PyQGIS ShapeViewer Demo

ShapeViewer.py

from qgis.core import *
from qgis.gui import *
from qgis.utils import iface

from PyQt5.QtCore import *
from PyQt5.QtWidgets import QApplication, QMainWindow, QFileDialog, QVBoxLayout
from PyQt5.QtGui import *

import sys
import os
# Import our GUI
from shapeviewer_gui import Ui_MainWindow

# Environment variable QGISHOME must be set to the install directory
# before running the application
qgis_prefix = os.getenv("QGISHOME")

class ShapeViewer(QMainWindow, Ui_MainWindow):

  def __init__(self):
    QMainWindow.__init__(self)

    # Required by Qt4 to initialize the UI
    self.setupUi(self)

    # Set the title for the app
    self.setWindowTitle("ShapeViewer")

    # Create the map canvas
    self.canvas = QgsMapCanvas()
    self.canvas.show()

    self.canvas.setCanvasColor(Qt.white)
    self.canvas.enableAntiAliasing(True)

    # Lay our widgets out in the main window using a 
    # vertical box layout
    self.layout = QVBoxLayout(self.frame)
    self.layout.addWidget(self.canvas)

    # layout is set - open a layer
    # Add an OGR layer to the map
    filename, filetype = QFileDialog.getOpenFileName(self, 
      "Open Shapefile", ".", "Shapefiles (*.shp)")
    #fileInfo = QFileInfo(file)

    # Add the layer
    layer = QgsVectorLayer(filename, "test", "ogr")
    if not layer.isValid():
      return

    QgsProject.instance().addMapLayer(layer)
    self.canvas.setExtent(layer.extent())
    self.canvas.setLayers([layer])
    layer.refresh()
    self.canvas.show()
    
def main(argv):
  # create Qt application
  app = QApplication(argv)

  # Initialize qgis libraries
  QgsApplication.setPrefixPath(qgis_prefix, True)
  QgsApplication.initQgis()

  # create main window
  wnd = ShapeViewer()
  # Move the app window to upper left
  wnd.move(100,100)
  wnd.show()

  # run!
  retval = app.exec_()
  
  # exit
  QgsApplication.exitQgis()
  sys.exit(retval)


if __name__ == "__main__":
  main(sys.argv)

that's great !!!
your code is the best for the standalone PyQgis sample.