Python Essential Reference, Second Edition

Exceptions

Exceptions indicate errors and break out of the normal control flow of a program. An exception is raised using the raise statement. The general format of the raise statement is raise exception [, value ] where exception is the exception type and value is an optional value giving specific details about the exception. For example:

raise RuntimeError, 'Unrecoverable Error' 

If the raise statement is used without any arguments, the last exception generated is raised again (although this works only while handling a previously raised exception).

To catch an exception, use the try and except statements, as shown here:

try: 
    f = open('foo') 
except IOError, e: 
    print "Unable to open 'foo': ", e 

When an exception occurs, the interpreter stops ...