Python "AttributeError" for instance declared in __init__

Question

I'm developping an application in Python and Qt and there is something that I'd like to clarify. Take this code example:

import serial

from PySide.QtGui import QMainWindow
from PySide import QtCore, QtGui
from PySide.QtCore import QTimer
from ZumaUI import Ui_MainWindow

class MainWindow(QMainWindow, Ui_MainWindow):
    '''
    MainWindow: this is the class that manages all the functionality.
    '''
    def __init__(self, parent = None):
        '''
        Default Constructor. It can receive a top window as parent. 
        '''
        QMainWindow.__init__(self, parent)
        self.setupUi(self)

        self.serial = serial.Serial()
        self.serial.port = 'COM1'
        self.serial.baudrate = 19200
        self.serial.timeout = 0.5   #make sure that the alive event can be checked from time to time

        self.on_PortSettings()

        self.timer = QTimer(self)            
        self.timer.timeout.connect(self.OnRefresh)
        self.timer.start(150)

        self.on_PortSettings()

    def OnRefresh(self):
        pass

    def on_PortSettings(self):
        self.serial.close()
        self.timer.stop()

When I run my code, I get

"AttributeError: 'MainWindow' object has no attribute 'timer'"

But I don't have this kind of error for self.serial which is called all over within the class. What is the difference between these two instances and how should I call timer so it can be accessed throughout the class?

Answer 1

You're calling onPortSettings before you set up timer.

    self.on_PortSettings() <-- here

    self.timer = QTimer(self)            

Answer 2

Do this:

def __init__(self, parent = None):
    self.timer = None

...

def on_PortSettings(self):
    self.serial.close()
    if self.timer is not None:
        self.timer.stop()