
data = [1, 2, 3] 
buffer = struct.pack("!ihb", *data)
print repr(buffer) 
print struct.unpack("!ihb", buffer)

 

 

Output:

'\x01\x00\x00\x00\x02\x00\x03'
(1, 2, 3)
'\x00\x00\x00\x01\x00\x02\x03'
(1, 2, 3)

View Code

If not appended, the default is @. Even if the native character order (big endian or little endian) is used, the size of the C structure and the alignment in memory are consistent with the native machine (native). For example, The integer of some machines is 2 bits, while some machines have four bits; some machine memories have four bits aligned, and some have n bits aligned (n is unknown, and I don't know how many).

There is also a standard option, which is described as: If standard is used, there will be no memory alignment for any type.

For example, in the second half of the applet just now, the first bit in the format string used is! , which is the standard alignment of big endian mode, so the output is '\x00\x00\x00\x01\x00\x02\x03', in which the high bit itself is placed in the high address bit of the memory.

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