ExceptionHandling Overview Exception when something unforeseen happens and

Exception-Handling Overview • Exception: when something unforeseen happens and causes an error • Exception handling – improves program clarity by removing error-handling code from the “main line” of the program – Improves a program’s fault tolerance • Exception handler catches a raised/thrown exception • try statement encloses code that may cause exception • use raise statement to indicate an exception occurred 1

number 1 = raw_input( "Enter numerator: " ) number 2 = raw_input( "Enter denominator: " ) # attempt to convert and divide values try: number 1 = float( number 1 ) number 2 = float( number 2 ) User might raise a Value. Error by entering a string that cannot be converted to float result = number 1 / number 2 # float raises a Value. Error exception except Value. Error: Division by zero raises Zero. Division. Error exception print "You must enter two numbers" # division by zero raises a Zero. Division. Error exception except Zero. Division. Error: print "Attempted to divide by zero" # else clause's suite executes if try suite raises no exceptions else: print "%. 3 f / %. 3 f = %. 3 f" % ( number 1, number 2, result )

How and where to handle exceptions Import codon translation module, look up codon not in the dictionary. What to do with the exception? • Should the module handle the exception? – return ‘? ’ ? – print error message? – exit? • . . or should the importing main program? – if translating long sequence, result will be meaningless – if outputting to file, print statements will corrupt it If user should know he’s doing something stupid, don’t catch exception (or raise one!), forcing him to realize his mistake If a meaningful or default response can be given which would not surprise the user, do that. 3

14. 3 Files and Streams • Python views files as sequential streams of bytes • Each file ends with an end-of-file marker • Opening a file creates an object associated with a stream 0 1 2 3 4 5 6 7 8 9 . . . Fig. 14. 2 n-1 end-of-file marker Python’s view of a file of n bytes. 4

Files and Streams • Three file streams created when Python program executes – sys. stdin (standard input stream) – sys. stdout (standard output stream) – sys. stderr (standard error stream) • Defaulted to keyboard, screen and screen, respectively (raw_input uses stdin, print uses stdout) • Redirect: print >> file, “Yes sir we have no bananas” • Force print now: sys. stdout. flush() 5

cd. py CD management 7

cd. py Implicitly calling the __str__ method of the cd class 8

cd. py Save cd collection to a file, line by line sys. exit method terminates the program and prints message 9

Test Prince Purple Rain 7. 4 Oscar Peterson File cdsave. dat Night Train 8. 0 Beatles cdtest. py Program output Sgt. Pepper 9. 6 Prince / Purple Rain (rating: 7. 4) Oscar Peterson / Night Train (rating: 8. 0) Beatles / Sgt. Pepper (rating: 9. 6) 10

Load cd collection from file, line by line cd. py Remove the trailing newline Create new CD object and add to collection 11

Testing the load method >>> from cd import CD, CDcollection >>> cds = CDcollection() >>> cds. load( 'cdsave. dat') >>> print cds Prince / Purple Rain (rating: 7. 4) Oscar Peterson / Night Train (rating: 8. 0) Beatles / Sgt. Pepper (rating: 9. 6) 12

cd 2. py Extending cd class with comments Write each comment on a line of its own, with a suitable number of spaces 13

cd 2 test. py Prince / Purple Rain (rating: 7. 4) [Groundbreaking!] Oscar Peterson / Night Train (rating: 8. 0) [Got it in a Starbucks in West L. A. ] [Good album, especially the Gershwin songs. ] Beatles / Sgt. Pepper (rating: 9. 6) [Groundbreaking AND evergreen!!] 14

cd 2. py New save method Write each comment on a line of its own after artist, title and rating 15

New load method. . ? Which attributes are which in the saved file? ? Prince Purple Rain 7. 4 Groundbreaking! No longer 3 lines per cd Oscar Peterson • Change save method or Night Train • Write super-intelligent load method 8. 0 Got it in a Starbucks in West L. A. Good album, especially the Gershwin songs. Beatles Sgt. Pepper 9. 6 Groundbreaking AND evergreen!! 16

New load/save methods using module c. Pickle cd_pickle. py Write entire object (list of CD objects) to file Easy to load object back into memory if you’re completely sure what kind of object the file 17 contains

cd_pickletest. py Testing the pickle save method Excerpt from file cd_picklesave. dat . . S'Purple Rain' p 6 s. S'comments' p 7 (lp 8 S'Groundbreaking!' p 9 as. S'artist' p 10 S'Prince' p 11 sba(icd_pickle CD. . 18

cd_pickletest_load. py Testing the load method We need a CDcollection object for which we can call the load method to retrieve the pickled object Prince / Purple Rain (rating: 7. 4) [Groundbreaking!] Oscar Peterson / Night Train (rating: 8. 0) [Got it in a Starbucks in West L. A. ] [Good album, especially the Gershwin songs. ] Beatles / Sgt. Pepper (rating: 9. 6) [Groundbreaking AND evergreen!!] Beatles / A Hard Day's Night (rating: 6. 8) [First album with all Lennon/Mc. Cartney songs] 19

. . on to the exercises 20