PySide2中backend_qt报错：self.window().windowHandle()返回None求助

问题描述 #

使用Python 3.8.10（无法自主选择版本）结合PySide2开发GUI窗口，嵌入matplotlib图表时，backend_qt包出现报错。示例代码中该问题影响不大，但集成到完整项目后会导致窗口崩溃或显示异常。报错在窗口打开和关闭时各出现一次，根源是self.window().windowHandle()返回了None，但不清楚原因。

示例代码 #

import sys
from matplotlib import pyplot as plt
from PySide2.QtWidgets import (QHBoxLayout, QWidget, QApplication)
from matplotlib.backends.backend_qt5agg import FigureCanvasQTAgg

class MplCanvas(FigureCanvasQTAgg):
    # Create canavs to display graph
    def __init__(self, parent, width=15, height=4, dpi=100):
        fig, self.axis = plt.subplots(figsize=(width, height), dpi=dpi)
        super().__init__(fig)
        self.setParent(parent)
        self.window = FigureCanvasQTAgg

class OutputWindow(QWidget):
    def __init__(self):
        super().__init__()

        self.chart = MplCanvas(self)
        self.create_params_box()

    def create_params_box(self):
        self.hbox = QHBoxLayout()
        self.hbox.addWidget(self.chart)

app = QApplication(sys.argv)
demo = OutputWindow()
demo.setLayout(demo.hbox)
demo.showMaximized()
sys.exit(app.exec_())

报错信息 #

Traceback (most recent call last):
  File "~\venv\lib\site-packages\matplotlib\backends\backend_qt.py", line 276, in showEvent
    window.screenChanged.connect(self._update_screen)
AttributeError: 'NoneType' object has no attribute 'screenChanged'

backend_qt中的相关代码 #

def showEvent(self, event):
    window = self.window().windowHandle()
    window.screenChanged.connect(self._update_screen)
    self._update_screen(window.screen())

已尝试操作 #

• 将原始代码精简为示例代码
• 更换PySide2版本至5.15.2、5.15.1、5.14.x
• 示例代码可正常运行，但集成到完整项目时窗口崩溃

解决方案 #

1. 修复MplCanvas错误赋值并增加空值判断 #

原代码中self.window = FigureCanvasQTAgg是错误赋值，会覆盖Qt的window()方法，这是引发问题的关键原因之一。同时重写showEvent方法增加空值判断：

import sys
from matplotlib import pyplot as plt
from PySide2.QtWidgets import (QHBoxLayout, QWidget, QApplication)
from matplotlib.backends.backend_qt5agg import FigureCanvasQTAgg

class MplCanvas(FigureCanvasQTAgg):
    def __init__(self, parent, width=15, height=4, dpi=100):
        fig, self.axis = plt.subplots(figsize=(width, height), dpi=dpi)
        super().__init__(fig)
        self.setParent(parent)
        # 移除错误的self.window赋值

    def showEvent(self, event):
        window = self.window().windowHandle()
        if window is not None:
            window.screenChanged.connect(self._update_screen)
            self._update_screen(window.screen())
        super().showEvent(event)

class OutputWindow(QWidget):
    def __init__(self):
        super().__init__()
        self.chart = MplCanvas(self)
        self.create_params_box()
        self.setLayout(self.hbox)  # 提前设置布局，避免时机问题

    def create_params_box(self):
        self.hbox = QHBoxLayout()
        self.hbox.addWidget(self.chart)

app = QApplication(sys.argv)
demo = OutputWindow()
demo.showMaximized()
sys.exit(app.exec_())

2. 延迟初始化画布确保窗口完全创建 #

在窗口的showEvent中初始化图表，保证窗口句柄已生成：

import sys
from matplotlib import pyplot as plt
from PySide2.QtWidgets import (QHBoxLayout, QWidget, QApplication)
from matplotlib.backends.backend_qt5agg import FigureCanvasQTAgg

class MplCanvas(FigureCanvasQTAgg):
    def __init__(self, parent, width=15, height=4, dpi=100):
        fig, self.axis = plt.subplots(figsize=(width, height), dpi=dpi)
        super().__init__(fig)
        self.setParent(parent)

class OutputWindow(QWidget):
    def __init__(self):
        super().__init__()
        self.chart = None
        self.create_params_box()
        self.setLayout(self.hbox)

    def create_params_box(self):
        self.hbox = QHBoxLayout()

    def showEvent(self, event):
        if self.chart is None:
            self.chart = MplCanvas(self)
            self.hbox.addWidget(self.chart)
        super().showEvent(event)

app = QApplication(sys.argv)
demo = OutputWindow()
demo.showMaximized()
sys.exit(app.exec_())

3. 避免使用pyplot全局状态，直接创建Figure对象 #

减少pyplot全局状态带来的潜在冲突：

import sys
from matplotlib.figure import Figure
from PySide2.QtWidgets import (QHBoxLayout, QWidget, QApplication)
from matplotlib.backends.backend_qt5agg import FigureCanvasQTAgg

class MplCanvas(FigureCanvasQTAgg):
    def __init__(self, parent, width=15, height=4, dpi=100):
        fig = Figure(figsize=(width, height), dpi=dpi)
        self.axis = fig.add_subplot(111)
        super().__init__(fig)
        self.setParent(parent)

    def showEvent(self, event):
        window = self.window().windowHandle()
        if window is not None:
            window.screenChanged.connect(self._update_screen)
            self._update_screen(window.screen())
        super().showEvent(event)

class OutputWindow(QWidget):
    def __init__(self):
        super().__init__()
        self.chart = MplCanvas(self)
        self.create_params_box()
        self.setLayout(self.hbox)

    def create_params_box(self):
        self.hbox = QHBoxLayout()
        self.hbox.addWidget(self.chart)

app = QApplication(sys.argv)
demo = OutputWindow()
demo.showMaximized()
sys.exit(app.exec_())

内容的提问来源于stack exchange，提问作者jtparm2