Python程序在检查变量[重复]的条件时重置

Question

我正在开发一个与串口通信的小程序。我使用pyQt5开发了一个GUI。

首先，程序将读取

点击按钮时出现问题

我的代码如下：

import PyQt5
from PyQt5 import QtCore, QtGui, uic, QtWidgets
from PyQt5.QtWidgets import QWidget
import serial
import serial.tools.list_ports

portOpen = 0 #variable to store the port open/close status.If open,value         will be 1
comPorts = list() #create a list named comPorts

mySer = serial.Serial()
mySer.baudrate = 9600
mySer.bytesize = 8
mySer.parity = 'N'
mySer.stopbits = 1
ports = serial.tools.list_ports.comports()

#read the available com ports & save to a list so the list will be like -    [COM0,COM1,COM2---]
def ReadComPorts():
    comPorts.clear() #clear the values of the list
    for port, desc, hwid in sorted(ports):
        comPorts.append(("{}".format(port)))

#These below mentioned part is used to adjust the view of the app to high resolution DPI of windows
if hasattr(QtCore.Qt, 'AA_EnableHighDpiScaling'):
    PyQt5.QtWidgets.QApplication.setAttribute(QtCore.Qt.AA_EnableHighDpiScaling, True)

if hasattr(QtCore.Qt, 'AA_UseHighDpiPixmaps'):
    PyQt5.QtWidgets.QApplication.setAttribute(QtCore.Qt.AA_UseHighDpiPixmaps, True)

#pyQt5 generated code
class Ui_MainWindow(object):

    def setupUi(self, MainWindow):
#read the com ports first
        ReadComPorts()
        MainWindow.setObjectName("MainWindow")
        MainWindow.resize(273, 200)
        self.centralwidget = QtWidgets.QWidget(MainWindow)
        self.centralwidget.setObjectName("centralwidget")
        self.listWidget = QtWidgets.QListWidget(self.centralwidget)
        self.listWidget.setGeometry(QtCore.QRect(10, 10, 91, 121))
        self.listWidget.setObjectName("listWidget")

#Now,add the read com ports into the list         
        for x in range(len(comPorts)):
            self.listWidget.addItem(comPorts[x])

#When,the an item on list will be clicked,call the item_click function
        self.listWidget.itemClicked.connect(self.item_click)

        self.pushButton = QtWidgets.QPushButton(self.centralwidget)
        self.pushButton.setGeometry(QtCore.QRect(30, 150, 56, 17))
        self.pushButton.setObjectName("pushButton")
        MainWindow.setCentralWidget(self.centralwidget)
        self.statusbar = QtWidgets.QStatusBar(MainWindow)
        self.statusbar.setObjectName("statusbar")
        MainWindow.setStatusBar(self.statusbar)

        self.retranslateUi(MainWindow)
        QtCore.QMetaObject.connectSlotsByName(MainWindow)

#When the pushbutton will be clicked,call portOpen/close function        
        self.pushButton.clicked.connect(self.PortOpenClose)

    def item_click(self, item):
        comPorts[0] = str(item.text()) #Store the selected port to comPorts  0 location
        print(comPorts[0])

#Open/close the port depending on the current port status
    def PortOpenClose(self):
        if(not(portOpen)):
            try:
                mySer.port = comPorts[0]
                mySer.open()
                self.pushButton.setText("Close Port")
                print('Port opened')
                print(portOpen)
                portOpen = 1 #this line is creating forcing the whole program to close
                                #If i omit this line,port is getting opened properly          
            except:
                print('Error occured')                



    def retranslateUi(self, MainWindow):
        _translate = QtCore.QCoreApplication.translate
        MainWindow.setWindowTitle(_translate("MainWindow", "MainWindow"))
        self.pushButton.setText(_translate("MainWindow", "Open Port"))


if __name__ == "__main__":
    import sys
    app = QtWidgets.QApplication(sys.argv)
    MainWindow = QtWidgets.QMainWindow()
    ui = Ui_MainWindow()
    ui.setupUi(MainWindow)
    MainWindow.show()
    sys.exit(app.exec_())

Answer 1

我已经在评论中回答了，但是我没有时间发表很长的解释为什么我的建议有效：

问题在于Python如何处理全局和局部变量。

基本上，如果不需要全局变量，则应避免全局变量。在许多语言中，意外更改全局变量的值太容易了，因此Python让我们显式地声明要更改它-使用global关键字。

如果只从中读取全局变量，则代码可以工作，因为Python按顺序查找变量：不是本地变量？进一步检查。。。以此类推，直到它触及全球范围。

但是，通过添加赋值（不带global关键字），Python在局部范围内看到它，并假设该变量在整个函数中是局部的-并使其失败，因为变量是在赋值之前读取的-失败的是print（portOpen），而不是赋值本身（！）。（谢天谢地，它们靠得太近了，错误消息意外地起到了帮助作用。）

global-portOpen向解释器显示赋值没有创建新的局部变量，因此所有读取和赋值都开始正常工作。