Error Handling

TurboGears has an extensive system for managing and redirecting errors above and beyond the standard Python try/except block. The TurboGears error system distinguishes between (form) validation errors and Python exceptions. They are handled by the @error_handler() and @exception_handler() directives, respectively.

Validation Errors

Validation errors are issued by the TurboGears validator framework. This is normally used with the widget library, but can also be helpful on its own when for example you just want to make sure one of your arguments is an integer. Validation errors are controlled using the @error_handler() decorator and are handled by a method with a tg_errors keyword parameter.

Here’s an example of how this decorator works:

from turbogears import (controllers, expose,
    validate, validators, error_handler)

class Root(controllers.RootController):

    @expose()
    def index(self, number=-1, tg_errors=None):
        """Display a number (but actually the argument can be anything)."""
        if tg_errors:
            errors = [(param, inv.msg, inv.value)
                for param, inv in tg_errors.items()]
            return dict(error_message=errors)
        else:
            return dict(number=number)

    @expose()
    @error_handler(index)
    @validate(validators=dict(number=validators.Int))
    def validated_number(self, number=2):
        """Display an Integer (other kinds of arguments not allowed)."""
        return dict(valid_number=number)

The above code creates two controller methods, one for the root / of the domain and one for /validated_number.

Both take a parameter number and display it. The index isn’t picky about its number parameter and can be passed anything, but validated_number is decorated with an Int validator, which will raise an error when it fails. The error is directed back to the index controller, which receives the error as tg_errors and handles them intelligently.

How Does it Work?

Here’s the output from a twill session (slightly formatted) demonstrating how this all works (twill is used here for demonstration only. Of course you can just try it out with your favorite browser):

-= Welcome to twill! =-

current page:  *empty page*
>> go http://localhost:8080/
==> at http://localhost:8080/
>> show
{"tg_flash": null, "number": -1}

>> go http://localhost:8080/?number=42
==> at http://localhost:8080/?number=42
>> show
{"tg_flash": null, "number": "42"}

>> go http://localhost:8080/?number=blue
==> at http://localhost:8080/?number=blue
>> show
{"tg_flash": null, "number": "blue"}

>> go http://localhost:8080/validated_number
==> at http://localhost:8080/validated_number
>> show
{"tg_flash": null, "valid_number": 2}

>> go http://localhost:8080/validated_number?number=42
==> at http://localhost:8080/validated_number?number=42
>> show
{"tg_flash": null, "valid_number": 42}

>> go http://localhost:8080/validated_number?number=blue
==> at http://localhost:8080/validated_number?number=blue
>> show
{"tg_flash": null, "error_messages": [["number", "Please enter an integer
value", "blue"]]}

The interesting part here is the last request. The validated_number controller has been passed a bad value blue, which is obviously not an integer.

The validator has detected this and passed control back up to the index controller as tg_errors, just like we wanted. At the same time, the normal valid calls also behave as expected. tg_errors itself is a standard dictionary with the failed parameters as keys and formencode.Invalid instances as values. The index method shows one (admittedly poor) way to handle errors. We can do better.

Using error_handler with Widgets Forms

Normally when you’re using @error_handler(), you’re working with Widget based forms. Here’s a demo using the same basic setup as above:

from turbogears import (controllers, expose,
    validate, validators, error_handler, widgets)

number_form = widgets.ListForm(
    fields=[
        widgets.TextField(
            name='number',
            label="Enter a Number",
            validator=validators.Int(not_empty=True))
    ],
    submit_text="Submit Number!"
)

## This is what the template looks like:
# <html xmlns:py="http://purl.org/kid/ns#">
#     <body>
#         <h1>This is a Form Page!</h1>
#         ${form(value_of('data', None), action=action, method='POST')}
#     </body>
# </html>

class Root(controllers.RootController):

    @expose(template=".templates.welcome")
    def index(self, number=-1, tg_errors=None):
        return dict(data=dict(number=number),
                    form=number_form,
                    action='/validated_number')

    @expose()
    @error_handler(index)
    @validate(form=number_form)
    def validated_number(self, number=2):
        return dict(valid_number=number)

The biggest change to the previous code (aside from the form being added) is that the index controller no longer explicitly handles tg_errors.

The convenient thing about forms is that they know how to handle their own validation errors, so all you have to do is set the error handler and forget about it.

Accessing Current Form Values

It should be noted that when you set a method as the @error_handler for a form submission, it can have extra variable keyword parameters, which will receive the current form field values in case of a validation error. This can be used, e.g. to manipulate the form values or options depending on the current value of some form field:

@expose(...)
def show_form(self, tg_errors=None, **kw):
    ...
    values = dict(...)
    params = dict()
    if kw.get('some_field'):
        params['some_option'] = dict(some_field='foo')
    return dict(form=some_form, values=values, params=params)

@error_handler(show_form)
@validate(form=some_form)
def save(self, ...):
    ...

Exceptions and Rules

Exceptions are handled separately from errors because validation errors are (somewhat) expected in a web application.

Exceptions, on the other hand, are a bit more serious, so you’ll probably want to handle them separately.

The syntax and usage is very similar except you use @exception_handler and tg_exceptions instead of @error_handler and tg_errors.

One notable difference is that you’ll probably want to handle different types of exceptions with different handlers. For example, you’ll probably want to handle SQLObject exceptions separately from ValueErrors.

This is done by using the rules parameter (also available on @error_handler()). rules takes a string containing a valid Python expression. The rule matches when the expression is true. Here’s a demonstration:

from turbogears import controllers, expose, exception_handler

class Root(controllers.RootController):
    # Note that the exception handlers don't need to be exposed
    # if they're not meant to be accessed directly. It's always a
    # wise decision to expose only what is absolutely needed to prevent
    # information leaking.

    def value_handler(self, tg_exceptions=None):
        """Only called for value errors."""
        return dict(handling_value=True, exception=str(tg_exceptions))

    def index_handler(self, tg_exceptions=None):
        """Only called for index errors."""
        return dict(handling_index=True, exception=str(tg_exceptions))

    @expose()
    @exception_handler(value_handler, "isinstance(tg_exceptions, ValueError)")
    @exception_handler(index_handler, "isinstance(tg_exceptions, IndexError)")
    def exceptional(self, number=2):
        number = int(number)
        if number < 42:
            raise IndexError("Number too Low!")
        if number == 42:
            raise IndexError("Wise guy, eh?")
        if number > 100:
            raise Exception("This number is exceptionally high!")
        return dict(result="No errors!")

And here’s the twill output:

current page:  *empty page*
>> go http://localhost:8080/exceptional
==> at http://localhost:8080/exceptional
>> show
{"exception": "Number too Low!", "handling_index": true, "tg_flash": null}

>> go http://localhost:8080/exceptional?number=42
==> at http://localhost:8080/exceptional?number=42
>> show
{"exception": "Wise guy, eh?", "handling_index": true, "tg_flash": null}

>> go http://localhost:8080/exceptional?number=blue
==> at http://localhost:8080/exceptional?number=blue
>> show
{"exception": "invalid literal for int(): blue", "tg_flash": null, "handling_value": true}

>> debug http 1
DEBUG: setting http debugging to level 1

>> go http://localhost:8080/exceptional?number=400
# ...
reply: 'HTTP/1.1 500 Internal error\r\n'
# ...

Notice that the calls for where number was less than 42 went to the index_handler (handling_index in the output) and that the non-int raised a value error. The final case shows that the built in CherryPy handling mechanisms will cover any unhandled case. You can, of course, redirect all the unhandled exceptions by using a @exception_handler() without the rules parameter.

Decorator Order

For the most part, the order of the decorators doesn’t matter too much. The one exception occurs when mixing error handling and the Identity framework and having a method serve as its own error handler. If your @error_handler() decorator is under your @identity_require() decorator, you’ll bypass the identity decorator completely and defeat your own security. Don’t do it.

@expose()
@identity.require(identity.not_anonymous())
@error_handler()                             #BAD, DON'T DO IT
def foo(self, tg_errors=None):
    pass

@expose()
@error_handler()                            #OK, be careful
@identity.require(identity.not_anonymous())
def bar(self, tg_errors=None):
    pass

def handler(self, tg_errors=None):
    pass

@expose()
@identity.require(identity.not_anonymous())
@error_handler(handler)                     #GOOD
def baz(self):
    pass

Further reading

See “How Does Error Handling Work?” for some more details and references.