QPixmap / QPainter 显示黑色 window 背景

Question

我正在按照 Martin Fitzpatrick 的 PyQt5 书中的示例进行操作。当我运行下面的代码时，背景是黑色的，没有画线：

import sys
from PyQt5 import QtCore, QtGui, QtWidgets, uic
from PyQt5.QtCore import Qt

class MainWindow(QtWidgets.QMainWindow):
    def __init__(self):
        super().__init__()
        
        self.label = QtWidgets.QLabel()

        canvas = QtGui.QPixmap(400, 300)

        self.label.setPixmap(canvas)
        self.setCentralWidget(self.label)
        self.draw_something()

    def draw_something(self):
        painter = QtGui.QPainter(self.label.pixmap())
        painter.drawLine(10, 10, 300, 200)
        painter.end()


app = QtWidgets.QApplication(sys.argv)
window = MainWindow()
window.show()
app.exec_()

左边是预期的结果：

Answer 1

默认情况下，QPixmap 使用的内存不会被效率清理，因此它的字节不会被更改，如 the docs 所示：

QPixmap::QPixmap(int width, int height)
Constructs a pixmap with the given width and height. If either width or height is zero, a null pixmap is constructed.

Warning: This will create a QPixmap with uninitialized data. Call fill() to fill the pixmap with an appropriate color before drawing onto it with QPainter.

_{（强调我的）}

解决方法是使用fill来设置背景色：

canvas = QtGui.QPixmap(400, 300)
canvas.fill(QtGui.QColor("white"))

QPixmap / QPainter 显示黑色 window 背景

QPixmap / QPainter showing black window background

python

pyqt5

qpainter

qpixmap

QPixmap::QPixmap(int width, int height)