110 CHAPTER7. CONTROLSTRUCTURES,PART 1

rarebutpossibleerrors.
Inthecaseofthequadraticsolver, wecheckedthedatabeforethecalltothesqrtfunction.Sometimes
functionsthemselvescheckforpossibleerrorsandreturna specialvaluetoindicatethattheoperationwas
unsuccessful. Forexample,a differentsquarerootoperationmightreturna negative number(say-1)to
indicateanerror. Sincethesquarerootsofrealnumbersarenever negative, thisvaluecouldbeusedtosignal
thatanerrorhadoccurred.Theprogramwouldchecktheresultoftheoperationwitha decision.

discRt = otherSqrt(bb- 4a*c)
if discRt < 0:
print "No realroots."
else:
...

Sometimesprogramsbecomesopepperedwithdecisionstocheckforspecialcasesthatthemainalgo-
rithmforhandlingtherun-of-the-millcasesseemscompletelylost.Programminglanguagedesignershave
comeupwithmechanismsforexceptionhandlingthathelptosolve thisdesignproblem.Theideaofan
exceptionhandlingmechanismis thattheprogrammercanwritecodethatcatchesanddealswitherrorsthat
arisewhentheprogramis running.Ratherthanexplicitlycheckingthateachstepinthealgorithmwassuc-
cessful,a programwithexceptionhandlingcaninessencesay“Dothesesteps,andif any problemcropsup,
handleit thisway.”
We’re notgoingtodiscussallthedetailsofthePythonexceptionhandlingmechanismhere,butI dowant
togive youa concreteexamplesoyoucanseehow exceptionhandlingworksandunderstandprogramsthat
useit.InPython,exceptionhandlingis donewitha specialcontrolstructurethatissimilartoa decision.
Let’s startwitha specificexampleandthentake a lookat thegeneralapproach.
Hereisa versionofthequadraticprogramthatusesPython’s exceptionmechanismtocatchpotential
errorsinthemath.sqrtfunction.

quadratic5.py

import math

def main():
print "This programfinds the real solutions to a quadratic\n"

try:

a, b, c = input("Please enter the coefficients (a, b, c):")

discRoot = math.sqrt(b* b - 4 * a * c)

root1 = (-b + discRoot)/ (2 * a)

root2 = (-b - discRoot)/ (2 * a)

print "\nThesolutions are:", root1, root2

except OverflowError:

print "\nNoreal roots"

Noticethatthisis basicallytheveryfirstversionofthequadraticprogramwiththeadditionofatry...except
aroundtheheartoftheprogram.Atrystatementhasthegeneralform:

try: