try: Handles exceptions

  The 'try' tag allows exception handling in DTML, mirroring the
  Python 'try/except' and 'try/finally' constructs.

  Syntax

    The 'try' tag has two different syntaxes, 'try/except/else' and
    'try/finally'.

    'try/except/else' Syntax::

      <dtml-try>
	<dtml-except [ExceptionName] [ExceptionName]...>
	... 
	[<dtml-else>]
      </dtml-try>

    The 'try' tag encloses a block in which exceptions can be caught and
    handled. There can be one or more 'except' tags that handles
    zero or more exceptions. If an 'except' tag does not specify an
    exception, then it handles all exceptions.

    When an exception is raised, control jumps to the first 'except'
    tag that handles the exception. If there is no 'except' tag to
    handle the exception, then the exception is raised normally.

    If no exception is raised, and there is an 'else' tag, then the
    'else' tag will be executed after the body of the 'try' tag.

    The 'except' and 'else' tags are optional.

    'try/finally' Syntax::

      <dtml-try>
	<dtml-finally>
      </dtml-try>

    The 'finally' tag cannot be used in the same 'try' block as the
    'except' and 'else' tags. If there is a 'finally' tag, its block
    will be executed whether or not an exception is raised in the
    'try' block.

  Attributes

    except -- Zero or more exception names. If no exceptions are
    listed then the except tag will handle all exceptions.

  Tag Variables

    Inside the 'except' block these variables
    are defined.

      error_type -- The exception type.

      error_value -- The exception value.

      error_tb -- The traceback.

  Examples

    Catching a math error::

      <dtml-try>
	<dtml-var expr="1/0">
      <dtml-except ZeroDivisionError>
	You tried to divide by zero.
      </dtml-try>

    Returning information about the handled exception::

      <dtml-try>
	<dtml-call dangerousMethod>
      <dtml-except>
	An error occurred.
	Error type: <dtml-var error_type>
	Error value: <dtml-var error_value>
      </dtml-try>

    Using finally to make sure to perform clean up regardless
    of whether an error is raised or not::

      <dtml-call acquireLock>
      <dtml-try>
	<dtml-call someMethod>
      <dtml-finally>
	<dtml-call releaseLock>
      </dtml-try>

  See Also

    "raise tag":dtml-raise.stx

    "Python Tutorial: Errors and
    Exceptions":http://www.python.org/doc/current/tut/node10.html

    "Python Built-in
    Exceptions":http://www.python.org/doc/current/lib/module-exceptions.html