#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

#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();
}

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

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_())