computers and programs the funny text " aecis the location (addressin computer memory where the greet function definition happens to be stored if you are trying this out on your own computeryou will almost certainly see different address one problem with entering functions interactively into python shell as we did with the hello and greet examples is that the definitions are lost when we quit the shell if we want to use them again the next timewe have to type them all over again programs are usually created by typing definitions into separate file called module or script this file is saved on disk so that it can be used over and over again module file is just text fileand you can create one using any program for editing textlike notepad or word processor program (provided you save your program as "plain textfilea special type of program known as programming environment simplifies the process programming environment is specifically designed to help programmers write programs and includes features such as automatic indentingcolor highlightingand interactive development the standard python distribution includes programming environment called idle that you may use for working on the programs in this book let' illustrate the use of module file by writing and running complete program our program will illustrate mathematical concept known as chaos here is the program as we would type it into idle or some other editor and save in module filefilechaos py simple program illustrating chaotic behavior def main()print("this program illustrates chaotic function" eval(input("enter number between and ")for in range( ) ( xprint(xmain(this file should be saved with the name chaos py the py extension indicates that this is python module you can see that this particular example contains lines to define new function called main (programs are often placed in function called main the last line of the file is the command to invoke this function don' worry if you don' understand what main actually doeswe will discuss it in the next section the point here is that once we have program in module filewe can run it any time we want this program can be run in number of different ways that depend on the actual operating system and programming environment that you are using if you are using windowing systemyou can run python program by clicking (or double-clickingon the module file' icon in command line situationyou might type command like python chaos py if you are using idle (or another programming environmentyou can run program by opening it in the editor and then selecting command like importrunor execute