Revision 12 as of 2003-11-23 21:25:28

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Handling Exceptions

The simplest way to handle exceptions is with a "try-except" block:

   1 (x,y) = (5,0)
   2 try:
   3   z = x/y
   4 except ZeroDivisionError:
   5   print "divide by zero"

If you wanted to examine the exception from code, you could have:

   1 (x,y) = (5,0)
   2 try:
   3   z = x/y
   4 except ZeroDivisionError, e:
   5   z = e # representation: "<exceptions.ZeroDivisionError instance at 0x817426c>"
   6 print z # output: "integer division or modulo by zero"

General Error Catching

Sometimes, you want to catch all errors that could possibly be generated, but usually you don't.In most cases, you want to be as specific as possible (CatchWhatYouCanHandle). In the first example above, if you were using a catch-all exception clause and a user presses Ctrl-C, generating a KeyboardInterrupt, you don't want the program to print "divide by zero".

However, there are some situations where it's best to catch all errors.

For example, suppose you are writing an extension module to a web service. You want the error information to output the output web page, and the server to continue to run, if at all possible. But you have no idea what kind of errors you might have put in your code.

In situations like these, you may want to code something like this:

   1 try:
   2   untrusted.execute()
   3 except Exception, e:
   4   write_to_page( "<p>Error: %s</p>" % str(e) )

MoinMoin software is a good example of where this is done. If you write MoinMoin extension macros, and trigger an error, MoinMoin will give you a detailed report of your error and the chain of events leading up to it.

To Write About...

Give example of IOError, and interpreting the IOError code.

Give example of multiple excepts. Handling multiple excepts in one line.

Show how to use "else" and "finally".

Show how to continue with a "raise".

See Also:

WritingExceptionClasses, TracebackModule, CoupleLeapingWithLooking

Questions

Is there an easy way to find all of the exceptions, and parameters to the exceptions, that a class has? -- LionKimbro

I'm not sure what you mean. Do you want to know what exceptions a class can raise or what parameters an exception class can take when raised? -- JohannesGijsbers

Both, actually. I'm thinking: "I'm writing some code, and I want to be reasonably aware of things that could go wrong, that I might not think of otherwise. Then, I want to know the parameters for the exceptions, when I learn what could go wrong."

I can look up the "Dict" builtin class in the Python manual, but it doesn't say, right there, that it emits the KeyError. Now, you and I know about the KeyError, but there are times when I need the exact word of the exception, and I want to know the parameters. So, where to people find it? Or is there just no such easy way to look up, yet? -- LionKimbro DateTime(2003-11-23T21:25:28Z)

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