A Comprehensive Introduction to Python Programming and ... - MSDL

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2.4 Nothing is perfect . . . 19 / 75 The first 2 lines allow to create a complete window. Compared to MFC programming, it makes no doubt that Tkinter is simple to use. The third line sets the caption of the window, and the fourth one makes it enter its event loop. 3. Portability Python scripts that use Tkinter do not require modifications to be ported from one platform to the other. Tkinter is available for any platform that Python is implemented for, namely Microsoft Windows, X Windows, and Macintosh. This gives it a great advantage over most competing libraries, which are often restricted to one or two platforms. Moreover, Tkinter will provide the native look-and-feel of the specific platform it runs on. 4. Availability Tkinter is now included in any Python distribution. Therefore, no supplementary modules are required in order to run scripts using Tkinter. 2.4 Nothing is perfect . . . Tkinter has many advantages that make it the primary choice for UI design. However, it has a drawback which originates directly from its imlementation: it has a significant overhead due to its layered implementation. While this could constitute a problem with older, slower machines, most modern computers are fast enough so as to cope with the extra processing in a reasonable amount of time. When speed is critical, however, proper care must be taken so as to write code that is as efficient as possible. Even if the overhead tends to become less relevant with time, there is still a disadvantage to this layered implementation that is not going to fade away: the source code for the Tkinter library is only a shell that provides Tk’s functionality to Python programs. Therefore, in order to understand what the Tkinter source code does, programmers need to understand the source code of the Tk library, which is in turn written in C. While certainly not impossible, this requires more work from programmers and can be quite time consuming. 2.5 Show me what you can do! In order to fully understand the advantages that Tkinter has to offer, it is necessary to see some examples demonstrating just how simple programming using this toolkit can be. Let’s start with a more detailed explanation of the example presented above. For simplicity, the listing of the complete source of the example is reproduced again below. from Tkinter import * root = Tk( ) root.title(’A simple application’) root.mainloop( ) Listing 20: A very simple application
2.5 Show me what you can do! 20 / 75 While this example is almost the smallest Tkinter program that can be written (the third line is optional, but was added to make the application slightly more interesting). Let’s analyze the code line by line. from Tkinter import * This line imports the whole Tkinter library. It must present in every Tkinter program. root = Tk() This line creates a complete window, called the Tk root widget. There should be only one root widget per application, and it must be created before all other widgets. This does not mean that Tkinter applications cannot have multiple windows at the same time. More complex examples showing how to create multiple windows within a single application will be discussed later. root.title(’A simple application’) This line invokes a method of the Tk root widget instance to set its title to ’A simple application’. For the purposes of this example, this is the simplest way of displaying text in the window. The next example will use another widget to achieve a similar goal. root.mainloop( ) The call to mainloop makes the application enter its event loop, ie it makes it able to respond to user events, such as mouse events and keyboard events. The next example is very similar to the first example, but it uses new widgets (visual components) to display some text and exit the application. Widgets will be discussed in the next chapter. from Tkinter import * Listing 21: Creating widgets def quit(): import sys; sys.exit() 5 root = Tk( ) lbl = Label(root, text="Press the button below to exit") lbl.pack() btn = Button(root, text="Quit", command=quit) btn.pack() 10 root.mainloop( ) This example introduces some new concepts. First of all, it creates new widgets to put on the root window. Also, it responds to a specific user event (a click on the “Quit” button) by exiting the program. def quit(): import sys; sys.exit()
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