Programming:Python Exceptions
Python handles all errors with exceptions.
An exception is a signal that an error or other unusual condition has occurred. There are a number of built-in exceptions, which indicate conditions like reading past the end of a file, or dividing by zero. You can also define your own exceptions.
Raising exceptions
Whenever your program attempts to do something erroneous or meaningless, Python raises exception to such conduct:
>>> 1 / 0
Traceback (most recent call last):
File "<stdin>", line 1, in ?
ZeroDivisionError: integer division or modulo by zero
This traceback indicates that the ZeroDivisionError exception is being raised. This is a built-in exception -- see below for a list of all the other ones.
Catching exceptions
In order to handle errors, you can set up exception handling blocks in your code. The keywords try and except are used to catch exceptions. When an error occurs within the try block, Python looks for a matching except block to handle it. If there is one, execution jumps there.
If you execute this code:
try:
print 1/0
except ZeroDivisionError:
print "You can't divide by zero, you silly."
Then Python will print this:
You can't divide by zero, you silly.
If you don't specify an exception type on the except line, it will cheerfully catch all exceptions. This is generally a bad idea in production code, since it means your program will blissfully ignore unexpected errors as well as ones which the except block is actually prepared to handle.
Exceptions can propagate up the call stack:
def f(x):
return g(x) + 1
def g(x):
if x < 0: raise ValueError, "I can't cope with a negative number here."
else: return 5
try:
print f(-6)
except ValueError:
print "That value was invalid."
In this code, the print statement calls the function f. That function calls the function g, which will raise an exception of type ValueError. Neither f nor g has a try/except block to handle ValueError. So the exception raised propagates out to the main code, where there is an exception-handling block waiting for it. This code prints:
That value was invalid.
Sometimes it is useful to find out exactly what went wrong, or to print the python error text yourself. For example:
try:
theFile = open("the_parrot")
except IOError, (ErrorNumber, ErrorMessage):
if ErrorNumber == 2: # file not found
print "Sorry, 'the_parrot' has apparently joined the choir invisible."
else:
print "Congratulations! You have managed to trip a #" + str(ErrorNumber) + " error:"
print ErrorMessage
Which of course will print:
Sorry, 'the_parrot' has apparently joined the choir invisible.
Trying over and over again
Recovering and continuing with finally
Builtin exception classes
All built-in Python exceptions (http://www.python.org/doc/current/lib/module-exceptions.html)
Exotic uses of exceptions
Exceptions are good for more than just error handling. If you have a complicated piece of code to choose which of several courses of action to take, it can be useful to use exceptions to jump out of the code as soon as the decision can be made. The Python-based mailing list software Mailman does this in deciding how a message should be handled. Using exceptions like this may seem like it's a sort of GOTO -- and indeed it is, but a limited one called an escape continuation. Continuations are a powerful functional-programming tool and it can be useful to learn them.
Just as a simple example of how exceptions make programming easier, say you want to add items to a list but you don't want to write clanky if statements to initialize the list; you can do:
for newItem in newItems:
try:
self.items.append(newItem)
except AttributeError:
self.items = [newItem]
this is also much more efficient then an if statement because it assumes the code will succeed. In fact it will work 99% of the time :) An if statement would continue to get executed even after the array has been initialized.
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