<code C++>
#ifndef VIEW_H
#define VIEW_H

#include <QWidget>

QT_BEGIN_NAMESPACE
namespace Ui { class View; }
QT_END_NAMESPACE

class View : public QWidget
{
    Q_OBJECT

public:
    View(QWidget *parent = nullptr);
    ~View();

private:
    Ui::View *ui;


// QWidget interface
protected:
    void paintEvent(QPaintEvent *event) override;
};

#endif // VIEW_H
</code>

<code C++>
#include "view.h"
#include "ui_view.h"
#include <QApplication>

View::View(QWidget *parent)
    : QWidget(parent)
    , ui(new Ui::View)
{
    ui->setupUi(this);
}

View::~View()
{
    delete ui;
}

#include <QPainter>

void View::paintEvent (QPaintEvent *event)
{
    int w = width ();
    int h = height ();
    QPainter p (this);

    QLinearGradient g (QPointF(0, 0), QPointF(w, h));
    g.setColorAt (0, QColor ("red"));
    g.setColorAt (1, QColor ("lime"));

    p.setPen (QPen (QColor ("blue"), 3));
    p.setBrush(QBrush (g));
    // p.drawLine (0, 0, 100, 100);
    p.drawEllipse (0, 0, w, h);
}

int main (int argc, char *argv[])
{
    QApplication a(argc, argv);
    View w;
    w.show();
    return a.exec();
}
</code>

<code>
QT       += core gui widgets
# CONFIG += c++11
SOURCES +=  view.cc
HEADERS += view.h
FORMS += view.ui
</code>

<code Python>
import sys
from PyQt5.QtCore import *
from PyQt5.QtGui import *
from PyQt5.QtWidgets import *

class Widget (QWidget) :

    def __init__ (self) :
        super().__init__()

    def color (self, painter, r, g, b) :
        painter.setPen (QColor.fromRgb (r, g, b))

    def line (self, painter, x1, y1, x2, y2) :
        painter.drawLine (x1, y1, x2, y2)

    def paintEvent (self, event) : # QPaintEvent *event
        p = QPainter ()
        p.begin (self)

        w = self.width ()
        h = self.height ()

        self.color (p, 255, 0, 0)
        self.line  (p, 0, 0, w-1, 0)

        self.color (p, 0, 255, 0)
        self.line  (p, w-1, 0, w-1, h-1)

        self.color (p, 0, 0, 255)
        self.line  (p, w-1, h-1, 0, h-1)

        self.color (p, 255, 200, 0)
        self.line  (p, 0, h-1, 0, 0)

        p.end ()

if __name__ == "__main__" :
    app = QApplication (sys.argv)
    win = Widget ()
    win.show ()
    sys.exit (app.exec_())
</code>