In Django, how can I get an exception's message?

Question

In a view function, I have something like:

try:
    url = request.POST.get('u', '')
    if len(url) == 0:
        raise ValidationError('Empty URL')
except ValidationError, err:
    print err

The output is a string: [u'Empty URL']

When I try to pass the error message to my template (stuffed in a dict, something like { 'error_message': err.value }), the template successfully gets the message (using {{ error_message }}).

The problem is that I get the exact same string as above, [u'Empty URL'], with the [u'...']!

How do I get rid of that?

(Python 2.6.5, Django 1.2.4, Xubuntu 10.04)

Solution

ValidationError actually holds multiple error messages.

The output of print err is [u'Empty URL'] because that is the string returned by repr(err.messages) (see ValidationError.__str__ source code).

If you want to print a single readable message out of a ValidationError, you can concatenate the list of error messages, for example:

    # Python 2 
    print '; '.join(err.messages)

    # Python 3
    print('; '.join(err.messages))

Comments

[Fred]

That is actually bad practise I'd say. BaseException catches everything, whereas you should only handle ValidationError. If another exception pops it's because there's an error in your code, which should be fixed.

[Nikki Erwin Ramirez]

Well, I did raise the error/exception myself. The code in the try block was very minimal, that I thought the chances of a different kind of error popping up was slim. I just wanted to reuse ValidationError for semantic purposes. I guess the more proper approach would be to subclass BaseException with my own kind of validation error? In any case, the code has changed a lot since I first posted this and my workaround is no longer applicable, so I accepted the answer above instead.

[Tomasz Gandor]

I figured that, as validators for a field is also an array. Good to know about the messages member of ValidationError. They never mention it here: docs.djangoproject.com/en/dev/ref/forms/validation as they primarily should!