When embedding VTK qt met VTK widget can’t move, began to directly copy the official case inside vtkScalarBarWidget wording, is shown in the QVTKRenderWindowInteractor abnormalities, cannot interact with the widget.
https://vtk.org/Wiki/VTK/Examples/Python/Widgets/ScalarBarWidget
in writing is as follows:

# create the scalar_bar_widget
scalar_bar_widget = vtk.vtkScalarBarWidget()
scalar_bar_widget.SetInteractor(interactor)
scalar_bar_widget.SetScalarBarActor(scalar_bar)
scalar_bar_widget.On()

Changed to:

# create the scalar_bar_widget
self.scalar_bar_widget = vtk.vtkScalarBarWidget()
self.scalar_bar_widget.SetInteractor(self.iren)
self.scalar_bar_widget.SetScalarBarActor(scalar_bar)
self.scalar_bar_widget.On()

This means that the widget should be prefixed with self when loading with classes.

>

from PyQt5 import QtWidgets
from PyQt5.QtWidgets import QApplication
from vtk.qt.QVTKRenderWindowInteractor import QVTKRenderWindowInteractor
import sys
import vtk
class MainPage(QtWidgets.QMainWindow):
    def __init__(self, parent=None):
        QtWidgets.QMainWindow.__init__(self, parent)
        self.setObjectName("MainWindow")
        self.resize(603, 553)
        self.centralWidget = QtWidgets.QWidget(self)
        self.gridlayout = QtWidgets.QGridLayout(self.centralWidget)
        self.vtkWidget = QVTKRenderWindowInteractor(self.centralWidget)
        self.gridlayout.addWidget(self.vtkWidget)
        self.setCentralWidget(self.centralWidget)
        self.ren = vtk.vtkRenderer()
        self.vtkWidget.GetRenderWindow().AddRenderer(self.ren)
        self.iren = self.vtkWidget.GetRenderWindow().GetInteractor()
        # Create source
        source = vtk.vtkSphereSource()
        source.SetCenter(0, 0, 0)
        source.SetRadius(5.0)
        # Create a mapper
        mapper = vtk.vtkPolyDataMapper()
        mapper.SetInputConnection(source.GetOutputPort())
        # Create an actor
        actor = vtk.vtkActor()
        actor.SetMapper(mapper)
        self.ren.AddActor(actor)
        # create the scalar_bar
        scalar_bar = vtk.vtkScalarBarActor()
        scalar_bar.SetOrientationToHorizontal()
        lut = vtk.vtkLookupTable()
        lut.Build()
        scalar_bar.SetLookupTable(lut)
        # create the scalar_bar_widget
        self.scalar_bar_widget = vtk.vtkScalarBarWidget()
        self.scalar_bar_widget.SetInteractor(self.iren)
        self.scalar_bar_widget.SetScalarBarActor(scalar_bar)
        self.scalar_bar_widget.On()
if __name__ == "__main__":
    app = QApplication(sys.argv)
    window = MainPage()
    window.show()
    window.iren.Initialize()
    sys.exit(app.exec_())