CSE : Python Programming. Homework 5 and Projects. Announcements. Course project: Overview. Course Project: Grading criteria

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Robyn McKenzie
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1 CSE : Python Programming Lecture 5: Course project and Exceptions February 12, 2007 Announcements Still working on grading Homeworks 3 and 4 (and 2 ) Homework 5 will be out by tomorrow morning I want to finish with Homework 4 before I hand out another coding assignment It's time to start thinking about projects 2 Course project: Overview Everyone here comes from different backgrounds and is interested in different things Homework 5 and Projects I'd like everyone to try their hand at working on a program of non-homework size Since there's no way a one-size-fits-all project will work, everyone will propose to me a project Homework 5: Writing your project proposal 4 Course project: Hard requirements Group size: no more than 3 students per/group The more students in the group, the more ambitious the project will need to be You will have to convince me the project is not too trivial for you nor too ambitious The project should be programming based, the major portion of which should be in Python Course Project: Grading criteria 60% Delivering what you proposed Program should work as advertised Bugs should be non-existent / "minimal" Code should be reasonably structured / written 40% Reasonable documentation Enough to understand and use your program without having read the source code 5 6

Course project: Grading mechanics I'll score the project out of 100 points Everyone in the group will get the same grade Recall: The project counts for 40% of your course grade Course project:

2 Course project: Grading mechanics I'll score the project out of 100 points Everyone in the group will get the same grade Recall: The project counts for 40% of your course grade Course project: Planning Time: 3 5 hours per week I may continue to give out short homeworks A couple of weeks before the end of classes: Plan on a short meeting with me to give a status report Last week of classes: Plan on a short meeting with me to go over your code and project I expect you to spread your work out through the term, and I don't want any unpleasant surprises at the end 7 8 Course project: Warning You're responsible for giving me reasonable notice if your project is going far more slowly than expected Lecture Lecture will go on, even as you work on the projects If you find yourself spending too much time on this project, you have some options: Bite the bullet (don't blame me then for problems) Talk to me (recommended) 9 10 Handling errors: Option #1 Exceptions (Tutorial, Section 8) (Kevin & Kell, ) 12

3 Handling errors: Option #2 Use special return values to indicate an error None Negative integers Debatable example: read() returning an empty string Advantages: Simple to implement and document Disadvantages: Requires an "invalid" value for a function to return Ad-hoc Propagating an error is not straightforward 13 Handling errors: Option #3 Use exceptions Exceptions are a form of non-local control flow An exception happens at one point in your code Control goes to some other point in your code Relationship between the points determined by what's happening at run-time Transfer control from where the error happened to some place that can actually deal with the error 14 When to look for and handle errors User input Users always make mistakes Absolutely pointless to tell users "don't do foo" Data from external sources Not really that different from user input Generally, any interaction with an external resource Errors not worth handling Random processor faults Not feasible to run all computations twice Out of memory Difficult to recover from Generally, anything rare and difficult to recover from The above example are rare things A non-existent file is, by comparison, common Network resources being down is also common What to do on an error Let your program die Sometimes there is nothing anyone can do But still provide a helpful message Deal with it yourself You did the thing which caused the error Caller isn't expecting anything to go wrong Raising (throwing) an exception >>> raise IndexError("silly message") IndexError: silly message Signal that someone else needs to handle the error You don't know how to proceed Caller should be expecting something to go wrong Provide as much data about the error as you can 17 18

4 Raising (throwing) an exception >>> raise IndexError("silly message") IndexError: silly message Raising (throwing) an exception >>> raise IndexError("silly message") IndexError: silly message Syntax: raise some_instance If the exception isn't "caught", it's printed out as a string Raising (throwing) an exception >>> raise IndexError("silly message") IndexError: silly message An exception backtrace is also printed out. This can be used to determine what was going on when the exception was raised. baz.py def : % python baz.py File "baz.py", line 4, in <module> File "baz.py", line 2, in foo KeyError: 'A silly example.' baz.py baz.py def : def : % python baz.py File "baz.py", line 4, in <module> File "baz.py", line 2, in foo KeyError: 'A silly example.' % python baz.py File "baz.py", line 4, in <module> File "baz.py", line 2, in foo KeyError: 'A silly example.' exception backtrace 23 The last item in the trace lists exactly where the exception was raised. 24

5 baz.py def : baz.py def : % python baz.py File "baz.py", line 4, in <module> File "baz.py", line 2, in foo KeyError: 'A silly example.' % python baz.py File "baz.py", line 4, in <module> File "baz.py", line 2, in foo KeyError: 'A silly example.' The other items in the trace tell us what we were executing at the time. 25 Reading the backtrace top to bottom, we can get an idea of exactly how we reached the line that raised the exception. 26 Built-in exceptions The exceptions module defines all built-in exceptions There is no need to import this module explicitly See Section 2.3 of the library reference for the full list Defining your own exceptions Define a class which derives from the built-in Exception class, either directly or indirectly Document it try and raise to your heart's content BaseException +-- SystemExit +-- KeyboardInterrupt +-- Exception +-- GeneratorExit +-- StopIteration +-- StandardError +-- ArithmeticError +-- FloatingPointError +-- OverflowError +-- ZeroDivisionError Handling exceptions Basic exception handling File "<stdin>", line 2, in divide ZeroDivisionError: integer division or modulo by zero try: except: print "Something went wrong." Something went wrong

6 Basic exception handling try: except: print "Something went wrong." Something went wrong. Basic exception handling try: except: print "Something went wrong." Something went wrong. "try except " statements are used to catch (i.e., handle) exceptions if the code in this try block raises an exception, then Basic exception handling try: except: print "Something went wrong." Something went wrong. Basic exception handling try: except: print "Something went wrong." Something went wrong. then execute the code in this except block no exception backtrace because we successfully caught the exception "try except " statements These statements come in many forms There can be multiple except blocks Each one names the type of instances it catches Only the last one may omit the type There can be an else block, which is executed when the try block finishes executing normally "Normal" excludes return, break, and continue, as well the case where an exception is raised try: result = x / int(y) except ValueError, inst: print "Oops, y wasn't an integer." print inst except ZeroDivisionError, inst: print "Caught and reraising division by zero." raise >>> divide(2,"hello") Oops, y wasn't an integer. invalid literal for int() with base 10: 'hello' There can be a finally block, which is executed no matter what happens 35 36

7 try: result = x / int(y) except ValueError, inst: print "Oops, y wasn't an integer." print inst except ZeroDivisionError, inst: print "Caught and reraising division by zero." raise >>> divide(2,"hello") Oops, y wasn't an integer. invalid literal for int() with base 10: 'hello' try: result = x / int(y) except ValueError, inst: print "Oops, y wasn't an integer." print inst except ZeroDivisionError, inst: print "Caught and reraising division by zero." raise >>> divide(2,"hello") Oops, y wasn't an integer. invalid literal for int() with base 10: 'hello' Syntax: except class_name, var_name inst here will be an instance of ValueError try: result = x / int(y) except ValueError, inst: print "Oops, y wasn't an integer." print inst except ZeroDivisionError, inst: print "Caught and reraising division by zero." raise Caught and reraising division by zero. File "<stdin>", line 3, in divide ZeroDivisionError: integer division or modulo by zero 39 try: result = x / int(y) except ValueError, inst: print "Oops, y wasn't an integer." print inst except ZeroDivisionError, inst: print "Caught and reraising division by zero." raise Caught and reraising division by zero. File "<stdin>", line 3, in divide ZeroDivisionError: integer division or modulo by zero if there is a pending exception, raise by itself will throw it again 40 try: result = x / int(y) except Exception: print "Oops." except ZeroDivisionError, inst: print "Caught division by zero." Oops. try: result = x / int(y) except Exception: print "Oops." except ZeroDivisionError, inst: print "Caught division by zero." Oops. 41 You don't have to give a name to the instance. 42

8 try: result = x / int(y) except Exception: print "Oops." except ZeroDivisionError, inst: print "Caught division by zero." Oops. try: result = x / int(y) except ValueError: print "Didn't get an integer." except: print "Caught something." Caught something. With multiple except blocks, only the first one that applies is executed try: result = x / int(y) except ValueError: print "Didn't get an integer." except: print "Caught something." Caught something. try: result = x / int(y) except ValueError: print "Didn't get an integer." except: print "Caught something." Caught something. Only the final except is allowed to leave off the type of the exception. In this case, it catches any exception at all. 45 This is dangerous! It's very easy to hide significant programming errors this way. 46 try: result = x / int(y) except ValueError: raise ZeroDivisionError() except: print "Caught something." >>> divide(2,"hello") File "<stdin>", line 5, in divide ZeroDivisionError try: result = x / int(y) except ValueError: raise ZeroDivisionError() except: print "Caught something." >>> divide(2,"hello") File "<stdin>", line 5, in divide ZeroDivisionError 47 Exceptions raised in an except block are not handled by other except blocks in the same "try except " statement. 48

9 Suppose an exception is raised somewhere. If that point is not surrounded by an appropriate "try except ", we start looking up the call-stack We execute the first handler that applies, if any. try: return x / int(y) except ValueError: print "Value error!" try: foo(x, y) except ZeroDivisionError: print "Div by zero!" >>> bar(2, "hello") Value error! try: return x / int(y) except ValueError: print "Value error!" try: foo(x, y) except ZeroDivisionError: print "Div by zero!" >>> bar(2, "hello") Value error! try: return x / int(y) except ValueError: print "Value error!" try: foo(x, y) except ZeroDivisionError: print "Div by zero!" >>> bar(2, 0) Div by zero! The ValueError here is handled by the immediately enclosing "try except " statement try: return x / int(y) except ValueError: print "Value error!" try: foo(x, y) except ZeroDivisionError: print "Div by zero!" >>> bar(2, 0) Div by zero! try: return x / int(y) except ValueError: print "Value error!" try: foo(x, y) except ZeroDivisionError: print "Div by zero!" >>> bar(2, 0) Div by zero! Nothing in foo handles ZeroDivisionError, so see if whoever called foo handles it. Now it looks like foo(x, y) raised ZeroDivisionError. Lucky us, bar handles this

10 try: return x / int(y) except ValueError: print "Value error!" try: foo(x, y) except ZeroDivisionError: print "Div by zero!" >>> bar("bogus", 3) File "<stdin>", line 2, in bar File "<stdin>", line 2, in foo TypeError: unsupported operand type(s) for /: 'str' and 'int' try: return x / int(y) except ValueError: print "Value error!" try: foo(x, y) except ZeroDivisionError: print "Div by zero!" >>> bar("bogus", 3) File "<stdin>", line 2, in bar File "<stdin>", line 2, in foo TypeError: unsupported operand type(s) for /: 'str' and 'int' 55 No one handles TypeError, so the toploop prints out the backtrace. 56 else and finally clauses try: except ZeroDivisionError: print "division by zero!" else: print "result is", result finally: print "executing finally clause" >>> divide(2, 1) result is 2 executing finally clause else and finally clauses try: except ZeroDivisionError: print "division by zero!" else: print "result is", result finally: print "executing finally clause" >>> divide(2, 1) result is 2 executing finally clause 57 Executed whenever we leave the try block "normally". 58 else and finally clauses try: except ZeroDivisionError: print "division by zero!" else: print "result is", result finally: print "executing finally clause" >>> divide(2, 1) result is 2 executing finally clause else and finally clauses try: except ZeroDivisionError: print "division by zero!" else: print "result is", result finally: print "executing finally clause" >>> divide(2, 0) division by zero! executing finally clause Executed no matter what happens in the try, except, and else clauses

11 else and finally clauses try: except ZeroDivisionError: print "division by zero!" else: print "result is", result finally: print "executing finally clause" >>> divide("2", "1") executing finally clause File "<stdin>", line 1, in? File "<stdin>", line 3, in divide TypeError: unsupported operand type(s) for /: 'str' and 'str' Next time??? If you drop me a short by Friday with what your project might be on, I might try to cover things which many people would find helpful 61 62

Outline. the try-except statement the try-finally statement. exceptions are classes raising exceptions defining exceptions

Outline. the try-except statement the try-finally statement. exceptions are classes raising exceptions defining exceptions Outline 1 Exception Handling the try-except statement the try-finally statement 2 Python s Exception Hierarchy exceptions are classes raising exceptions defining exceptions 3 Anytime Algorithms estimating