Forms ValidationError and error code

Question

In Django documentation https://docs.djangoproject.com/en/dev/ref/forms/validation/#raising-validationerror said that it is good practice to prodive error code while raising ValidationError exception.

# Good ValidationError(_('Invalid value'), code='invalid')  # Bad ValidationError(_('Invalid value')) 

I have API in my application and I'm using form to validate input data.
If form is not valid, I whant to get these error codes (not error messages).

So I looked at source code of _clean_fields method of BaseForm:
https://github.com/django/django/blob/master/django/forms/forms.py#L278

except ValidationError as e:     self._errors[name] = self.error_class(e.messages)     if name in self.cleaned_data:         del self.cleaned_data[name] 

As I understand this parameter (self.code) is not passed anywhere and can not be obtained after the form validation.

Can someone explain what the purpose of using this error code?

Answer 1

In Django 1.7, you can now access the original error data from the form. You can call the as_data() method on an ErrorList or ErrorDict. For example: my_form.errors.as_data(). This basically gives you the original ValidationError object instead of the message itself. From this you can access the .code property, eg: my_form.errors["__all__"].as_data()[0].code.

You can also serialize form errors, great for APIs:

>>> print(form.errors.as_json()) {"__all__": [     {"message": "Your account has not been activated.", "code": "inactive"} ]} 

Answer 2

Take a look at ValidationError definition in django src, it's used as a convenient way to pass additional identifier (similar to e.errno in standard python exception), you can use it like this:

try:     ...     raise ValidationError(u'Oops', code=0x800)     ...  except ValidationError as e:     print "Error code: ", e.code