Page - 82 - in Programming for Computations – Python - A Gentle Introduction to Numerical Simulations with Python 3.6, Volume Second Edition

82 4 FunctionsandtheWritingofCode Here, the comment # First line after function definition is after the function. The first line in the function body shown here, is an optional documentation string, a docstring. This string is meant for a human interpreter, so it should say something about the purpose with the function and explain input parameters and return values, unless obvious. By convention, a docstring is enclosed in triple double quotes ("""), which allows the string to run over several lines. When present, the docstringmust appear immediatelyafter the functionheader, as shown above. The code lines of a function are executed only when the function is called (or invoked), as explained above with ball_function.py. Usually, a function ends with a return statement, starting with the reserved word return, which “returns” one or more results back to the place where the function was called. However, the explicit return statement may be dropped if not needed.3 How to receive multiple returnvalues froma functioncallwill beexemplified inSect.4.1.6. Afunctionbodycanhaveloops,branchingandcalls toother functions,andmay containasmanycodelinesasyouneed.Sometimes,a functionbodycontainsnoth- ing more than a return statement of the form return <some calculation>, wheresomecalculation is carriedoutbeforereturningthe result.Note that function inputparametersarenot required tobenumbers.Anyobjectwill do,e.g., stringsor other functions. 4.1.3 FunctionsandtheMainProgram An expression you will often encounter in programming, is main program, or that some code is “in main”. This is nothing particular to Python, and simply refers to that part of the programwhich is outside functions, taking functionheadersas part of the mainprogram. We may exemplify this withball_function.py from above, using comments to tell which linesarenot “in main”. def y(v0, t): g = 9.81 # not in main return v0*t - 0.5*g*t**2 # not in main v0 = 5 time = 0.6 print(y(v0, time)) time = 0.9 print(y(v0, time)) Thus,everything is inmainexcept the two linesof the functionbody. 3 In that case, something calledNone is returned, being a certain object that represents “nothing”.