# Error Handling

There are two ways to handle runtime errors (exceptions) in Cheetah. The first is with the Cheetah directives that mirror Python’s structured exception handling statements. The second is with Cheetah’s {ErrorCatcher} framework. These are described below.

## #try … #except … #end try, #finally, and #assert

Cheetah’s exception-handling directives are exact mirrors Python’s exception-handling statements. See Python’s documentation for details. The following Cheetah code demonstrates their use:

    #try
      $mightFail()
    #except
      It failed
    #end try

    #try
      #assert $x == $y
    #except AssertionError
      They're not the same!
    #end try

    #try
      #raise ValueError
    #except ValueError
      #pass
    #end try

    #try
      $mightFail()
    #except ValueError
      Hey, it raised a ValueError!
    #except NameMapper.NotFound
      Hey, it raised a NameMapper.NotFound!
    #else
      It didn't raise anything!
    #end try

    #try
      $mightFail()
    #finally
      $cleanup()
    #end try

Like Python, {#except} and {#finally} cannot appear in the same try-block, but can appear in nested try-blocks.

## #errorCatcher and ErrorCatcher objects

Syntax:

    #errorCatcher CLASS
    #errorCatcher $PLACEHOLDER_TO_AN_ERROR_CATCHER_INSTANCE

{ErrorCatcher} is a debugging tool that catches exceptions that occur inside {$placeholder} tags and provides a customizable warning to the developer. Normally, the first missing namespace value raises a {NameMapper.NotFound} error and halts the filling of the template. This requires the developer to resolve the exceptions in order without seeing the subsequent output. When an {ErrorCatcher} is enabled, the developer can see all the exceptions at once as well as the template output around them.

The {Cheetah.ErrorCatchers} module defines the base class for ErrorCatchers:

    class ErrorCatcher:
        _exceptionsToCatch = (NameMapper.NotFound,)

        def __init__(self, templateObj):
            pass

        def exceptions(self):
            return self._exceptionsToCatch

        def warn(self, exc_val, code, rawCode, lineCol):
            return rawCode

This ErrorCatcher catches {NameMapper.NotFound} exceptions and leaves the offending placeholder visible in its raw form in the template output. If the following template is executed:

    #errorCatcher Echo
    #set $iExist = 'Here I am!'
    Here's a good placeholder: $iExist
    Here's bad placeholder: $iDontExist

the output will be:

    Here's a good placeholder: Here I am!
    Here's bad placeholder: $iDontExist

The base class shown above is also accessible under the alias {Cheetah.ErrorCatchers.Echo}. {Cheetah.ErrorCatchers} also provides a number of specialized subclasses that warn about exceptions in different ways. {Cheetah.ErrorCatchers.BigEcho} will output

    Here's a good placeholder: Here I am!
    Here's bad placeholder: ===============&lt;$iDontExist could not be found&gt;===============

ErrorCatcher has a significant performance impact and is turned off by default. It can also be turned on with the {Template} class’ {‘errorCatcher’} keyword argument. The value of this argument should either be a string specifying which of the classes in {Cheetah.ErrorCatchers} to use, or a class that subclasses {Cheetah.ErrorCatchers.ErrorCatcher}. The {#errorCatcher} directive can also be used to change the errorCatcher part way through a template.

{Cheetah.ErrorCatchers.ListErrors} will produce the same output as {Echo} while maintaining a list of the errors that can be retrieved later. To retrieve the list, use the {Template} class’ {‘errorCatcher’} method to retrieve the errorCatcher and then call its {listErrors} method.

ErrorCatcher doesn’t catch exceptions raised inside directives.