如何从bytes对象提取指定字节生成新bytes对象用于Modbus CRC校验

我有一台运行修改版Modbus协议的设备，会向树莓派3的串口发送消息。消息总长14字节：开头1个同步字节，接着11个数据字节，最后2个Modbus CRC-16校验字节。我需要把这11个数据字节传入CRC校验函数来验证消息有效性，但不知道怎么提取这些字节并生成校验函数能接受的新bytes对象。以下是我的代码和运行报错（报错是因为未创建newData变量）：

import serial
from time import sleep
from modbus_crc import check_crc
    
    
ser = serial.Serial("/dev/ttyS0", 9600)
    
print("waiting for  message from the serial port ......\n")
rxData = ser.read()
sleep(0.03)
data_left = ser.inWaiting()
rxData += ser.read(data_left)
    
print("Message has been received\n")
print("The 'rxData' type from ser.read() is ",type(rxData), " and length is ", len(rxData))
print("'rxData - ", [hex(i) for i in rxData], "\n")
    
print("Now show only bytes 1 to 11 of rxData\n")
    
x = range(1,12,1)
for i in x:
    print((hex(rxData[i])), end=" ")
print("\n")
    
#####################
#### Missing code to make newData  with only the bytes (1 to 11 in rxData
#####################
        
print("\nThe 'newData' type is ",type(newData), " and length is ", len(newData))
print("'newData' - ", [hex(i) for i in newData], "\n")
    
print("\n")
print("check if newData CRC is OK\n")
    
if not check_crc(newData):
    print("CRC is NOT OK")
else:
    print("CRC is OK!")

运行输出： #

waiting for  message from the serial port ......

Message has been received

The 'rxData' type from ser.read() is  <class 'bytes'>  and length is  14
'rxData -  ['0xff', '0xd', '0x77', '0x2', '0x1', '0x1', '0x12', '0x33', '0x30', '0x2e', '0x38', '0x39', '0xfd', '0x78'] 

Now show only bytes 1 to 11 of rxData

0xd 0x77 0x2 0x1 0x1 0x12 0x33 0x30 0x2e 0x38 0x39 

Traceback (most recent call last):
  File "/home/stevev/Projects/TKinter/20240309-operation on Bytes class.py", line 29, in <module>
    print("\nThe 'newData' type is ",type(newData), " and length is ", len(newData))
NameError: name 'newData' is not defined

解决方案 #

直接利用Python的bytes切片特性就能提取指定范围的字节，在缺失代码的位置添加以下一行即可：

newData = rxData[1:12]

说明：

Python的序列切片采用左闭右开规则，rxData[1:12]会提取索引从1到11的所有元素（结束索引12不包含在内），正好对应你需要的11个数据字节。切片返回的结果本身就是bytes类型，无需额外转换，完全符合check_crc函数的参数要求。

修改后运行代码，就能正常生成newData并完成CRC校验。

内容的提问来源于stack exchange，提问作者slv026