Exceptions - Page 2

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 executing statements in the try block and looks for an except clause that matches the exception that has occurred. If found, control is passed to the first statement in the except clause. Otherwise, the exception is propagated up to the block of code in which the try statement appeared.This code may itself be enclosed in a try-except that can handle the exception. If an exception works its way up to the top level of a program without being caught, the interpreter aborts with an error message. If desired, uncaught exceptions can also be passed to a user-defined function sys.excepthook() as described in Appendix A,"The Python Library," sys module.

The optional second argument to the except statement is the name of a variable in which the argument supplied to the raise statement is placed if an exception occurs. Exception handlers can examine this value to find out more about the cause of the exception.

Multiple exception-handling blocks are specified using multiple except clauses, such as in the following example:

try:
  do something
except IOError,e:
  # Handle I/O error
  ...
except TypeError,e:
  # Handle Type error
  ...
except NameError,e:
  # Handle Name error
  ...

A single handler can catch multiple exception types like this:

try:
  do something
except (IOError,TypeError,NameError),e:
  # Handle I/O,Type, or Name errors
  ...

To ignore an exception, use the pass statement as follows:

try:
  do something
except IOError:
  pass   # Do nothing (oh well).

To catch all exceptions, omit the exception name and value:

try:
  do something
except:
  print 'An error occurred '

Control Flow - Page 1
Python Essential Reference, Second Edition
Built-in Exceptions - Page 3