Seite - 114 - in Programming for Computations – Python - A Gentle Introduction to Numerical Simulations with Python 3.6, Band Second Edition

114 5 SomeMorePythonEssentials moveswithout any air resistance.7 As with built-in modules, we will see that func- tionality may be imported in the usual ways, e.g., asimport vertical_motion, whichallowseasy reuseofmodule functions. 5.4.1 ANaiveImport Before turning to the making of our vertical motion module, we will do some “warm-up”testingwitha previousscriptofours, just toenlightenourselvesa bit. Let us pick ball_function.py from Sect.4.1.1 (which addresses vertical motion), and argue that, if this script is a module, we should be able to “import it” asimport ball_function, right? This sounds like a reasonable expectation, sowithout toodeep reasoning, let us just start there. First, however, we better take another look at that code (after all, it has been a while).Foreasyreference,wejust repeat thefewcodelinesofball_function.py here: def y(v0, t): g = 9.81 # Acceleration of gravity return v0*t - 0.5*g*t**2 v0 = 5 # Initial velocity time = 0.6 # Just pick one point in time print(y(v0, time)) time = 0.9 # Pick another point in time print(y(v0, time)) We recognize the function definition of y and the two applications of that function, involving a function call and a printout for each of the chosen points in time. Now, we previously thought of this code as a program, executed it, and got the printouts.Whatwillhappennow,whenweratherconsideritamoduleandimportit? Here iswhathappens: In [1]: import ball_function 1.2342 0.52695 What?Printingofnumbers?Weaskedforan import,8 not somethingthat looks like programexecution! Thethingis, thatwhenanymoduleis imported,Pythondoesactuallyexecutethe modulecode(!), i.e., functiondefinitionsarereadandstatements(outsidefunctions) executed. This is Python’s way of bringing module content “to life”, so that, e.g., 7 The reader who is into physics, will know that the computations done here, work equally well if the object has some simultaneous horizontal motion. In that case, our computations apply to the vertical component of the motion only, not the motionas a whole. 8 Doingtheimportdifferently, e.g., likefrom vertical_motion import y,wouldstillgive the printouts! Also if the import were done ina script.