Errors and Exceptions

1. Handle Exceptions

try ... except

while True:
	try:
		x = int(raw_input("Please enter a number: "))
		break
	except ValueError:
		print "Oops! That was no valid number. Try again..."

try may have more than one except

except clause may have multiple exceptions as a tuple

 try:
 	# something
 except (RuntimeError, TypeError, NameError): 
 	pass

last except clause may omit exception name, wildcard. Use to print error message, re-raise the exception

 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:
 	print "Could not convert data to an integer."
 except:
 	print "Unexpected error:", sys.exc_info()[0]
 	raise

optional else
- must follow all except clauses
- useful for code that must be executed if the try clause does not raise an exception

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

Exceptions's argument: an associated value with an exception
- Not mandatory
- presence and type of the argument depend on the exception type

A except clause may specify a variable after the exception name (or tuple), and the variable is bound to an exception instance with the arguments stored in instance.args

 try:
 	raise Exception('spam', 'eggs')
 except Exception as inst:
 	print type(inst)
 	print inst.args
 	print inst
 	x, y = inst.args
 	print 'x =', x
 	print 'y =', y

2. Raising Exceptions

raise allows programmer to force a specified exception to occur
```
 raise NameError('HiThere')
```

3. User-defined Exceptions

Derived from Exception class, directly or indirectly

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

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

A module raises serveral distinct errors
- create a base class for exceptions
- subclass the base to create specific exception classes
Most exceptions' names end with "Error"

4. Clean-up Actions

finally, always executed before leaving try statement

useful for releasing external resources

 def divide(x, y):
 	try:
 		result = x / y
 	except ZeroDivisionError:
 		print "division by zero!"
 	else:
 		print "result is", result
 	finally:
 		print "executing finally clause"

 divide(2, 1)
 divide(2, 0)
 divide("2", "1")

5. Predefined Clean-up Actions

with open("myfile.txt") as f:
	for line in f:
		print line,

# f is always closed after the statement