Python PyQt5 drawing application

Last update on December 21 2024 07:44:51 (UTC/GMT +8 hours)

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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