如何消除Python脚本运行结束时的‘Lost access to message queue’提示

问题描述 #

运行以下Python脚本读取DHT11温湿度传感器时，正常输出数据后会出现Lost access to message queue提示，需要消除该提示：

运行输出：

pi@raspberrypi:$ peder.py
25.0  41.0 
Temp: 77.0 F / 25.0 C    Humidity: 41% 
Lost access to message queue

脚本代码：

#!/usr/bin/python3
import sys
import board
import time

import adafruit_dht

dhtDevice = adafruit_dht.DHT11(board.D17)

try:
        # Print the values to the serial port
        temperature_c = dhtDevice.temperature
        temperature_f = temperature_c * (9 / 5) + 32
        humidity = dhtDevice.humidity
        print("%-3.1f " % temperature_c + " " + "%-3.1f " % humidity)
        print(
            "Temp: {:.1f} F / {:.1f} C    Humidity: {}% ".format(
                temperature_f, temperature_c, humidity
            )
        )
       
except RuntimeError as error:
        # Errors happen fairly often, DHT's are hard to read, just keep going
        print(error.args[0])
        time.sleep(2.0)

except Exception as error:
        dhtDevice.exit()
        raise error         

解决方法 #

这个提示是因为脚本结束时没有正确释放DHT设备占用的底层消息队列资源导致的，以下两种方式都能修复：

方法1：用finally块确保资源释放

在try-except结构后添加finally块，不管是否触发异常，都会执行设备退出操作来释放资源：

#!/usr/bin/python3
import sys
import board
import time

import adafruit_dht

dhtDevice = adafruit_dht.DHT11(board.D17)

try:
        # Print the values to the serial port
        temperature_c = dhtDevice.temperature
        temperature_f = temperature_c * (9 / 5) + 32
        humidity = dhtDevice.humidity
        print("%-3.1f " % temperature_c + " " + "%-3.1f " % humidity)
        print(
            "Temp: {:.1f} F / {:.1f} C    Humidity: {}% ".format(
                temperature_f, temperature_c, humidity
            )
        )
       
except RuntimeError as error:
        # Errors happen fairly often, DHT's are hard to read, just keep going
        print(error.args[0])
        time.sleep(2.0)

except Exception as error:
        raise error
finally:
        dhtDevice.exit()

方法2：使用上下文管理器（with语句）

adafruit_dht支持上下文管理器，会自动在代码块结束时释放资源，写法更简洁：

#!/usr/bin/python3
import sys
import board
import time

import adafruit_dht

with adafruit_dht.DHT11(board.D17) as dhtDevice:
    try:
            # Print the values to the serial port
            temperature_c = dhtDevice.temperature
            temperature_f = temperature_c * (9 / 5) + 32
            humidity = dhtDevice.humidity
            print("%-3.1f " % temperature_c + " " + "%-3.1f " % humidity)
            print(
                "Temp: {:.1f} F / {:.1f} C    Humidity: {}% ".format(
                    temperature_f, temperature_c, humidity
                )
            )
           
    except RuntimeError as error:
            # Errors happen fairly often, DHT's are hard to read, just keep going
            print(error.args[0])
            time.sleep(2.0)

    except Exception as error:
            raise error

两种方法都能确保脚本结束时正确释放资源，消除Lost access to message queue的提示。

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