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 ...