exceptions try except Exception as error traceback stack trace

Exception handling is relatively easy in Python
"A try/except block is extremely efficient. Actually executing an exception is expensive."


try:
    example_divide( dividend , divisor )   # may raise FloatingPointError or ZeroDivisionError
except ( FloatingPointError, ZeroDivisionError ) as e:
    repr( e )       # e.g. prints ZeroDivisionError( "divisor cannot be 0" ), whereas print str( e ) will only print "divisor cannot be 0"


import traceback
logging.error( traceback.print_exc( ) )     # log the stack trace

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try:
    raise Exception('spam', 'eggs')
except Exception as inst:
    print type(inst)     # the exception instance
    print inst.args      # arguments stored in .args
    print inst           # __str__ allows args to printed directly
    x, y = inst.args
    print 'x =', x
    print 'y =', y


for arg in sys.argv[1:]:
    try:
        f = open(arg, 'r')
    except IOError:
        print 'cannot open', arg
    else:
        print arg, 'has', len(f.readlines()), 'lines'
        f.close()

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# multiple exceptions on the same line or on different lines
import sys

try:
    f = open('myfile.txt')
    s = f.readline()
    i = int(s.strip())
except IOError as e:
    print "I/O error({0}): {1}".format(e.errno, e.strerror)
except (ValueError,KeyError) as error:
    print "Could not convert data to an integer."
except:
    print "Unexpected error:", sys.exc_info()[0]
    raise




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try:
     raise NameError('HiThere')
except NameError:
     print 'An exception flew by!'
     raise              # re-raise the exception so not handling it yet


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try:
     raise MyErrorExtended( 2*2 )
except MyErrorExtended as e:
     print 'My exception occurred, value:', e.value



# Best practice to use a base exception class and inherit from it
class MyError( Exception ):
    """Base class for exceptions in this module."""
    pass

class InputError( MyError ):
    """Exception raised for errors in the input.

    Attributes:
        expr -- input expression in which the error occurred
        msg  -- explanation of the error
    """

    def __init__(self, expr, msg):
        self.expr = expr
        self.msg = msg

class MyErrorExtended( MyError )
     def __init__(self, value):
        self.value = value
    def __str__(self):
        return repr(self.value)



try:
     raise KeyboardInterrupt
finally:
     print 'Goodbye, world!'                # always executes


# python uses "finally" when using the keyword with for opening a file
with open("myfile.txt") as f:
    for line in f:
        print line,


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#!/usr/bin/env python

import logging
import sys

def throws():
    raise RuntimeError('this is the error message')

def main():
    logging.basicConfig(level=logging.WARNING)
    log = logging.getLogger('example')
    try:
        throws()
        return 0
    except Exception as err:
        log.exception('Error from throws():')
        return 1

if __name__ == '__main__':
    sys.exit(main())

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python 2.7.3 not comprehensive list of built in exceptions:

BaseException
 +-- SystemExit
 +-- KeyboardInterrupt
 +-- GeneratorExit
 +-- 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