How to Except When You’re Excepting by Esther Nam

Transcript

1. from The New Book of Etiquette (1925) by Lillian Eichler
   

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2. How to Except When
   You’re Excepting
   Esther Nam PyCon 2013
   Santa Clara, CA
   Text
   

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3. •Python developer
   About Me
   

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4. About Me
   •Python developer
   •Pro since 2011
   

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5. •Python developer
   •Pro since 2011
   •No CS degree
   About Me
   

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6. You’ve Met Me At...
   •LA Hackathons
   

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7. •LA Hackathons
   •PyLadies
   You’ve Met Me At...
   

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8. •LA Hackathons
   •PyLadies
   •LA Girl Geek
   Dinners
   You’ve Met Me At...
   

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9. In this talk
   

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10. In this talk
    •What are exceptions?
    

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11. In this talk
    •What are exceptions?
    •Handling exceptions
    

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12. In this talk
    •What are exceptions?
    •Handling exceptions
    •Exceptions in Python
    

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13. In this talk
    •What are exceptions?
    •Handling exceptions
    •Exceptions in Python
    •Defensive coding
    

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14. What is an exception?
    

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15. def  square_this(number):
           """  Return  the  number  squared  """
           return  number  *  number
     >>> square_this(3)
    9
    

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16. def  square_this(number):
           """  Return  the  number  squared  """
           return  number  *  number
     >>> square_this(3)
    9
    >>> square_this(“number”)
    Traceback (most recent call last):
    File "", line 1, in
    File "", line 2, in square_this
    TypeError: can't multiply sequence by non-int of type
    'str'
    

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17. "the function's assumptions
    about its inputs are violated"
    

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18. def  square_this(number):
           return  number  *  number
     
    >>> square_this(3)
    9
    >>> square_this(“number”)
    Traceback (most recent call last):
    File "", line 1, in
    File "", line 2, in square_this
    TypeError: can't multiply sequence by non-int of type
    'str'
    

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19. def  square_this(number):
           return  number  *  number
     
    >>> square_this(3)
    9
    >>> square_this(“number”)
    Traceback (most recent call last):
    File "", line 1, in
    File "", line 2, in square_this
    TypeError: can't multiply sequence by non-int of type
    'str'
    

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20. def  square_this(number):
           return  number  *  number
     
    >>> square_this(3)
    9
    >>> square_this(“number”)
    Traceback (most recent call last):
    File "", line 1, in
    File "", line 2, in square_this
    TypeError: can't multiply sequence by non-int of type
    'str'
    

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21. def  make_sandwich(self):
           """  Make  a  sandwich  """
           sandwich.add_meat()
           sandwich.add_cheese()
           sandwich.add_pickle()
           return  sandwich
    

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22. def  make_sandwich(self):
           """  Make  a  sandwich  """
           sandwich.add_meat()
           sandwich.add_cheese()
           sandwich.add_pickle()
           return  sandwich
     
    def  add_meat(self):
           """  Slice  meat  """
           if  sandwich.meat  in  ['ham',  'beef']:  
                   raise  ValueError
    

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23. def  make_sandwich(self):
           """  Make  a  sandwich  """
           sandwich.add_meat()
           sandwich.add_cheese()
           sandwich.add_pickle()
           return  sandwich
     
    def  add_meat(self):
           """  Slice  meat  """
           if  sandwich.meat  in  ['ham',  'beef']:  
                   raise  ValueError
    

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24. >>> sandwich = Sandwich("ham")
    >>> sandwich.make_sandwich()
    def  make_sandwich(self):
           """  Make  a  sandwich  """
           sandwich.add_meat()
           sandwich.add_cheese()
           sandwich.add_pickle()
           return  sandwich
     
    def  add_meat(self):
           """  Slice  meat  """
           if  sandwich.meat  in  ['ham',  'beef']:  
                   raise  ValueError
    

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25. >>> sandwich = Sandwich("ham")
    >>> sandwich.make_sandwich()
    Traceback (most recent call last):
    […]
    ValueError
    def  make_sandwich(self):
           """  Make  a  sandwich  """
           sandwich.add_meat()
           sandwich.add_cheese()
           sandwich.add_pickle()
           return  sandwich
     
    def  add_meat(self):
           """  Slice  meat  """
           if  sandwich.meat  in  ['ham',  'beef']:  
                   raise  ValueError
    

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26. What is exception
    handling?
    

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27. def  make_sandwich(self):
           """  Make  a  sandwich  """
           try:
                   sandwich.add_meat()
           except  ValueError:
                   print  "Meat  is  murder!  Cheese  only"
           sandwich.add_cheese()
           sandwich.add_pickle()
           return  sandwich
     
    

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28. def  make_sandwich(self):
           """  Make  a  sandwich  """
           try:
                   sandwich.add_meat()
           except  ValueError:
                   print  "Meat  is  murder!  Cheese  only"
           sandwich.add_cheese()
           sandwich.add_pickle()
           return  sandwich
     
    def  add_meat(self):
           """  Slice  meat  """
           if  sandwich.meat  in  ['ham',  'beef']:  
                   sandwich.meat  =  None
                   raise  ValueError
    

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29. >>> sandwich.make_sandwich()
    def  make_sandwich(self):
           """  Make  a  sandwich  """
           try:
                   sandwich.add_meat()
           except  ValueError:
                   print  "Meat  is  murder!  Cheese  only"
           sandwich.add_cheese()
           sandwich.add_pickle()
           return  sandwich
     
    def  add_meat(self):
           """  Slice  meat  """
           if  sandwich.meat  in  ['ham',  'beef']:  
                   sandwich.meat  =  None  
                   raise  ValueError
    

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30. >>> sandwich.make_sandwich()
    Meat is murder! Cheese only
    >>> sandwich.meat
    >>>
    def  make_sandwich(self):
           """  Make  a  sandwich  """
           try:
                   sandwich.add_meat()
           except  ValueError:
                   print  "Meat  is  murder!  Cheese  only"
           sandwich.add_cheese()
           sandwich.add_pickle()
           return  sandwich
     
    def  add_meat(self):
           """  Slice  meat  """
           if  sandwich.meat  in  ['ham',  'beef']:  
                   sandwich.meat  =  None  
                   raise  ValueError
    

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31. What causes exceptions?
    

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32. •Programming errors
    

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33. •Programming errors
    •Client code errors
    

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34. •Programming errors
    •Client code errors
    •Resource failures
    

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35. Exiting may be preferable to
    compounding errors
    

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36. Why not use error codes?
    

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37. ERROR  =  -­‐1
    def  a():
           value  =  b()
           
           if  value  !=  ERROR:
                   do_the_next_thing()
           else:
                   barf()
     
    

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38. ERROR  =  -­‐1
    def  a():
           value  =  b()
           
           if  value  !=  ERROR:
                   do_the_next_thing()
           else:
                   barf()
     
    def  b():
           bvalue  =  c()
           
           if  bvalue  !=  ERROR:
                   return  bvalue  +  1
           else:
                   barf()
    

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39. ERROR  =  -­‐1
    def  a():
           value  =  b()
           
           if  value  !=  ERROR:
                   do_the_next_thing()
           else:
                   barf()
     
    def  b():
           bvalue  =  c()
           
           if  bvalue  !=  ERROR:
                   return  bvalue  +  1
           else:
                   barf()
    def  c():
           return  ERROR
    

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40. def  a():
           try:  
                   value  =  b()
                   do_the_next_thing()
           except  ValueError:
                   barf()
     
    def  b():
           bvalue  =  c()      
           return  bvalue  +  1
    def  c():
           something_that_causes_ValueError()
           #  Oh  no,  a  ValueError  was  raised!
           return  cvalue
    

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41. enum  {
           CMDERR_OPTION_UNKNOWN  =  -­‐3,  /*  unknown  -­‐option  */
           CMDERR_OPTION_AMBIGUOUS  =  -­‐2,  /*  ambiguous  -­‐option  */
           CMDERR_OPTION_ARG_MISSING  =  -­‐1,  /*  argument  missing  for  -­‐option  */
           CMDERR_UNKNOWN,  /*  unknown  command  */
           CMDERR_AMBIGUOUS,  /*  ambiguous  command  */
           CMDERR_ERRNO,  /*  get  the  error  from  errno  */
           CMDERR_NOT_ENOUGH_PARAMS,  /*  not  enough  parameters  given  */
           CMDERR_NOT_CONNECTED,  /*  not  connected  to  server  */
           CMDERR_NOT_JOINED,  /*  not  joined  to  any  channels  in  this  window  */
           CMDERR_CHAN_NOT_FOUND,  /*  channel  not  found  */
           CMDERR_CHAN_NOT_SYNCED,  /*  channel  not  fully  synchronized  yet  */
           CMDERR_ILLEGAL_PROTO,  /*  requires  different  chat  protocol  than  the  active  server  */
           CMDERR_NOT_GOOD_IDEA,  /*  not  good  idea  to  do,  -­‐yes  overrides  this  */
           CMDERR_INVALID_TIME,  /*  invalid  time  specification  */
           CMDERR_INVALID_CHARSET,  /*  invalid  charset  specification  */
           CMDERR_EVAL_MAX_RECURSE,  /*  eval  hit  recursion  limit  */
           CMDERR_PROGRAM_NOT_FOUND  /*  program  not  found  */
    };
    

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42. Why handle exceptions?
    

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43. Error may not be fatal
    

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44. What kinds of exceptions
    can I catch?
    

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45. BaseException
    +-- SystemExit
    +-- KeyboardInterrupt
    +-- GeneratorExit
    +-- Exception
    

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46. BaseException
    +-- SystemExit
    +-- KeyboardInterrupt
    +-- GeneratorExit
    +-- Exception
    

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47. +-- Exception
    +-- StopIteration
    +-- StandardError
    | +-- BufferError
    | +-- ArithmeticError
    | | +-- FloatingPointError
    | | +-- OverflowError
    | | +-- ZeroDivisionError
    | +-- AssertionError
    | +-- AttributeError
    | +-- EnvironmentError
    | | +-- IOError
    | | +-- OSError
    | | +-- WindowsError (Windows)
    | | +-- VMSError (VMS)
    | +-- EOFError
    | +-- ImportError
    | +-- LookupError
    | | +-- IndexError
    | | +-- KeyError
    | +-- MemoryError
    | +-- NameError
    | | +-- UnboundLocalError
    | +-- ReferenceError
    | +-- RuntimeError
    | | +-- NotImplementedError
    | +-- SyntaxError
    | | +-- IndentationError
    | | +-- TabError
    | +-- SystemError
    | +-- TypeError
    | +-- ValueError
    | +-- UnicodeError
    | +-- UnicodeDecodeError
    | +-- UnicodeEncodeError
    | +-- UnicodeTranslateError
    +-- Warning
    +-- DeprecationWarning
    +-- PendingDeprecationWarning
    +-- RuntimeWarning
    +-- SyntaxWarning
    +-- UserWarning
    +-- FutureWarning
    +-- ImportWarning
    +-- UnicodeWarning
    +-- BytesWarning
    http://docs.python.org/2/library/exceptions.html#exception-hierarchy
    

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48. Who should catch the
    exception?
    

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49. Where things break
    •Application
    •Framework
    •System-level
    

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50. Can I just catch all of them?
    

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51. Catch all the exceptions?
    try:
           results  =  run_query()
    except:
           handle_error()
     
    

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52. BaseException
    +-- SystemExit
    +-- KeyboardInterrupt
    +-- GeneratorExit
    +-- Exception
    

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53. Catch all the exceptions?
    try:
           results  =  run_query()
    except:
           handle_error()
    

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54. Catch all the exceptions?
    try:
           results  =  run_query()
    except  Exception:
           handle_error()
     
    

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55. try:
           results  =  run_query()
    except  Exception:
           return_empty_rows()
     
    def  run_query():
           connect_to_db()
           sort_rows()
           
    def  connect_to_db():
           if  connection_is_dead:
                   raise  DatabaseNotConnectedError
    

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56. try:
           results  =  run_query()
    except  Exception:
           handle_error()
    def  run_query():
           connect_to_db()
           sort_rows()
           
    def  connect_to_db():
           if  connection_is_dead:
                   raise  DatabaseNotConnectedError
    

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57. try:
           results  =  run_query()
    except  Exception:
           handle_error()
    def  run_query():
           connect_to_db()
           sort_rows()
           
    def  connect_to_db():
           if  connection_is_dead:
                   raise  DatabaseNotConnectedError
    

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58. What if there’s more than one
    possible exception?
    

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59. try:
    do_something()
    do_the_next_thing()
    except AnException:
    handle_exception()
    Catching multiple exceptions
    

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60. try:
    do_something()
    do_the_next_thing()
    except (ExceptionA, ExceptionB):
    handle_exception()
    Catching multiple exceptions
    

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61. try:
    do_something()
    do_the_next_thing()
    except AnException:
    handle_exception()
    except FatalException:
    just_die()
    Catching multiple exceptions
    

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62. try:
    do_some_math()
    # ZeroDivisionError is a subclass of
    # ArithmeticError
    except ZeroDivisionError:
    just_die()
    except ArithmeticError:
    try_to_handle_this()
    Catching granular exceptions
    

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63. Catching lots of exceptions?
    try:
    do_some_math()
    except OneException:
    just_die()
    except AnotherException:
    try_to_handle_this()
    except YetAnotherException:
    just_log_this_one()
    except YetAnotherException:
    i_dunno_what_to_do_anymore()
    

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64. try:
    do_some_math()
    except OneException:
    just_die()
    except AnotherException:
    try_to_handle_this()
    except YetAnotherException:
    just_log_this_one()
    except YetAnotherException:
    i_dunno_what_to_do_anymore()
    Catching lots of exceptions?
    Re-think your inputs
    

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65. So you’ve caught an exception
    

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66. try:
    do_something()
    except ValueError:
    pass
    Handling exceptions
    

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67. Continue as though nothing
    went wrong?
    

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68. Exceptions are always handled
    

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69. try:
    do_something()
    except ValueError:
    logger.error()
    

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70. Delay the exception
    try:
    open_file()
    read_the_file()
    except AnException as exception:
    clean_up()
    

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71. try:
    open_file()
    read_the_file()
    except AnException as exception:
    clean_up()
    logger(“Log: %s”” % exception)
    Delay the exception
    

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72. try:
    open_file()
    read_the_file()
    except AnException as exception:
    clean_up()
    logger(“Log: %s”” % exception)
    raise
    Delay the exception
    

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73. Re-try
    for attempt in number_of_attempts:
    connected = False
    try:
    connect_to_server()
    connected = True
    except ConnectionFail:
    wait_one_second()
    return connected
    

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74. try:
    do_something()
    do_the_next_thing()
    except:
    # Something bad happened, not sure what
    Re-raising an exception
    

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75. try:
    do_something()
    do_the_next_thing()
    except:
    # Something bad happened, not sure what
    raise
    Re-raising an exception
    

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76. try:
    do_something()
    do_the_next_thing()
    except ValueError:
    # Something bad happened, not sure what
    raise ValueError
    Re-raising an exception
    

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77. try:
    do_something()
    do_the_next_thing()
    except IOError:
    raise
    Raising another exception
    

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78. try:
    do_something()
    do_the_next_thing()
    except MyCustomError:
    raise ValueError
    Raising another exception
    

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79. Why would I create
    my own exceptions?
    

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80. Isolate exceptions caused by
    your application from
    exceptions caused by other
    code
    

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81. Custom exceptions
    class SandwichException(Exception):
    pass
    

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82. +-- Exception
    +-- StopIteration
    +-- StandardError
    | +-- BufferError
    | +-- ArithmeticError
    | | +-- FloatingPointError
    | | +-- OverflowError
    | | +-- ZeroDivisionError
    | +-- AssertionError
    | +-- AttributeError
    | +-- EnvironmentError
    | | +-- IOError
    | | +-- OSError
    | | +-- WindowsError (Windows)
    | | +-- VMSError (VMS)
    | +-- EOFError
    | +-- ImportError
    | +-- LookupError
    | | +-- IndexError
    | | +-- KeyError
    | +-- MemoryError
    | +-- NameError
    | | +-- UnboundLocalError
    | +-- ReferenceError
    | +-- RuntimeError
    | | +-- NotImplementedError
    | +-- SyntaxError
    | | +-- IndentationError
    | | +-- TabError
    | +-- SystemError
    | +-- TypeError
    | +-- ValueError
    | +-- UnicodeError
    | +-- UnicodeDecodeError
    | +-- UnicodeEncodeError
    | +-- UnicodeTranslateError
    +-- Warning
    +-- DeprecationWarning
    +-- PendingDeprecationWarning
    +-- RuntimeWarning
    +-- SyntaxWarning
    +-- UserWarning
    +-- FutureWarning
    +-- ImportWarning
    +-- UnicodeWarning
    +-- BytesWarning
    http://docs.python.org/2/library/exceptions.html#exception-hierarchy
    

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83. +-- Exception
    +-- StopIteration
    +-- StandardError
    | +-- BufferError
    | +-- ArithmeticError
    | | +-- FloatingPointError
    | | +-- OverflowError
    | | +-- ZeroDivisionError
    | +-- AssertionError
    | +-- AttributeError
    | +-- EnvironmentError
    | | +-- IOError
    | | +-- OSError
    | | +-- WindowsError (Windows)
    | | +-- VMSError (VMS)
    | +-- EOFError
    | +-- ImportError
    | +-- LookupError
    | | +-- IndexError
    | | +-- KeyError
    | +-- MemoryError
    | +-- NameError
    | | +-- UnboundLocalError
    | +-- ReferenceError
    | +-- RuntimeError
    | | +-- NotImplementedError
    | +-- SyntaxError
    | | +-- IndentationError
    | | +-- TabError
    | +-- SystemError
    | +-- TypeError
    | +-- ValueError
    | +-- UnicodeError
    | +-- UnicodeDecodeError
    | +-- UnicodeEncodeError
    | +-- UnicodeTranslateError
    +-- Warning
    +-- DeprecationWarning
    +-- PendingDeprecationWarning
    +-- RuntimeWarning
    +-- SyntaxWarning
    +-- UserWarning
    +-- FutureWarning
    +-- ImportWarning
    +-- UnicodeWarning
    +-- BytesWarning
    http://docs.python.org/2/library/exceptions.html#exception-hierarchy
    

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84. +-- Exception
    +-- StopIteration
    +-- StandardError
    | +-- BufferError
    | +-- ArithmeticError
    | | +-- FloatingPointError
    | | +-- OverflowError
    | | +-- ZeroDivisionError
    | +-- AssertionError
    | +-- AttributeError
    | +-- EnvironmentError
    | | +-- IOError
    | | +-- OSError
    | | +-- WindowsError (Windows)
    | | +-- VMSError (VMS)
    | +-- EOFError
    | +-- ImportError
    | +-- LookupError
    | | +-- IndexError
    | | +-- KeyError
    | +-- MemoryError
    | +-- NameError
    | | +-- UnboundLocalError
    | +-- ReferenceError
    | +-- RuntimeError
    | | +-- NotImplementedError
    | +-- SyntaxError
    | | +-- IndentationError
    | | +-- TabError
    | +-- SystemError
    | +-- TypeError
    | +-- ValueError
    | +-- UnicodeError
    | +-- UnicodeDecodeError
    | +-- UnicodeEncodeError
    | +-- UnicodeTranslateError
    +-- Warning
    +-- DeprecationWarning
    +-- PendingDeprecationWarning
    +-- RuntimeWarning
    +-- SyntaxWarning
    +-- UserWarning
    +-- FutureWarning
    +-- ImportWarning
    +-- UnicodeWarning
    +-- BytesWarning
    http://docs.python.org/2/library/exceptions.html#exception-hierarchy
    | +-- RuntimeError
    | | +-- NotImplementedError
    | +-- SyntaxError
    | | +-- IndentationError
    | | +-- TabError
    | +-- SystemError
    | +-- TypeError
    | +-- ValueError
    | +-- UnicodeError
    | +-- UnicodeDecodeError
    | +-- UnicodeEncodeError
    | +-- UnicodeTranslateError
    +-- Warning
    +-- DeprecationWarning
    

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85. exception ValueError
    •Raised when a built-in operation
    or function receives an argument
    that has the right type but an
    inappropriate value, and the
    situation is not described by a
    more precise exception such as
    IndexError.
    

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86. Custom exceptions
    class SandwichException(Exception):
    pass
    

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87. Custom exceptions
    class SandwichException(ValueError):
    pass
    

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88. Granular custom exceptions
    class SandwichException(ValueError):
    pass
    class MeatException(SandwichException):
    pass
    

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89. def make_sandwich(self):
    try:
    sandwich.get_bread()
    sandwich.get_meat()
    except SandwichException:
    return_failed_sandwich()
    def get_bread(self):
    bread = this_raisesValueError()
    # oh no! that raised a ValueError!
    return bread
    

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90. def make_sandwich(self):
    try:
    sandwich.get_bread()
    sandwich.get_meat()
    except SandwichException:
    return_failed_sandwich()
    def get_bread(self):
    bread = this_raisesValueError()
    # oh no! that raised a ValueError!
    return bread
    

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91. Don't expose
    implementation details
    

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92. from Bakeries import NewYorkBakery
    from Bakeries import NewYorkBakeryException
    def make_sandwich(self):
    try:
    sandwich.get_bread()
    sandwich.get_meat()
    except MeatException:
    return_cheese_sandwich()
    except SandwichException:
    return_failed_sandwich()
    def get_bread(self):
    try:
    bread = NewYorkBakery()
    except NewYorkBakeryException:
    raise SandwichException
    

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93. from Bakeries import CaliforniaBakery
    from Bakeries import CaliforniaBakeryException
    def make_sandwich(self):
    try:
    sandwich.get_bread()
    sandwich.get_meat()
    except MeatException:
    return_cheese_sandwich()
    except SandwichException:
    return_failed_sandwich()
    def get_bread(self):
    try:
    bread = CaliforniaBakery()
    except CaliforniaBakeryException:
    raise SandwichException
    

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94. try:
    file = open(‘file.txt’, ‘r’)
    except IOError as error:
    print “Can’t open file, screw it”
    else:
    # We can do this only if the exception
    # was not raised. If read() raises an
    # IOError, we can handle it differently
    contents = file.read()
    else
    

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95. finally
    try:
    file = open(‘file.txt’, ‘r’)
    except IOError as error:
    print “Can’t open file, screw it”
    finally:
    file.close()
    

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96. The context manager:
    Cleaner code with with
    

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97. def my_function():
    try:
    do_something()
    do_the_next_thing()
    except AnException as exception:
    handle_that_exception(exception)
    finally:
    do_this_no_matter_what()
    

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98. from contextlib import contextmanager
    @contextmanager
    def try_this():
    try:
    yield # calls functions in try_this()
    except (ValueError, TypeError):
    handle_that_exception()
    finally:
    clean_up_step()
    def my_function():
    with try_this(): # this is the context
    do_something()
    do_something_else()
    

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99. Look Before You Leap
    v.
    Easier to Ask for
    Forgiveness than Permission
    

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100. View Slide

101. if foo:
     do_foo()
     elif bar:
     do_bar()
     elif baz:
     do_baz()
     else:
     barf()
     

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102. •__iter__() raises
     StopIteration
     •__getitem__() raises
     IndexError
     

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103. Defensive Programming
     

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104. Expecting the unexpected
     

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105. known
     knowns
     Expecting the unexpected
     

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106. known
     knowns
     known
     unknowns
     Expecting the unexpected
     

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107. known
     knowns
     unknown
     unknowns
     known
     unknowns
     Expecting the unexpected
     

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108. unknown
     knowns
     known
     knowns
     unknown
     unknowns
     known
     unknowns
     Expecting the unexpected
     

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109. Think through your assumptions
     

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110. Every bug is 2 bugs
     

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111. Every bug is 2 bugs
     1. the bug in the code
     

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112. Every bug is 2 bugs
     1. the bug in the code
     2. the test you didn’t write
     

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113. How do you know you’ve
     anticipated necessary
     exceptions?
     

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114. Test your error-handling
     

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115. All exceptions are handled
     

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116. http://newcars.com/about
     Thanks
     

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117. NewCars Tech Team
     SoCal Python
     Audrey Roy
     Danny Greenfeld
     Chris McDonough
     Doug Napoleone
     Rachel Sanders
     Thanks
     

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118. estherbester on:
     •Twitter
     •GitHub
     •IRC (freenode)
     

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119. Related topics
     •Logging
     •Debugging exceptions
     •sys.exc_info()
     •Exception chaining (Python 3)
     •Testing
     

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120. References
     http://blog.ianbicking.org/2007/09/12/re-raising-exceptions/
     http://codemonkeyism.com/7-good-rules-to-log-exceptions/
     http://docs.python.org/2/tutorial/errors.html
     http://docs.python.org/3.0/whatsnew/3.0.html
     http://doughellmann.com/2008/05/pymotw-contextlib.html
     http://en.wikipedia.org/wiki/Defensive_programming
     http://en.wikipedia.org/wiki/Exception_safety
     http://jeffknupp.com/blog/2013/02/06/write-cleaner-python-use-exceptions/
     http://jessenoller.com/blog/2009/02/03/get-with-the-program-as-contextmanager-completely-
     different
     http://politechnosis.kataire.com/2008/02/all-exceptions-are-handled.html
     http://stackoverflow.com/questions/696047/re-raising-exceptions-with-a-different-type-and-
     message-preserving-existing-inf
     http://stackoverflow.com/questions/7108193/frequently-repeated-try-except-in-python?rq=1
     http://stackoverflow.com/questions/77127/when-to-throw-an-exception
     http://today.java.net/article/2006/04/04/exception-handling-antipatterns
     http://www.blog.pythonlibrary.org/2012/08/02/python-101-an-intro-to-logging/
     http://www.itmaybeahack.com/homepage/books/nonprog/html/p09_exc_iter/
     p09_c01_exception.html#designing-exceptionshttps://en.wikipedia.org/wiki/
     There_are_known_knowns#Quoted_from_Persian_literature
     

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