python programming theturtle pendown(for cmd in self graphicscommandscmd draw(theturtlescreen update(screen listen( screen onkeypress(undohandler" "screen listen( the main function in our gui program is very simple it creates the root window then it creates the drawingapplication frame which creates all the widgets and has the logic for the event handlers calling mainloop on the frames makes it start listening for events the mainloop function will return when the application is exited def main()root tkinter tk(drawingapp drawingapplication(root drawingapp mainloop(print("program execution completed " if __name__ ="__main__"main( xml files reading standard text filelike the graphics commands file we read using the loop and half pattern in sect is common task in computer programs the only problem is that the program must be written to read the specific format of the input file if we later wish to change the format of the input file to includefor examplea new option like fill color for circlethen we are stuck updating the program and updating all the files it once read the input file format and the program must always be synchronized this means that all old formatted input files must be converted to the new format or they must be thrown away that is simply not acceptable to most businesses because data is valuable to deal with this problemcomputer programmers designed language for describing data input files called xml which stands for extensible markup language xml is meta-language for data description meta-language is language for describing other languages the xml meta-language is universally accepted in factthe xml format is governed by standards committeewhich means that we can count on the xml format remaining very stable and backwards compatible forever any additions to xml will have to be compatible with what has already been defined an xml document begins with special line to identify it as an xml file this line looks like this the rest of an xml file consists of elements or nodes each node is identified by tag or pair of beginning and ending tags each tag is delimited ( surroundedby angle brackets for instancehere is one such tag this copy belongs to 'acha