painter

(You can also check this code in the repository)

from PySide6.QtWidgets import (
    QWidget,
    QMainWindow,
    QApplication,
    QFileDialog,
    QStyle,
    QColorDialog,
)
from PySide6.QtCore import QPoint, Qt, QDir, Slot, QStandardPaths
from PySide6.QtGui import (
    QMouseEvent,
    QPaintEvent,
    QPen,
    QAction,
    QPainter,
    QColor,
    QPixmap,
    QIcon,
    QKeySequence,
)
import sys


class PainterWidget(QWidget):
    """A widget where user can draw with their mouse

    The user draws on a QPixmap which is itself paint from paintEvent()

    """

    def __init__(self, parent=None):
        super().__init__(parent)

        self.setFixedSize(680, 480)
        self.pixmap = QPixmap(self.size())
        self.pixmap.fill(Qt.white)

        self.previous_pos = None
        self.painter = QPainter()
        self.pen = QPen()
        self.pen.setWidth(10)
        self.pen.setCapStyle(Qt.RoundCap)
        self.pen.setJoinStyle(Qt.RoundJoin)

    def paintEvent(self, event: QPaintEvent):
        """Override method from QWidget

        Paint the Pixmap into the widget

        """
        painter = QPainter()
        painter.begin(self)
        painter.drawPixmap(0, 0, self.pixmap)
        painter.end()

    def mousePressEvent(self, event: QMouseEvent):
        """Override from QWidget

        Called when user clicks on the mouse

        """
        self.previous_pos = event.position().toPoint()
        QWidget.mousePressEvent(self, event)

    def mouseMoveEvent(self, event: QMouseEvent):
        """Override method from QWidget

        Called when user moves and clicks on the mouse

        """
        current_pos = event.position().toPoint()
        self.painter.begin(self.pixmap)
        self.painter.setRenderHints(QPainter.Antialiasing, True)
        self.painter.setPen(self.pen)
        self.painter.drawLine(self.previous_pos, current_pos)
        self.painter.end()

        self.previous_pos = current_pos
        self.update()

        QWidget.mouseMoveEvent(self, event)

    def mouseReleaseEvent(self, event: QMouseEvent):
        """Override method from QWidget

        Called when user releases the mouse

        """
        self.previous_pos = None
        QWidget.mouseReleaseEvent(self, event)

    def save(self, filename: str):
        """ save pixmap to filename """
        self.pixmap.save(filename)

    def load(self, filename: str):
        """ load pixmap from filename """
        self.pixmap.load(filename)
        self.pixmap = self.pixmap.scaled(self.size(), Qt.KeepAspectRatio)
        self.update()

    def clear(self):
        """ Clear the pixmap """
        self.pixmap.fill(Qt.white)
        self.update()


class MainWindow(QMainWindow):
    """An Application example to draw using a pen """

    def __init__(self, parent=None):
        QMainWindow.__init__(self, parent)

        self.painter_widget = PainterWidget()
        self.bar = self.addToolBar("Menu")
        self.bar.setToolButtonStyle(Qt.ToolButtonTextBesideIcon)
        self._save_action = self.bar.addAction(
            qApp.style().standardIcon(QStyle.SP_DialogSaveButton), "Save", self.on_save
        )
        self._save_action.setShortcut(QKeySequence.Save)
        self._open_action = self.bar.addAction(
            qApp.style().standardIcon(QStyle.SP_DialogOpenButton), "Open", self.on_open
        )
        self._open_action.setShortcut(QKeySequence.Open)
        self.bar.addAction(
            qApp.style().standardIcon(QStyle.SP_DialogResetButton),
            "Clear",
            self.painter_widget.clear,
        )
        self.bar.addSeparator()

        self.color_action = QAction(self)
        self.color_action.triggered.connect(self.on_color_clicked)
        self.bar.addAction(self.color_action)

        self.setCentralWidget(self.painter_widget)

        self.set_color(Qt.black)

        self.mime_type_filters = ["image/png", "image/jpeg"]

    @Slot()
    def on_save(self):

        dialog = QFileDialog(self, "Save File")
        dialog.setMimeTypeFilters(self.mime_type_filters)
        dialog.setFileMode(QFileDialog.AnyFile)
        dialog.setAcceptMode(QFileDialog.AcceptSave)
        dialog.setDefaultSuffix("png")
        dialog.setDirectory(
            QStandardPaths.writableLocation(QStandardPaths.PicturesLocation)
        )

        if dialog.exec() == QFileDialog.Accepted:
            if dialog.selectedFiles():
                self.painter_widget.save(dialog.selectedFiles()[0])

    @Slot()
    def on_open(self):

        dialog = QFileDialog(self, "Save File")
        dialog.setMimeTypeFilters(self.mime_type_filters)
        dialog.setFileMode(QFileDialog.ExistingFile)
        dialog.setAcceptMode(QFileDialog.AcceptOpen)
        dialog.setDefaultSuffix("png")
        dialog.setDirectory(
            QStandardPaths.writableLocation(QStandardPaths.PicturesLocation)
        )

        if dialog.exec() == QFileDialog.Accepted:
            if dialog.selectedFiles():
                self.painter_widget.load(dialog.selectedFiles()[0])

    @Slot()
    def on_color_clicked(self):

        color = QColorDialog.getColor(Qt.black, self)
        if color:
            self.set_color(color)

    def set_color(self, color: QColor = Qt.black):

        # Create color icon
        pix_icon = QPixmap(32, 32)
        pix_icon.fill(color)

        self.color_action.setIcon(QIcon(pix_icon))
        self.painter_widget.pen.setColor(color)
        self.color_action.setText(QColor(color).name())


if __name__ == "__main__":

    app = QApplication(sys.argv)

    w = MainWindow()
    w.show()
    sys.exit(app.exec())