Python PyQt5 drawing application

Python PyQt Event Handling: Exercise-8 with Solution

Write a Python program to build a PyQt5 application that allows users to draw shapes and freehand lines on a canvas.

From doc.qt.io:

QApplication Class: The QApplication class manages the GUI application's control flow and main settings.

QMainWindow Class: The QMainWindow class provides a main application window.

QWidget Class: The QWidget class is the base class of all user interface objects.

QPainter Class: The QPainter class performs low-level painting on widgets and other paint devices.

QPen Class: The QPen class defines how a QPainter should draw lines and outlines of shapes.

Qt module: PyQt5 is a set of Python bindings for the Qt application framework. It allows us to use Qt, a popular C++ framework, to create graphical user interfaces (GUIs) in Python.

QPoint Class: The QPoint class defines a point in the plane using integer precision.

Sample Solution:

Python Code:

import sys
from PyQt5.QtWidgets import QApplication, QMainWindow, QWidget
from PyQt5.QtGui import QPainter, QPen
from PyQt5.QtCore import Qt, QPoint

class DrawingApp(QMainWindow):
    def __init__(self):
        super().__init__()

        self.setWindowTitle("Drawing Application")
        self.setGeometry(100, 100, 800, 600)

        self.canvas = CanvasWidget(self)
        self.setCentralWidget(self.canvas)
        
class CanvasWidget(QWidget):
    def __init__(self, parent=None):
        super().__init__(parent)
        self.parent = parent
        self.setGeometry(0, 0, 800, 600)

        self.drawing = False
        self.last_point = QPoint()
        self.current_point = QPoint()
        self.shapes = []

    def mousePressEvent(self, event):
        if event.button() == Qt.LeftButton:
            self.drawing = True
            self.last_point = event.pos()
            self.current_point = event.pos()

    def mouseMoveEvent(self, event):
        if self.drawing:
            self.current_point = event.pos()
            self.update()

    def mouseReleaseEvent(self, event):
        if event.button() == Qt.LeftButton:
            self.drawing = False
            shape = (self.last_point, self.current_point)
            self.shapes.append(shape)
            self.update()

    def paintEvent(self, event):
        painter = QPainter(self)
        pen = QPen()
        pen.setWidth(2)
        pen.setColor(Qt.black)
        painter.setPen(pen)

        for shape in self.shapes:
            painter.drawLine(shape[0], shape[1])

        if self.drawing:
            painter.drawLine(self.last_point, self.current_point)

def main():
    app = QApplication(sys.argv)
    window = DrawingApp()
    window.show()
    sys.exit(app.exec_())

if __name__ == "__main__":
    main()

Explanation:

In the exercise above -

• Import the necessary PyQt5 modules.
• "mousePressEvent" starts drawing when the left mouse button is clicked, and "mouseReleaseEvent" stops drawing when the button is released.
• "mouseMoveEvent" updates the current line endpoint while drawing.
• Maintain a list of line shapes in the self.shapes list.
• In the "paintEvent()" method, we use a QPainter to draw the shapes and lines on the canvas.
• The main function sets up the application and runs it.

Output:

Flowchart:

Python Code Editor:

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