Development with Python

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A Little Digression on Queryset Ordering and String Representations
When we run the tests they reveal an unexpected failure:
======================================================================
FAIL: test_saving_and_retrieving_items
(lists.tests.test_models.ListAndItemModelsTest)
---------------------------------------------------------------------
Traceback (most recent call last):
File "/workspace/superlists/lists/tests/test_models.py", line 31, in
test_saving_and_retrieving_items
self.assertEqual(first_saved_item.text, 'The first (ever) list item')
AssertionError: 'Item the second' != 'The first (ever) list item'
- Item the second
[...]
Depending on your platform and its SQLite installation, you may not
see this error. You can follow along anyway; the code and tests are
interesting in their own right.
That’s a bit of a puzzler. A bit of print-based debugging:
lists/tests/test_models.py.
first_saved_item = saved_items[0]
print(first_saved_item .text)
second_saved_item = saved_items[1]
print(second_saved_item.text)
self .assertEqual(first_saved_item .text, 'The first (ever) list item')
Will show us…
.....Item the second
The first (ever) list item
F.....
It looks like our uniqueness constraint has messed with the default ordering of queries
like Item.objects.all(). Although we already have a failing test, it’s best to add a new
test that explicitly tests for ordering:
lists/tests/test_models.py (ch09l032).
def test_list_ordering(self):
list1 = List.objects.create()
item1 = Item.objects.create(list=list1, text='i1' )
item2 = Item.objects.create(list=list1, text='item 2')
item3 = Item.objects.create(list=list1, text='3')
self.assertEqual(
Item.objects.all(),
[item1, item2, item3]
)
That gives us a new failure, but it’s not a very readable one:
214 | Chapter 12: More Advanced Forms