Running Linux, 5th Edition - Matthias Kalle Dalheimer [424]
The QApplication instance we create on line 87 is the magical object that makes Qt process events and so forth. We start this event processing by calling the method enter_loop() on line 90. Details on this are beyond the scope of this book and are not needed for understanding the example.
On line 88 we create an instance of the class Calculator, and on line 89 we call the method show(), which takes care of mapping the dialog on the screen.
Now let's have a look at the class Display on lines 5 to 25. This class inherits the class QTextEdit, which is a widget for editing multiple lines of text. Inheriting is done on line 5, where we specify the superclass in parentheses. Multiple inheritance is also possible, in which case you would simply write each superclass as a comma-separated list within the parentheses.
Line 6 is the constructor of the class. The constructor is implicitly called whenever an instance of the class is created. The first argument is a reference to the instance itself. This reference is needed whenever a method of the instance is to be called.[*]
Line 7 is a call to the constructor of the superclass. We need to call the constructor of the superclass to be able to hand on the parent reference to the superclass. The parent reference is, among other things, needed to get the layout working in our GUI.
On lines 8 and 9 you see methods called on the object itself. These methods are defined in the superclass, but could of course have been from the class itself.[*]
On lines 11 to 22 we have a definition of the method pop(). Again notice how we get a reference to the object itself as the first argument of the function. When calling this method, you do not hand it a reference as the first argument, Python will take care of doing so itself. On line 58 you can see such a call.
The implementation of the pop() and push() methods involves mostly Qt issues that we do not care about in this context. So let's just shortly outline what they do. The push() method appends a number to the end of the text edit, whereas pop() does the opposite — namely taking the last line of the text edit, converting it into a number, and returning it.
Now let's turn our focus to the class Calculator, which is our GUI class that you see on screen. To be a Qt dialog, it must inherit QDialog, as we see it do on line 27.
Once again, the code is mostly about Qt, and as such not too interesting in this context. There is one Python construct we still haven't seen, though: instance variables . On lines 32, 55, 61, 65, and 71 we assign to the variable self.insertMode, and on line 68 we read this variable. Due to the self. part of the variable name, it is a variable that is local to the object, which is called an instance variable. Had we had several instances of the Calculator class, then each of these objects would have had its own copy of the insertMode variable. In contrast to languages such as C++ and Java, you do not need to declare an instance variable. The first time you assign to it, it will jump into existence, just like local variables do.
You can read all about Python at http://www.python.org or in Learning Python and Programming Python. If you are interested in learning more about Qt, then Programming with Qt (O'Reilly) might be interesting to you. There is also at least one book dedicated to developing Qt programs in Python: Gui Programming With Python: Using the Qt Toolkit (Opendocs).
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[*] In languages such as C++ and Java, you do not need to explicitly specify the object when calling methods on it from within member functions. This is needed in Python, however, where we couldn't have replaced line 8 with setAlignment( Qt.AlignRight ).
[*] All methods in Python are virtual, as is the case in Java, and unlike how it is in C++.
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Many other popular (and not-so-popular) languages are available for Linux. For the most part, however, these work identically on Linux as on other Unix systems, so