Think Python - Denison University

36 Chapter 4. Case study: interface designFor example, polyline requires four arguments: t has to be a Turtle; n is the number of linesegments, so it has to be an integer; length should be a positive number; and angle has to be anumber, which isunderstood tobe indegrees.These requirements are called preconditions because they are supposed to be true before the functionstartsexecuting.Conversely,conditionsattheendofthefunctionarepostconditions. Postconditionsinclude the intended effect of the function (like drawing line segments) and any side effects(likemoving theTurtleor making other changes intheWorld).Preconditions are the responsibility of the caller. If the caller violates a (properly documented!)precondition and thefunction doesn’t workcorrectly, thebug isinthe caller, not thefunction.4.11 Glossaryinstance: A member of a set. The TurtleWorld in this chapter is a member of the set of Turtle-Worlds.loop: A part of aprogram that can execute repeatedly.encapsulation: The process oftransforming asequence ofstatements intoafunction definition.generalization: The process of replacing something unnecessarily specific (like a number) withsomething appropriately general (likeavariable or parameter).keyword argument: An argument that includes thename ofthe parameter as a“keyword.”interface: Adescriptionofhowtouseafunction,includingthenameanddescriptionsoftheargumentsand returnvalue.refactoring: Theprocessofmodifyingaworkingprogramtoimprovefunctioninterfacesandotherqualities ofthe code.development plan: Aprocess forwritingprograms.docstring: A stringthat appears inafunction definition todocument the function’s interface.precondition: A requirement that should be satisfiedby thecaller before a function starts.postcondition: A requirement that should besatisfied bythe function before itends.4.12 ExercisesExercise 4.1 Download thecode inthischapter fromthinkpython.com/code/polygon.py.1. Writeappropriate docstrings forpolygon,arcandcircle.2. Draw a stack diagram that shows the state of the program while executing circle(bob,radius). You can do the arithmeticby hand or addprintstatements tothecode.3. TheversionofarcinSection4.7isnotveryaccuratebecausethelinearapproximationofthecircle is always outside the true circle. As a result, the turtle ends up a few units away fromthe correct destination. My solution shows a way to reduce the effect of this error. Read thecode and see ifitmakes sensetoyou. Ifyou draw a diagram, you might seehow itworks.