QPixmap / QPainter showing black window background

Question

I am following an example from the PyQt5 book by Martin Fitzpatrick. When I run the following code, the background is black and the line is not drawn:

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_()

The expected result is on the left:

Answer 1

By default the memory that a QPixmap uses is not cleaned by efficiency so it has the bytes not altered as indicated by the docs:

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.

_{(emphasis mine)}

The solution is to use fill to set the background color:

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