Seite - 33 - in Programming for Computations – Python - A Gentle Introduction to Numerical Simulations with Python 3.6, Band Second Edition

1.7 ErrorMessagesandWarnings 33 Thus, if the user name was Paul, for example, he would get this question up on his screen Hello Paul! What is your age? Hewould typehis age,pressenter, and thecodewouldproceed likebefore. Thereareotherwaysofprovidinginput toaprogramaswell,e.g.,viaagraphical interface(asmanyreaderswillbeusedto)orat thecommandline(i.e.,asparameters succeeding, on the same line, the command that starts the program). Reading data fromafile isyet anotherway. 1.7 ErrorMessagesandWarnings Allprogrammersexperienceerrormessages,andusually toa largeextentduringthe early learning process. Sometimes error messages are understandable, sometimes theyarenot.Anyway, it is important togetused to them. Oneidea is tostartwithaprogramthat initially isworking,andthendeliberately introduce errors in it, one by one (but remember to take a copy of the original working code!). For each error, you try to run the program to see what Python’s response is. Then you know what the problem is and understand what the error message is about. This will greatly help you when you get a similar error message orwarning later. Debugging Veryoften,youwillexperiencethat thereareerrors in theprogramyou havewritten. This is normal,but frustrating in the beginning.You then have to find the problem, try to fix it, and then run the program again. Typically, you fix one error just to experience that another error is waiting around the corner. However, after some time you start to avoid the most common beginner’s errors, and things run more smoothly. The process of finding and fixing errors, called debugging, is very important to learn.Thereare differentwaysofdoingit too. A special program(debugger)may be used to help youcheck (anddo) different things in the program you need to fix. A simpler procedure, that often brings you a long way, is to print information to the screen from different places in the program.Firstofall, this issomethingyoushoulddo(several times)duringprogram development anyway, so that things get checked as you go along. However, if the final program still ends up with error messages, you may save a copy of it, and do some testing on the copy.Useful testing may then be to remove,e.g., the latter half of the program (e.g., by inserting comment signs#), and insert print commands at clever places to see what is the case. When the first half looks ok, possibly after some corrections, insert parts of what was removedand repeat the processwith the new code. Using simple numbers and doing this in parallel with hand calculations ona pieceofpaper (forcomparison) isoftena verygood idea. Exception Handling Python also offers means to detect and handle errors by the program itself! The programmer must then foresee (when writing the code) that there is a potential for error at some particular point. If, for example, a running programasks theuser togiveanumber, thingsmaygoverywrongif theuser inputs the word five in stead of the number 5. In Python, such cases may be handled