遇到‘endswith first arg must be bytes...’错误，如何正确拼接字节？

在使用PyVISA与MKS946质量流量控制器通信时，遇到错误：endswith first arg must be bytes or a tuple of bytes, not str，报错指向comm方法中的self.mks946.write(prestring + command.encode('ascii'))行。

原代码如下：

class MKS946:
    def __init__(self, com_port):
        self.port = int(com_port)

    def __enter__(self):
        rm = pyvisa.ResourceManager()
        self.mks946 = rm.open_resource(f"ASRL{self.port}::INSTR")
        self.mks946.baud_rate = 9600
        sleep(0.1)
    
        return self

    def __exit__(self, exc_type, exc_val, exc_tb):
        self.mks946.write("@253LOCK!OFF")
        self.mks946.close()


    def comm(self, command):
        """ Implement communication protocol """
        prestring = b'@253'
        #endstring = b';FF' 
   
        self.mks946.write(prestring + command.encode('ascii'))
        sleep(0.1)
    
        return_string =   self.mks946.read_bytes(self.mks946.bytes_in_buffer)
        return return_string
 
    def get_id(self):
        return self.comm('SN')

if __name__ == "__main__":
    with MKS946(4) as mfc_box:
        print(mfc_box.get_id())

错误原因 #

错误的根源并非comm方法里的字节拼接（这部分本身合法），而是__exit__方法中调用write时传入了字符串类型的命令"@253LOCK!OFF"。PyVISA在串口通信模式下，要求写入内容必须是字节类型，混合字符串和字节的写入操作会触发内部类型检查失败，导致endswith报错。

解决方案 #

统一所有write调用的参数为字节类型，同时优化comm方法的字节拼接逻辑，提升代码可读性：

1. 修改__exit__方法中的写入命令，前缀加b转为字节：

def __exit__(self, exc_type, exc_val, exc_tb):
    self.mks946.write(b"@253LOCK!OFF")
    self.mks946.close()

2. 优化comm方法的字节拼接，先拼接完整命令字符串再统一编码：

def comm(self, command):
    """ Implement communication protocol """
    full_command = f"@253{command}".encode('ascii')
    self.mks946.write(full_command)
    sleep(0.1)
    
    return_string = self.mks946.read_bytes(self.mks946.bytes_in_buffer)
    return return_string

修改后的完整代码 #

from time import sleep
import pyvisa

class MKS946:
    def __init__(self, com_port):
        self.port = int(com_port)

    def __enter__(self):
        rm = pyvisa.ResourceManager()
        self.mks946 = rm.open_resource(f"ASRL{self.port}::INSTR")
        self.mks946.baud_rate = 9600
        sleep(0.1)
    
        return self

    def __exit__(self, exc_type, exc_val, exc_tb):
        self.mks946.write(b"@253LOCK!OFF")
        self.mks946.close()


    def comm(self, command):
        """ Implement communication protocol """
        full_command = f"@253{command}".encode('ascii')
        self.mks946.write(full_command)
        sleep(0.1)
    
        return_string = self.mks946.read_bytes(self.mks946.bytes_in_buffer)
        return return_string
 
    def get_id(self):
        return self.comm('SN')

if __name__ == "__main__":
    with MKS946(4) as mfc_box:
        print(mfc_box.get_id())

内容的提问来源于stack exchange，提问作者luiz_iniciante