Color change for new drawings without changing the color of existing drawings

Question

The given code below is derived from another question on SO. It displays a QMainWindow with 4 QGraphicsView to draw with the mouse in it, a QPushButton to clear the 4 QGraphicsView, and a QPushButton to get a new random color for drawing.

Goal: I would like to change the color of future drawings without changing the color of existing drawings.

Attempt: Changing the color of a QPen object immediately changed the color of the drawings already drawn. Therefore I created new GraphicsPathItem objects with new QPen objects with the new color to avoid this.

Problem: After a color change the color of existing drawings in a QGraphicsView changes as soon as you draw something in it again.

How can this problem be solved?

---

main.py

import sys
from random import choice

from PyQt5.QtWidgets import QApplication, QMainWindow, QGraphicsView, QGraphicsScene, QGraphicsPathItem
from PyQt5.QtGui import QPainterPath, QPen, QColor
from PyQt5.QtCore import Qt
from PyQt5.uic import loadUi

# Based on code from https://stackoverflow.com/a/44248794/7481773


class MainWindow(QMainWindow):
    COLORS = ("black", "red", "green", "blue", "yellow")

    def __init__(self):
        super().__init__()
        loadUi("mainwindow.ui", self)
        self.color = "black"

        self.layouts = (self.verticalLayout_top_left, self.verticalLayout_top_right,
                        self.verticalLayout_bottom_left, self.verticalLayout_bottom_right)

        self._views = []

        for layout in self.layouts:
            graphics_view = GraphicsView(self.color)
            self._views.append(graphics_view)
            layout.addWidget(graphics_view)

        self.clear_button.clicked.connect(self.clear_views)
        self.color_button.clicked.connect(self.update_color)

    def _get_new_random_color(self):
        new_colors = list(self.COLORS)
        new_colors.remove(self.color)
        return choice(new_colors)

    def clear_views(self):
        for view in self._views:
            view.clear_view()

    def update_color(self):
        new_color = self._get_new_random_color()
        self.color = new_color
        for view in self._views:
            view.new_item(new_color)


class GraphicsView(QGraphicsView):
    def __init__(self, color):
        super().__init__()
        self.start = None
        self.end = None
        self.item = None

        self.setScene(QGraphicsScene())
        self.path = QPainterPath()
        self.new_item(color)

        self.contents_rect = self.contentsRect()
        self.setSceneRect(0, 0, self.contents_rect.width(), self.contents_rect.height())
        self.setHorizontalScrollBarPolicy(Qt.ScrollBarAlwaysOff)
        self.setVerticalScrollBarPolicy(Qt.ScrollBarAlwaysOff)

    def clear_view(self):
        try:
            self.path.clear()
        except AttributeError:
            self.path = QPainterPath()
        self.item.setPath(self.path)

    def new_item(self, color):
        self.item = GraphicsPathItem(color)
        self.scene().addItem(self.item)

    def mousePressEvent(self, event):
        self.start = self.mapToScene(event.pos())
        self.path.moveTo(self.start)

    def mouseMoveEvent(self, event):
        self.end = self.mapToScene(event.pos())
        self.path.lineTo(self.end)
        self.start = self.end
        self.item.setPath(self.path)


class GraphicsPathItem(QGraphicsPathItem):

    def __init__(self, color):
        super().__init__()
        self.pen = QPen()
        self.pen.setColor(QColor(color))
        self.pen.setWidth(5)
        self.setPen(self.pen)


def main():
    app = QApplication(sys.argv)
    main_window = MainWindow()
    main_window.show()
    app.exec_()
    del main_window, app


if __name__ == "__main__":
    main()

---

mainwindow.ui

<?xml version="1.0" encoding="UTF-8"?>
<ui version="4.0">
 <class>MainWindow</class>
 <widget class="QMainWindow" name="MainWindow">
  <property name="geometry">
   <rect>
    <x>0</x>
    <y>0</y>
    <width>800</width>
    <height>600</height>
   </rect>
  </property>
  <property name="windowTitle">
   <string>Paint and Clear</string>
  </property>
  <property name="locale">
   <locale language="English" country="UnitedKingdom"/>
  </property>
  <widget class="QWidget" name="centralwidget">
   <layout class="QGridLayout" name="gridLayout">
    <item row="2" column="0">
     <layout class="QVBoxLayout" name="verticalLayout_bottom_left"/>
    </item>
    <item row="2" column="1">
     <layout class="QVBoxLayout" name="verticalLayout_bottom_right"/>
    </item>
    <item row="0" column="0">
     <layout class="QVBoxLayout" name="verticalLayout_top_left"/>
    </item>
    <item row="0" column="1">
     <layout class="QVBoxLayout" name="verticalLayout_top_right"/>
    </item>
    <item row="1" column="0">
     <widget class="QPushButton" name="clear_button">
      <property name="text">
       <string>Clear</string>
      </property>
     </widget>
    </item>
    <item row="1" column="1">
     <widget class="QPushButton" name="color_button">
      <property name="text">
       <string>New random color</string>
      </property>
     </widget>
    </item>
   </layout>
  </widget>
  <widget class="QMenuBar" name="menubar">
   <property name="geometry">
    <rect>
     <x>0</x>
     <y>0</y>
     <width>800</width>
     <height>24</height>
    </rect>
   </property>
  </widget>
  <widget class="QStatusBar" name="statusbar"/>
 </widget>
 <resources/>
 <connections/>
</ui>

Answer 1

Each time you create a new item, you are still using the same QPainterPath, so what you are seeing is not the new color applied on existing "drawings", but a new QGraphicsPathItem item that still uses the previous path in it.

You can easily solve your issue by creating a new path each time a new item is created:

    def new_item(self, color):
        self.item = GraphicsPathItem(color)
        self.scene().addItem(self.item)
        self.path = QPainterPath()