how to add arguments to a Qt PySide built-in Signal when connecting to a Slot

This is the class where I'm trying to implement a simple expression evaluator:

class mainWindow(QtGui.QMainWindow, mainGui.Ui_MainWindow):

    def __init__(*args, **kwargs)
        super(mainWindow, self).__init__(*args, **kwargs)
        self.setupUi(self)
        self.connecting_range_displays()

    def connecting_range_displays(self):    
        ranges = num_ranges()
        first_range = num_ranges.ones_range
        second_range = num_ranges.tens_range

        print first_range
        print second_range

        self.top_lineEdit.setText(str(first_range))
        self.bottom_lineEdit.setText(str(second_range))

        ex_a = first_range + second_range
        print "this is expressions answer", ex_a

        self.answer_lineEdit.returnPressed.connect(self.ex_evaluator)

    def ex_evaluator(self, right_answer):
        answer = self.answer_lineEdit.text()

        if ex_a == right_answer:
            print "Correct answer bucko"

In the above code for a simple arithmetic practice application I use two lines to display random numbers in a certain range, and the third QlineEdit i.e answer_lineEdit I use it to get user input and then to check if its correct I use the Signal returnPressed and connect it to the ex_evaluator which will then tell you if you have given a correct response, the problem seems to be that I cannot pass the ex_a argument to the function to be able to evaluate it, could it be namespace issue, I'm a pretty new at coding and my philosophy is to only learn the least amount to get me to where I want so I'm wondering if I will have to learn how to subclass the QLineEdit which I have no idea how to do, I saw that maybe decorators might help which is another bucket of trouble I don't want to get into.

Solution

You need to do one of the following:

make ex_a a class attribute by changing it to be "self.ex_a". Then it can be used from anywhere within the class
pass it to the slot (ex_evaluator) using lambda or functools.partial

For the second option, I wrote a tutorial that describes both ways of doing this: http://www.blog.pythonlibrary.org/2013/04/10/pyside-connecting-multiple-widgets-to-the-same-slot/

To use a lambda, do something like this:

self.answer_lineEdit.returnPressed.connect(lambda ans=ex_a: self.ex_evaluator(ans))

For the functools.partial, it would be something like this:

import functools
callback = partial(self.ex_evaluator, ex_a)
self.answer_lineEdit.returnPressed.connect(callback)

If you don't understand that last one, you might want to take a look at the documentation.