2.5.ASSIGNMENTSTATEMENTS 19

2.5.3 SimultaneousAssignment

Thereis analternative formoftheassignmentstatementthatallowsustocalculateseveralvaluesallatthe
sametime.It lookslike this:

, , ..., = , , ...,

Thisiscalledsimultaneousassignment. Semantically, thistellsPythontoevaluatealltheexpressionson
theright-handsideandthenassignthesevaluestothecorrespondingvariablesnamedontheleft-handside.
Here’s anexample.

sum, diff = x+y, x-y

Heresumwouldgetthesumofxandyanddiffwouldgetthedifference.
Thisformofassignmentseemsstrangeatfirst,butit canprove remarkablyuseful.Here’s anexample.
Supposeyouhave two variablesxandyandyouwanttoswapthevalues. Thatis,youwantthevalue
currentlystoredinxtobeinyandthevaluethatis currentlyinytobestoredinx. Atfirst,youmightthink
thiscouldbedonewithtwo simpleassignments.

x = y
y = x

Thisdoesn’t work.We cantracetheexecutionofthesestatementsstep-by-steptoseewhy.
Supposexandystartwiththevalues2 and4. Let’s examinethelogicoftheprogramtoseehowthe
variableschange.Thefollowingsequenceusescommentstodescribewhathappenstothevariablesasthese
two statementsareexecuted.

variables x y

initial values 2 4

x = y

now 4 4

y = x

final 4 4

Seehow thefirststatementclobberstheoriginalvalueofxbyassigningtoit thevalueofy? Whenwethen
assignxtoyinthesecondstep,wejustendupwithtwo copiesoftheoriginalyvalue.
Onewaytomake theswapworkis tointroduceanadditionalvariablethattemporarilyremembersthe
originalvalueofx.

temp = x
x = y
y = temp

Let’s walk-throughthissequencetoseehow it works.

variables x y temp

initial values 2 4 no valueyet

temp = x

2 4 2

x = y

4 4 2

y = temp

4 2 2

Asyoucanseefromthefinalvaluesofxandy, theswapwassuccessfulinthiscase.
Thissortofthree-wayshuffleis commoninotherprogramminglanguages.InPython,thesimultaneous
assignmentstatementoffersanelegantalternative. Hereis a simplerPythonequivalent:

x, y = y, x