Think Python - Denison University

110 Chapter 11. Dictionariesknown[n] = resreturn resknownis a dictionary that keeps track of the Fibonacci numbers we already know. It starts with twoitems: 0maps to0and 1maps to1.Wheneverfibonacciiscalled,itchecksknown. Iftheresultisalreadythere,itcanreturnimmediately.Otherwiseithas tocompute thenew value, add ittothe dictionary, and returnit.Exercise 11.6 Run this version of fibonacci and the original with a range of parameters andcompare their runtimes.11.6 Global variablesIn the previous example, known is created outside the function, so it belongs to the special framecalled__main__. Variablesin__main__aresometimescalledglobalbecausetheycanbeaccessedfrom any function. Unlike local variables, which disappear when their function ends, global variablespersistfrom one function call tothenext.Itiscommontouseglobalvariablesforflags;thatis,booleanvariablesthatindicate(“flag”)whethera condition is true. For example, some programs use a flag named verbose to control the level ofdetail inthe output:verbose = Truedef example1():if verbose:print 'Running example1'If you try to reassign a global variable, you might be surprised. The following example is supposedtokeep trackof whether the function has been called:been_called = Falsedef example2():been_called = True# WRONGBut if you run it you will see that the value of been_called doesn’t change. The problem is thatexample2 creates a new local variable named been_called. The local variable goes away whenthefunction ends, and has noeffect on theglobal variable.To reassign a global variable inside a function you have to declare the global variable before youuseit:been_called = Falsedef example2():global been_calledbeen_called = TrueThe global statement tells the interpreter something like, “In this function, when I saybeen_called,Imean the global variable; don’t create alocal one.”Here’s an example that triestoupdate aglobal variable: