Source: 📖 Test Driven Development with Python ch14 p237

Testing forms

We can test that forms are being created the way we want by checking if desired attributes exist in the form's HTML when we call as_p() on it. If we want to test the ItemForm from Creating Django ModelForms, we can do so in the following manner:

# test_forms.py
from django.test import TestCase

from my_app.forms import ItemForm


class ItemFormTest(TestCase):
	
	def test_html_attributes(self):
		form = ItemForm()
		self.assertIn('placeholder="Enter an item"', form.as_p())
		self.assertIn('class="form-control input-lg"', form.as_p())

We can also test sending data to a form by providing a data argument upon creating the form as a dictionary containing the form fields to which we want to post data.

# test_forms.py
...

class ItemFormTest(TestCase):
	
	...
	
	def test_data_sent_to_form(self):
		form = ItemForm(data={'text': ''})
		self.assertFalse(form.is_valid())
		self.assertEqual(
			form.errors['text'],
			["You can't have an empty list item"]
		)

Here we test that trying to send and empty string raises an error which displays the text "You can't have an empty list item". Calling is_valid() on the form returns True or False, validates the input data and populates the errors attribute which we then access with form.errors. The errors attribute being accessed is a dictionary that maps the names of the form's fields to any errors for those fields upon validation. Note that the errors are stored as a list as there could be multiple errors, so when using assertEqual be sure to place the error message inside square brackets as shown.