Development with Python

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def test_form_validation_for_duplicate_items(self):
list_ = List.objects.create()
Item.objects.create(list=list_, text='no twins!')
form = ExistingListItemForm (for_list=list_, data={'text': 'no twins!'})
self.assertFalse(form.is_valid())
self.assertEqual(form.errors['text'], [DUPLICATE_ITEM_ERROR ])
We can iterate through a few TDD cycles (I won’t show them all, but I’m sure you’ll do
them, right? Remember, theGoat sees all.) until we get a formwith a customconstructor,
which just ignores its for_list argument:
lists/forms.py (ch09l071).
DUPLICATE_ITEM_ERROR = "You've already got this in your list"
[...]
class ExistingListItemForm(forms.models. ModelForm):
def __init__(self, for_list, *args, **kwargs ):
super(). __init__(*args, **kwargs)
Gives:
ValueError: ModelForm has no model class specified.
Now let’s see if making it inherit from our existing form helps:
lists/forms.py (ch09l072).
class ExistingListItemForm(ItemForm):
def __init__(self, for_list, *args, **kwargs ):
super(). __init__(*args, **kwargs)
That takes us down to just one failure:
FAIL: test_form_validation_for_duplicate_items
(lists.tests.test_forms.ExistingListItemFormTest)
self.assertFalse(form.is_valid())
AssertionError: True is not false
The next step requires a little knowledge of Django’s internals, but you can read up on
it in the Django docs on and
Django uses a method called validate_unique, both on forms and models, and we can
use both, in conjunction with the instance attribute:
lists/forms.py.
from django.core.exceptions import ValidationError
[...]
class ExistingListItemForm(ItemForm):
def __init__(self, for_list, *args, **kwargs ):
super(). __init__(*args, **kwargs)
self.instance.list = for_list
def validate_unique(self):
try:
220 | Chapter 12: More Advanced Forms