does not show icons

I just installed Python3 (3.5.2) and Pyqt5 (5.8.2) and I am following this tutorial to learn and make a GUI: http://zetcode.com/gui/pyqt5/firstprograms/

I'm trying to run the 2nd example but program is returning an error (which also happened on the 1st one, but since it had no image i took no notice) which is the following:

QApplication: invalid style override passed, ignoring it.
No XVisualInfo for format QSurfaceFormat(version 2.0, options QFlags<QSurfaceFormat::FormatOption>(), depthBufferSize -1, redBufferSize 1, greenBufferSize 1, blueBufferSize 1, alphaBufferSize -1, stencilBufferSize -1, samples -1, swapBehavior QSurfaceFormat::SwapBehavior(SingleBuffer), swapInterval 1, profile  QSurfaceFormat::OpenGLContextProfile(NoProfile))
Falling back to using screens root_visual.

What is the meaning of this? Am i missing some packages?

I installed pyqt first with this command:

sudo -H pip3 install PyQt5

but Python3 was not acknowledging its existence so i searched the apt ubuntu repos and installed with:

sudo apt install python3-PyQt5

I also tried to reference the image by full path /foo/bar/image.png and nothing

What is the problem?

EDIT #1

The code that i am using is from example 2:

#!/usr/bin/python3
# -*- coding: utf-8 -*-

"""
ZetCode PyQt5 tutorial

This example shows an icon
in the titlebar of the window.

author: Jan Bodnar
website: zetcode.com
last edited: January 2015
"""

import sys
import os
from PyQt5.QtWidgets import QApplication, QWidget
from PyQt5.QtGui import QIcon

base_dir = os.path.dirname(os.path.abspath(__file__))
os.chdir(base_dir)

class Example(QWidget):

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

        self.initUI()

    def initUI(self):

        self.setGeometry(300, 300, 300, 220)
        self.setWindowTitle('Icon')
        self.setWindowIcon(QIcon('image.png'))

        self.show()


if __name__ == '__main__':

    app = QApplication(sys.argv)
    ex = Example()
    sys.exit(app.exec_())

After your post i reinstalled all my packages. The error is slightly different but the result is the same:

python3 example_02.py 
QApplication: invalid style override passed, ignoring it.

Screencapture:

Notice that you are having no icons at all for all applications, not just for the PyQt icon example. This is because by default, certain environments turn off the icons in the titlebar. You have to enable them.

For instance in Xfce Desktop Environment, we can use the xfce4-settings-editor tool. In Settings/Settings Editor select xfwm4. Find the show_app_icon option and check it. Change a theme back and forth to see the changes; they are not visible right away.

After this, you will see the icon in the titlebar of the PyQt5 example.

As for the warning; it is a recent thing and it has to do something with the incopatibilities between Qt and GTK theming. I have not found a solution to remove the warning so far.