Pass a variable from one Class to another in PyQt

I want to pass a string variable from Main_Window class to another QDialog class in PyQt. I can't understand what I do wrong. I want to pass host_mac variable from Main Class to QDialog Class. Here the main part of my code.

Here is the QDialog class:

class Client(QDialog):
    def __init__(self, parent=None):
        super(Client, self).__init__(parent)
        self.pb = QPushButton()
        self.pb.setObjectName("connect")
        self.pb.setText("Connect") 

        layout = QFormLayout()
        layout.addWidget(self.pb)

        self.setLayout(layout)
        self.connect(self.pb, SIGNAL("clicked()"),self.set_client)
        self.setWindowTitle("Learning")

    def set_client(self):
        self.client = self.le.text()
        print 'provided client mac is ' + self.client + 'and host_mac is ' + Main_window_ex.host_mac

And here the Main_Window Class:

class Main_window_ex(QMainWindow, Ui_Main_window):   
    def __init__(self, parent = None):
    """
    Default Constructor. It can receive a top window as parent. 
    """
    QMainWindow.__init__(self, parent)
    self.setupUi(self)


    self.host_mac = 'blah blah'

#more code beneeth

But I get the following error:

AttributeError: type object 'Main_window_ex' has no attribute 'host_mac'

Solution

Main_window_ex.host_mac refers to a class variable (as Main_window_ex is only a class), but you are wanting to access the instance variable. In other words, host_mac is not defined until the class is instantiated.

There are a few ways around this. Assuming Main_window_ex is responsible for creating Client, then one simple way is to pass the variable to Client:

class Client(QDialog):
    def __init__(self, host_mac, parent=None):
        self.host_mac = host_mac
        ...

And use it like:

    def set_client(self):
        self.client = self.le.text()
        print 'provided client mac is ' + self.client + 'and host_mac is ' + self.host_mac

As a side note you may want to use the new style connection syntax:

# old style
# self.connect(self.pb, SIGNAL("clicked()"),self.set_client)

# new style
self.pb.clicked.connect(self.set_client)