Page - 119 - in Programming for Computations – Python - A Gentle Introduction to Numerical Simulations with Python 3.6, Volume Second Edition

5.4 MakingOurOwnModule 119 5.4.3 ModuleorProgram? We know how a .py file can be executed as a program, and we have seen how functions may be collected in a .py file, so that imports do not trigger any undesirableprintouts.However,wehavealreadyrealizedthatPythondoesnot force a.pyfile to be either a program, or a module. No, it can be both, and thanks to a clever construction, Python allows a very flexible switch between the two ways of usinga.pyfile. Thiscleverconstructionisbasedonanif test,whichtestswhetherthefileshould be run as a program, or act as a module only. This is doable by use of the variable __name__, which (behindthescenes)Pythonsets to’__main__’only if thefile is executed as a program(note the compulsory two underscores to each side ofname andmainhere). We may put up a rather general form of the construction, that we place in the.pyfile, as < function definitions > if __name__ == ’__main__’: # note double underscores (and colon) < statement 1 > < statement 2 > ... ... So, if the file is run as a program, Python immediately sets __name__ to ’__main__’. When reaching the if test, it will thus evaluate to True, which in turn causes the corresponding (indented) statements, i.e., the statements of the so-called test block, to be executed. To the contrary, if the file is used for imports only, __name__ will not be set to ’__main__’, the if test will consequently evaluate toFalse, and thecorrespondingstatementsare notexecuted. Often, thestatements in the test blockarebest placed inoneor several functions (thendefinedabovetheif test, togetherwith theother functiondefinitions), so that when the if test evaluates to True, one or more function calls will follow. This is particularly important when different tasks are handled, so that each function containsstatements that logicallybelong together. As a simple illustration, when one function is natural (e.g., named application), theconstructionmaybereformulatedas < function definitions > def application(): < statement 1 > < statement 2 > ... ... if __name__ == ’__main__’: application() Our .py File as Both Module and Program We will now incorporate this construction in vertical_motion.py. This allows us to use the functions from vertical_motion.pyalso inaprogram(ourapplication) thatasks theuser for