Think Python - Denison University

3.10. Stack diagrams 23Thisfunctiontakestwoarguments,concatenatesthem,andprintstheresulttwice. Hereisanexamplethatusesit:>>> line1 = 'Bing tiddle '>>> line2 = 'tiddle bang.'>>> cat_twice(line1, line2)Bing tiddle tiddle bang.Bing tiddle tiddle bang.Whencat_twiceterminates,thevariablecatisdestroyed. Ifwetrytoprintit,wegetanexception:>>> print catNameError: name 'cat' is not definedParameters are alsolocal. For example, outsideprint_twice,thereisno such thingasbruce.3.10 StackdiagramsTokeeptrackofwhichvariablescanbeusedwhere,itissometimesusefultodrawastackdiagram.Likestatediagrams,stackdiagramsshowthevalueofeachvariable,buttheyalsoshowthefunctioneach variable belongs to.Eachfunctionisrepresentedbyaframe. Aframeisaboxwiththenameofafunctionbesideitandthe parameters and variables of the function inside it. The stack diagram for the previous examplelooks like this:__main__line1line2’Bing tiddle ’’tiddle bang.’cat_twicepart1part2cat’Bing tiddle ’’tiddle bang.’’Bing tiddle tiddle bang.’print_twicebruce’Bing tiddle tiddle bang.’The frames are arranged in a stack that indicates which function called which, and so on. In thisexample,print_twicewas called bycat_twice,andcat_twicewas called by__main__,whichis a special name for the topmost frame. When you create a variable outside of any function, itbelongs to__main__.Each parameter refers to the same value as its corresponding argument. So, part1 has the samevalue asline1,part2has thesamevalue asline2,andbrucehas the samevalue ascat.Ifanerroroccursduringafunctioncall,Pythonprintsthenameofthefunction,andthenameofthefunction that called it,and thename of the function that called that, all theway back to__main__.For example, ifyou trytoaccesscatfrom withinprint_twice,you get aNameError: