Python3.8+PySide2：QMainWindow实例closeEvent内置方法替换失效问题

问题背景 #

尝试替换QMainWindow类实例的内置closeEvent方法时，出现两种代码表现不一致的情况：

CODE #1（不生效） #

import sys
from PySide2.QtUiTools import QUiLoader
from PySide2.QtWidgets import QApplication
from PySide2.QtCore import QFile, QIODevice

app = QApplication(sys.argv)
ui_file_name = "ui\Main.ui"
ui_file = QFile(ui_file_name)
if not ui_file.open(QIODevice.ReadOnly):
    print("Cannot open {}: {}".format(ui_file_name, ui_file.errorString()))
    sys.exit(-1)
loader = QUiLoader()
window = loader.load(ui_file)
ui_file.close()
if not window:
    print(loader.errorString())
    sys.exit(-1)

def MainFormCloseEvent(event):
    print(event)
    event.ignore()

print(window.closeEvent)
window.closeEvent=MainFormCloseEvent
print(window.closeEvent)
window.show()
sys.exit(app.exec_())

运行输出：

<built-in method closeEvent of PySide2.QtWidgets.QMainWindow object at 0x000000000573BF80>
<function MainFormCloseEvent at 0x0000000002C37430>

窗体关闭时不会调用MainFormCloseEvent。

CODE #2（正常生效） #

import sys
from PySide2.QtUiTools import QUiLoader
from PySide2.QtWidgets import QApplication
from PySide2.QtCore import QFile, QIODevice
from PySide2.QtWidgets import QMainWindow

def MainFormCloseEvent(event):
    print(event)
    event.ignore()

class MainWindow(QMainWindow):
    def __init__(self):
        super().__init__()
    def closeEvent(self, event):
        print('Original class method')

app = QApplication(sys.argv)
window = MainWindow()
print(window.closeEvent)
window.closeEvent=MainFormCloseEvent
print(window.closeEvent)
window.show()

sys.exit(app.exec_())

运行输出：

<bound method MainWindow.closeEvent of <__main__.MainWindow(0x51522b0) at 0x0000000004E8AF40>>
<function MainFormCloseEvent at 0x0000000002C37430>
<PySide2.QtGui.QCloseEvent object at 0x0000000004E8F340>

窗体关闭时正常调用MainFormCloseEvent。

根本差异解析 #

两种情况的核心区别在于closeEvent方法的底层实现：

1. CODE #1中的closeEvent是C++内置方法：通过QUiLoader加载的是Qt原生的QMainWindow实例，其closeEvent是C层面实现的内置方法。Qt的事件分发系统会直接调用C方法，不会触发Python层面的实例属性查找，因此Python赋值的新方法无法被调用。
2. CODE #2中的closeEvent是Python绑定方法：自定义的MainWindow类继承自QMainWindow，并重写了closeEvent，此时该方法是Python层面的绑定方法。Qt的元对象系统会将事件转发到Python的方法实现，Python的实例属性优先级高于类方法，因此直接赋值实例的closeEvent可以覆盖原方法。

让CODE #1生效的解决方案 #

方案1：使用事件过滤器拦截CloseEvent #

通过给窗口安装事件过滤器，直接拦截CloseEvent事件：

import sys
from PySide2.QtUiTools import QUiLoader
from PySide2.QtWidgets import QApplication
from PySide2.QtCore import QFile, QIODevice, QObject, QEvent

app = QApplication(sys.argv)
ui_file_name = "ui\Main.ui"
ui_file = QFile(ui_file_name)
if not ui_file.open(QIODevice.ReadOnly):
    print("Cannot open {}: {}".format(ui_file_name, ui_file.errorString()))
    sys.exit(-1)
loader = QUiLoader()
window = loader.load(ui_file)
ui_file.close()
if not window:
    print(loader.errorString())
    sys.exit(-1)

class CloseEventFilter(QObject):
    def eventFilter(self, obj, event):
        if obj == window and event.type() == QEvent.Close:
            print(event)
            event.ignore()
            return True  # 返回True表示事件已处理，不再传递
        return super().eventFilter(obj, event)

# 安装事件过滤器
event_filter = CloseEventFilter()
window.installEventFilter(event_filter)

window.show()
sys.exit(app.exec_())

方案2：继承QMainWindow并加载UI到自定义类 #

自定义QMainWindow子类，将UI文件加载到子类实例中，这样可以直接重写closeEvent：

import sys
from PySide2.QtUiTools import QUiLoader
from PySide2.QtWidgets import QApplication, QMainWindow
from PySide2.QtCore import QFile, QIODevice

class CustomMainWindow(QMainWindow):
    def closeEvent(self, event):
        print(event)
        event.ignore()

app = QApplication(sys.argv)
ui_file_name = "ui\Main.ui"
ui_file = QFile(ui_file_name)
if not ui_file.open(QIODevice.ReadOnly):
    print("Cannot open {}: {}".format(ui_file_name, ui_file.errorString()))
    sys.exit(-1)

loader = QUiLoader()
window = CustomMainWindow()
# 将加载的UI设置为窗口的中央部件
ui_widget = loader.load(ui_file, window)
window.setCentralWidget(ui_widget)

ui_file.close()
if not ui_widget:
    print(loader.errorString())
    sys.exit(-1)

window.show()
sys.exit(app.exec_())

内容的提问来源于stack exchange，提问作者Nikolas