Python Programming: An Introduction to Computer Science

1.6.THEMAGICOFPYTHON 5

(Program)

Compiler Machine

Code

Running

Inputs

Outputs

Source

Code

Program

Figure1.2:Compilinga High-Level Language

Code Computer

Inputs

Outputs

Source

(Program)

Running an

Interpreter

Figure1.3:Interpretinga High-Level Language.

Eachkindofcomputerhasitsownmachinelanguage.A programfora PentiumCPUwon’t runona Mac-
intoshthatsportsa PowerPC.Ontheotherhand,a programwrittenina high-level languagecanberunon
many differentkindsofcomputersaslongasthereis a suitablecompilerorinterpreter(whichis justanother
program).Forexample,if I designa new computer, I canalsoprograma Pythoninterpreterforit,andthen
any programwritteninPythoncanberunonmynew computer, asis.

1.6 TheMagicofPython

Nowthatyouhave allthetechnicaldetails,it’s timetostarthavingfunwithPython.Theultimategoalis
tomake thecomputerdoourbidding.To thisend,wewillwriteprogramsthatcontrolthecomputational
processesinsidethemachine.Youhave alreadyseenthatthereis nomagicinthisprocess,butinsomeways
programmingfeelslike magic.
Thecomputationalprocessesinsidethecomputerarelike magicalspiritsthatwecanharnessforour
work.Unfortunately, thosespiritsonlyunderstanda veryarcanelanguagethatwedonotknow. Whatwe
needis a friendlyGeniethatcandirectthespiritstofulfillourwishes.OurGenieis a Pythoninterpreter. We
cangive instructionstothePythoninterpreter, andit directstheunderlyingspiritstocarryoutourdemands.
We communicatewiththeGeniethrougha speciallanguageofspellsandincantations(i.e.,Python).The
bestwaytostartlearningaboutPythonis toletourGenieoutofthebottleandtrysomespells.
YoucanstartthePythoninterpreterinaninteractive modeandtypeinsomecommandstoseewhat
happens.Whenyoufirststarttheinterpreterprogram,youmayseesomethinglike thefollowing:

Python 2.1 (#1, Jun 21 2001,11:39:00)
[GCC pgcc-2.91.66 19990314 (egcs-1.1.2 release)] on linux
Type "copyright", "credits"or "license" for more information.

The

is a PythonpromptindicatingthattheGenieis waitingforusto give it a command.Inprogramming
languages,a completecommandis calledastatement.
Hereis a sampleinteractionwiththePythoninterpreter.