on-error: Handle errors

Syntax

tal:on-error syntax:

      argument ::= (['text'] | 'structure') expression

Description

The tal:on-error statement provides error handling for your template. When a TAL statement produces an error, the TAL interpreter searches for a tal:on-error statement on the same element, then on the enclosing element, and so forth. The first tal:on-error found is invoked. It is treated as a tal:content statement.

A local variable error is set. This variable has these attributes:

type

the exception type

value

the exception instance

traceback

the traceback object

The simplest sort of tal:on-error statement has a literal error string or nothing for an expression. A more complex handler may call a script that examines the error and either emits error text or raises an exception to propagate the error outwards.

Examples

Simple error message:

      <b tal:on-error="string: Username is not defined!" 
         tal:content="here/getUsername">Ishmael</b>

Removing elements with errors:

      <b tal:on-error="nothing"
         tal:content="here/getUsername">Ishmael</b>

Calling an error-handling script:

      <div tal:on-error="structure here/errorScript">
        ...
      </div>

Here's what the error-handling script might look like:

      ## Script (Python) "errHandler"
      ##bind namespace=_
      ##
      error=_['error']
      if error.type==ZeroDivisionError:
          return "<p>Can't divide by zero.</p>"
      else
          return """<p>An error ocurred.</p>
                    <p>Error type: %s</p>
                    <p>Error value: %s</p>""" % (error.type,
                                                 error.value)

See Also

Python Tutorial: Errors and Exceptions

Python Built-in Exceptions