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Error Handling
==============
(errorHandling)
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
----------------------------------------------------
(errorHandling.directives)
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
--------------------------------------
(errorHandling.errorCatcher)
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: $iDontExist
The base class shown above is also accessible under the alias
{Cheetah.ErrorCatchers.Echo}. {Cheetah.ErrorCatchers} also provides
a number of specialized subclasses that warn about exceptions in
different ways. {Cheetah.ErrorCatchers.BigEcho} will output
::
Here's a good placeholder: Here I am!
Here's bad placeholder: ===============<$iDontExist could not be found>===============
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 ouput 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.
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