7.4.EXCEPTIONHANDLING 111

Traceback (innermostlast):
File "", line1, in?
File "quadratic.py",line 13, in?
discRoot = math.sqrt(b b - 4 a * c)
OverflowError: mathrange error

Thelastlineofthiserrormessageindicatesthetypeoferrorthatwasgenerated,namelyanOverflowError.
TheupdatedversionoftheprogramprovidesanexceptclausetocatchtheOverflowError.
Hereis theerrorhandlingversioninaction:

This program findsthe real solutions to a quadratic

Please enter the coefficients(a, b, c): 1,2,3

No real roots

Insteadofcrashing,theexceptionhandlercatchestheerrorandprintsa messageindicatingthattheequation
doesnothave realroots.
Thenicethingaboutthetry...exceptstatementis thatit canbeusedto catchany kindoferror, even
onesthatmightbedifficulttotestfor, andhopefully, providea gracefulexit.Forexample,inthequadratic
solver, therearelotsofotherthingsthatcouldgowrongbesideshavinga badsetofcoefficients.If theuser
failstotypethecorrectnumberofinputs,theprogramgeneratesaValueError. Iftheuseraccidently
typesanidentifierinsteadofa number, theprogramgeneratesaNameError. If theusertypesina valid
Pythonexpressionthatproducesnon-numericresults,theprogramgeneratesaTypeError. A singletry
statementcanhave multipleexceptclausestocatchvariouspossibleclassesoferrors.
Here’s onelastversionoftheprogramdesignedtorobustlyhandleany possibleerrorsintheinput.

quadratic6.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"

except ValueError:

print "\nYoudidn’t give me three coefficients."

except NameError:

print "\nYoudidn’t enter three numbers"

except TypeError:

print "\nYourinputs were not all numbers"

except:

print "\nSomethingwent wrong, sorry!"

Themultipleexcepts aresimilartoelifs. Ifanerroroccurs,Pythonwilltryeachexceptinturn
lookingforonethatmatchesthetypeoferror. Thebareexceptatthebottomactslike anelseandwill
beusedif noneoftheothersmatch.If thereis nodefaultat thebottomandnoneoftheexcepttypesmatch
theerror, thentheprogramcrashesandPythonreportstheerror.
Youcanseehow thetry...exceptstatementallowsusto writereallybullet-proofprograms.Whether
youneedtogotothismuchtroubledependsonthetypeofprogramthatyouarewriting.Inyourbeginning