![Introduction to
computational thinking
Module 12 : Exceptions
Module 12 : Exceptions
Asst Prof Michael Lees
Office: N4‐02c‐76
email: mhlees[at]ntu.edu.sg
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Lecture 12 exceptions
- 2. Contents 1. Why & What are exceptions 2. Try/Except group 3. Exception Philosophy 4. else and finally suites Module 12 : Exceptions Chapter 14 2
- 3. Dealing with problems • Most modern languages provide ways to deal with ‘exceptional’ situations • Try to capture certain situations/failures and deal with them gracefully • All about being a good programmer! Module 12 : Exceptions 3
- 4. What counts as an exception • Errors: Indexing past the end of a list, trying to open a nonexistent file, fetching a nonexistent key from a dictionary, etc. • Events: Search algorithm doesn’t find a value (not really an error), mail message arrives, queue event occurs. • Ending conditions: File should be closed at the end of processing, list should be sorted after being filled. • Weird stuff: For rare events, keep from clogging your code with lots of if statements. Module 12 : Exceptions 4
- 5. General idea 4 general steps: 1. Keep watch on a particular section of code 2. If we get an exception, raise/throw that exception (let it be known) 3. Look for a catcher that can handle that kind of exception 4. If found, handle it, otherwise let Python handle it (which usually halts the program) Module 12 : Exceptions 5
- 6. Bad input • In general, we have assumed that the input we receive is correct (from a file, from the user). • This is almost never true. There is always the chance that the input could be wrong. • Our programs should be able to handle this. • "Writing Secure Code,” by Howard and LeBlanc – “All input is evil until proven otherwise.” Module 12 : Exceptions 6
- 8. Try suite • The try suite contains code that we want to monitor for errors during its execution. • If an error occurs anywhere in that try suite, Python looks for a handler that can deal with the error. • If no special handler exists, Python handles it, meaning the program halts and with an error message as we have seen so many times Module 12 : Exceptions try: Code to run 8
- 9. Except suite • An except suite (perhaps multiple except suites) is associated with a try suite. • Each exception names a type of exception it is monitoring for (can handle). • If the error that occurs in the try suite matches the type of exception, then that except suite is activated. Module 12 : Exceptions except aParticularError: Stuff to do on error 9
- 10. try/except group • If no exception in the try suite, skip all the try/except to the next line of code. • If an error occurs in a try suite, look for the right exception. • If found, run that except suite, and then skip past the try/except group to the next line of code. • If no exception handling found, give the error to Python. Module 12 : Exceptions 10
- 12. An example try: print('Entering try suite') # trace dividend = float(input('dividend:')) divisor = float(input('divisor:')) result = dividend/divisor print('{:2.2f} divided by {:2.2f} = ’ '{:2.2f}'.format(dividend,divisor,result)) except ZeroDivisionError: print('Divide by 0 error') except ValueError: print("Couldn't convert to a float") print('Continuing with the rest of the program') Module 12 : Exceptions Try Suite Except Suite 1 Except Suite 1 Try/Except Group 12
- 13. What exceptions are there? • In the present Python, there is a set of exceptions that are pre‐labeled. • To find the exception for a case you are interested in, easy enough to try it in the interpreter and see what comes up. 3/0 => ZeroDivisionError: integer division or modulo by zero Module 12 : Exceptions 13
- 17. Look before you leap • Super cautious! • Check all aspects before executing – If string required : check that – if values should be positive : check that • What happens to length of code? • And readability of code – code is hidden in checking. Module 12 : Exceptions 17
- 18. Easier to ask forgiveness than permission • Run anything you like! • Be ready to clean up in case of error. • The try suite code reflects what you want to do, and the except code what you want to do on error. Cleaner separation! Module 12 : Exceptions 18
- 19. A Choice • Python programmers support the EAFP approach: – Run the code (in try) and used except suites to deal with errors (don’t check first) Module 12 : Exceptions if not isinstance(s, str) or not s.isdigit: return None elif len(s) > 10: # too many digits to convert return None else: return int(str) try: return int(str) except (TypeError, ValueError, OverflowError): return None LBYL EAFP 19
- 24. Thought process • What errors can the code generate? • Capture the common ones and give clear instruction • Use the else condition to print out answer only when no error Module 12 : Exceptions 24
- 25. Take home lessons • Using exception handling to help users is important! • Different types of exceptions: events, errors, etc. • LYBL vs. EAFP • try‐except‐else‐finally suites (in many languages) Module 12 : Exceptions 25
- 26. Further reading • http://docs.python.org/tutorial/errors.html • http://diveintopython.org/file_handling/index .html • http://oranlooney.com/lbyl‐vs‐eafp/ Module 12 : Exceptions 26