struct 模块可以把一个类型，如数字，转成固定长度的bytes类型

Format (格式)	C Type (c语言类型)	Python type (Python类型)	Standard size (bytes长度)	Notes
x	pad byte	no value
c	char	string of length 1	1
b	signed char	int	1	(3)
B	unsigned char	int	1	(3)
?	_Bool	bool	1	(1)
h	short	int	2	(3)
H	unsigned shoort	int	2	(3)
i	int	int	4	(3)
I	unsigned int	int	4	(3)
l	long	int	4	(3)
L	unsigned long	int	4	(3)
q	long long	int	8	(2),(3)
Q	unsigned long long	int	8	(2),(3)
f	float	float	4	(4)
d	double	float	8	(4)
s	char[]	str
p	char[]	str
P	void*	int		(5),(3)

import struct

num = 10

bytes_int = struct.pack('i', num)  # 将数字转化为固定长度的bytes类型 -> 'i' 是上面对照表的格式，代表 int

print(bytes_int)  # b'\n\x00\x00\x00'

print(len(bytes_int))  # 4

decoding_num = struct.unpack('i', bytes_int)  # 对固定长度的bytes类型进行解码

print(decoding_num)  # (10,)