django-testdata 1.0.3
Django application providing isolation for model instances created during `setUpTestData`.
License: MIT
Keywords: django test testdata

django-testdata
===============

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Django application providing isolation for model instances created during
`setUpTestData`.

**Note:**  This package has been merged into Django and released in version
3.2 (see `PR #12608 <https://github.com/django/django/pull/12608>`__).

Installation
------------

.. code:: sh

    pip install django-testdata

Motivation
----------

Django 1.8 introduced ``TestCase.setUpTestData`` to allow costly generation of
model fixtures to be executed only once per test class in order to speed up
testcase instances execution.

One gotcha of ``setUpTestData`` though is that test instances all share the same
model instances and have to be careful not to alter them to prevent breaking
test isolation. Per Django's `documentation`_::

    Be careful not to modify any objects created in setUpTestData() in your
    test methods. Modifications to in-memory objects from setup work done at
    the class level will persist between test methods. If you do need to modify
    them, you could reload them in the setUp() method with refresh_from_db(),
    for example.

Reloading objects in ``setUp()`` certainly works but it kind of defeats the
purpose of avoiding database hits to speed up tests execution in the first
place. It makes little sense to fetch model instances from the database
given all the data is available in memory.

This package offers a different alternative to work around this quirk of
``setUpTestData``. Instead of reloading objects from the database the model
instances assigned as class attributes during ``setUpTestData`` are lazily deep
copied on test case instance accesses. All deep copying during a test is done
with a shared `memo`_ which makes sure in-memory relationships between objects
are preserved.

.. _documentation: https://docs.djangoproject.com/en/2.1/topics/testing/tools/#django.test.TestCase.setUpTestData
.. _memo: https://docs.python.org/3/library/copy.html?highlight=memo#copy.deepcopy

Usage
-----

The test data can be either wrapped manually by using ``testdata``:

.. code:: python

    from django.test import TestCase
    from testdata import testdata

    from .models import Author, Book

    class BookTests(TestCase):
        @classmethod
        def setUpTestData(cls):
            cls.author = testdata(Author.objects.create(
                name='Milan Kundera',
            ))
            cls.book = testdata(cls.author.books.create(
                title='Nesnesitelná lehkost bytí',
            ))

Or automatically by using the ``wrap_testdata`` decorator:

.. code:: python

    from django.test import TestCase
    from testdata import wrap_testdata

    from .models import Author, Book

    class BookTests(TestCase):
        @classmethod
        @wrap_testdata
        def setUpTestData(cls):
            cls.author = Author.objects.create(
                name='Milan Kundera',
            )
            cls.book = cls.author.books.create(
                title='Nesnesitelná lehkost bytí',
            )

Under the hood ``wrap_testdata`` wraps all attributes added to ``cls``
during the execution of ``setUpTestData()`` into ``testdata(attr, name=name)``.

Once test data is wrapped the testcase instance methods can alter objects
retrieved from ``self`` without worrying about cross-tests isolation:

.. code:: python

    from django.test import TestCase
    from testdata import wrap_testdata

    from .models import Author, Book

    class BookTests(TestCase):
        @classmethod
        @wrap_testdata
        def setUpTestData(cls):
            cls.author = Author.objects.create(
                name='Milan Kundera',
            )
            cls.book = cls.author.books.create(
                title='Nesnesitelná lehkost bytí',
            )

        def test_book_name_english(self):
            self.assertEqual(self.book.title, 'Nesnesitelná lehkost bytí')
            self.book.title = 'The Unbearable Lightness of Being'
            self.book.save()

        def test_book_name_french(self):
            self.assertEqual(self.book.title, 'Nesnesitelná lehkost bytí')
            self.book.title = "L'Insoutenable Légèreté de l'être"
            self.book.save()