Errors During Conversion

When conversion fails, the default behavior of DMP is to raise an Http404 exception, indicating to the browser that the given url does not exist. Most of the time, this makes sense because it is, in effect, an invalid URL.

Resolution is as follows:

• If a converter function raises ValueError on any parameter, an Http 404 response is returned to the browser, indicating that the conversion failed for “normal” reasons.
• If a converter function raises any other type of exception, a ConverterException is raised, indicating that an unexpected error occurred during conversion. This generally returns an Http 500 server error to the browser.

Customizing

Enough of this 404 rudeness! Suppose we want to be more forgiving of converter failures. When converter functions raise ValueError, we’ll use the view function defaults and let processing continue normally.

For example, consider the following view function in index.py:

from django_mako_plus import view_function

@view_function
def process_request(request, age:int=0):
    ...

With this view function:

• /homepage/index/5/ works perfectly: the function is called with age=5.
• /homepage/index/asdf/ fails the conversion to integer: the Http 404 page is returned.

Let’s extend the ParameterConverter class to catch the 404 and return the default. Add the following to /homepage/apps.py:

from django.apps import AppConfig
from django.http import Http404
from django_mako_plus import ParameterConverter
import inspect

class HomepageConfig(AppConfig):
    name = 'homepage'


class ForgivingConverter(ParameterConverter):
    '''Uses defaults for values that cannot be converted (instead of the usual 404)'''

    def convert_value(self, value, parameter, request):
        try:                                # try the normal conversion process
            return super().convert_value(value, parameter, request)

        except Http404:                     # converter function raised a ValueError
            if parameter.default is not inspect.Parameter.empty:
                return parameter.default    # return the default specified in process_request() signature
            return None                     # return None if signature has no default

Activate the custom converter in settings.py:

TEMPLATES = [
    {
        'NAME': 'django_mako_plus',
        'BACKEND': 'django_mako_plus.MakoTemplates',
        'OPTIONS': {
            'PARAMETER_CONVERTER': 'homepage.apps.ForgivingConverter',
            ...
        }
    }
]

With the above setup, converter failures will no longer trigger a 404. Instead, the default value is used anytime a conversion fails.