Cannot change value of a QSlider from a QWidget

Question

I am trying to change the value of a QWidget from another QWidget, but nothing is happening. I am using this line to call the changeValue function from each Filter object:

self.filter1.valueChanged.connect(self.filter1.changeValue)

I have put the print(value) statement inside the changeValue method to see if the method is being called, but it is not.

This is my code:

import sys
import numpy as np
import cv2
from PyQt5.QtCore import Qt
from PyQt5.QtGui import QPixmap, QImage
from PyQt5.QtWidgets import (QWidget, QLabel, QHBoxLayout, QVBoxLayout,
                             QApplication, QPushButton, QSlider,
                             QFileDialog)


class Filter(QSlider):
    """Common base for all filters"""
    defaultK = 3
    filterCount = 0

    def __init__(self):
        super(Filter, self).__init__()

        # Variable for the constant of the OpenCV filter
        self.k = 3

        # Label for the slider
        self.k_lbl = QLabel(str(self.k))

        # Increase the number of filters created
        Filter.filterCount += 1

        # Slider for the first OpenCV filter, with min, max, default and step values
        self.thresh_sld = QSlider(Qt.Horizontal, self)
        self.thresh_sld.setFocusPolicy(Qt.NoFocus)
        self.thresh_sld.setMinimum(3)
        self.thresh_sld.setMaximum(51)
        self.thresh_sld.setValue(self.k)
        self.thresh_sld.setSingleStep(2)

    def changeValue(self, value):
        # Function for setting the value of k1

        print(value)

        if value % 2 == 1:
            self.k = value
        else:
            self.k = value + 1

        self.thresh_sld.setValue(self.k)
        self.k_lbl.setText(str(self.k))

class MainWindow(QWidget):
    def __init__(self):
        super(MainWindow, self).__init__()

        self.filter1 = Filter()
        self.filter2 = Filter()

        # Function sending the slider signal to the processing function
        self.filter1.valueChanged.connect(self.filter1.changeValue)
        self.filter2.valueChanged.connect(self.filter2.changeValue)

        # Creates the main layout (vertical)
        v_main_lay = QVBoxLayout()
        # Adds the sliders and their labels to the bottom of the main layout
        v_main_lay.addWidget(self.filter1.k_lbl)
        v_main_lay.addWidget(self.filter1.thresh_sld)
        v_main_lay.addWidget(self.filter2.k_lbl)
        v_main_lay.addWidget(self.filter2.thresh_sld)

        # Sets the main layout
        self.setLayout(v_main_lay)

        # Sets the geometry, position, window title and window default mode
        self.setGeometry(300, 300, 350, 300)
        self.setWindowTitle('Review')
        self.showMaximized()

if __name__ == '__main__':
    app = QApplication(sys.argv)
    window = MainWindow()
    sys.exit(app.exec_())

Answer 1

The problem is that you are inheriting from QSlider and at the same time you are creating within that class an attribute called thresh_sld, and this is the element that is showing in the view, but not connected to changeValue, the slider that is connected to ChangeValue is the Filter class Slider.

Use simple solution is to connect the signal of the attribute thresh_sld to the slot provided, you must change the following:

self.filter1.valueChanged.connect(self.filter1.changeValue)
self.filter2.valueChanged.connect(self.filter2.changeValue)

to:

self.filter1.thresh_sld.valueChanged.connect(self.filter1.changeValue)
self.filter2.thresh_sld.valueChanged.connect(self.filter2.changeValue)

A more elegant solution is to change the Filter base class from QSlider to QWidget, and add the label and QSlider to its layout.

class Filter(QWidget):
    """Common base for all filters"""
    defaultK = 3
    filterCount = 0

    def __init__(self):
        super(Filter, self).__init__()
        lay = QVBoxLayout(self)

        # Variable for the constant of the OpenCV filter
        self.k = 3

        # Label for the slider
        self.k_lbl = QLabel(str(self.k))

        # Increase the number of filters created
        Filter.filterCount += 1

        # Slider for the first OpenCV filter, with min, max, default and step values
        self.thresh_sld = QSlider(Qt.Horizontal, self)
        self.thresh_sld.setFocusPolicy(Qt.NoFocus)
        self.thresh_sld.setMinimum(3)
        self.thresh_sld.setMaximum(51)
        self.thresh_sld.setValue(self.k)
        self.thresh_sld.setSingleStep(2)
        self.thresh_sld.valueChanged.connect(self.changeValue)

        lay.addWidget(self.k_lbl)
        lay.addWidget(self.thresh_sld)

    def changeValue(self, value):
        # Function for setting the value of k1

        print(value)

        if value % 2 == 1:
            self.k = value
        else:
            self.k = value + 1

        self.thresh_sld.setValue(self.k)
        self.k_lbl.setText(str(self.k))

class MainWindow(QWidget):
    def __init__(self):
        super(MainWindow, self).__init__()

        self.filter1 = Filter()
        self.filter2 = Filter()

        # Creates the main layout (vertical)
        v_main_lay = QVBoxLayout()
        # Adds the sliders and their labels to the bottom of the main layout
        v_main_lay.addWidget(self.filter1)
        v_main_lay.addWidget(self.filter2)

        # Sets the main layout
        self.setLayout(v_main_lay)

        # Sets the geometry, position, window title and window default mode
        self.setGeometry(300, 300, 350, 300)
        self.setWindowTitle('Review')
        self.show()

if __name__ == '__main__':
    app = QApplication(sys.argv)
    window = MainWindow()
    sys.exit(app.exec_())