Add configuration for Django assertion diff length

docs/changelog.rst

@@ -9,6 +9,12 @@ Compatibility
 
 * Official Django 6.1 support.
 
+Improvements
+^^^^^^^^^^^^
+
+* Added the ``django_asserts_max_diff`` configuration option to set the
+  maximum diff length emitted by ``pytest_django.asserts`` (`#1155 <https://github.com/pytest-dev/pytest-django/issues/1155>`__).
+
 v4.14.0 (2026-08-10)
 --------------------
 

docs/configuring_django.rst

@@ -59,6 +59,24 @@ The order of precedence is, from highest to lowest:
 If you want to use the highest precedence in the configuration file, you can
 use ``addopts = --ds=yourtestsettings``.
 
+Configuring Django assertion diffs
+----------------------------------
+
+The assertion helpers in ``pytest_django.asserts`` use Django's
+``TestCase`` assertions. To change the maximum amount of assertion diff shown,
+set ``django_asserts_max_diff`` in your pytest configuration. The default is
+``640``, matching ``unittest.TestCase.maxDiff``. Set it to ``None`` to show the
+complete diff::
+
+    [pytest]
+    django_asserts_max_diff = None
+
+In ``pyproject.toml``, use a string because TOML does not have a ``None``
+literal::
+
+    [tool.pytest.ini_options]
+    django_asserts_max_diff = "None"
+
 Using django-configurations
 ---------------------------
 

pytest_django/asserts.py

@@ -16,7 +16,11 @@ class MessagesTestCase(MessagesTestMixin, TestCase):
     pass
 
 
-test_case = MessagesTestCase("run")
+test_case: Any = MessagesTestCase("run")
+
+
+def _set_max_diff(max_diff: int | None) -> None:
+    test_case.maxDiff = max_diff
 
 
 def _wrapper(name: str) -> Callable[..., Any]:

pytest_django/plugin.py

@@ -142,6 +142,11 @@ def pytest_addoption(parser: pytest.Parser) -> None:
         "How to set the Django DEBUG setting (default `False`). Use `keep` to not override.",
         default="False",
     )
+    parser.addini(
+        "django_asserts_max_diff",
+        "Maximum diff length for pytest_django.asserts (use `None` for no limit).",
+        default="640",
+    )
     group.addoption(
         "--fail-on-template-vars",
         action="store_true",
@@ -410,6 +415,22 @@ def pytest_configure(config: pytest.Config) -> None:
     # it's fully initialized here.
     _setup_django(config)
 
+    if "django" not in sys.modules:
+        return
+
+    from pytest_django.asserts import _set_max_diff
+
+    max_diff = config.getini("django_asserts_max_diff")
+    if max_diff.lower() == "none":
+        _set_max_diff(None)
+    else:
+        try:
+            _set_max_diff(int(max_diff))
+        except ValueError as error:
+            raise pytest.UsageError(
+                "django_asserts_max_diff must be an integer or None"
+            ) from error
+
 
 @pytest.hookimpl()
 def pytest_report_header(config: pytest.Config) -> list[str] | None:

tests/test_asserts.py

@@ -69,3 +69,51 @@ def test_sanity() -> None:
         pass
 
     assert assertContains.__doc__
+
+
+def test_django_asserts_max_diff_can_be_unlimited(
+    django_pytester: pytest.Pytester,
+) -> None:
+    django_pytester.makeini(
+        """
+        [pytest]
+        django_asserts_max_diff = None
+        """
+    )
+    django_pytester.makepyfile(
+        """
+        from pytest_django.asserts import assertXMLEqual
+
+
+        def test_assert_xml_equal_uses_the_configured_max_diff():
+            expected = "<root>" + "".join(f"<item>{i}</item>" for i in range(100)) + "</root>"
+            actual = "<root>" + "".join(f"<item>{i + 1}</item>" for i in range(100)) + "</root>"
+
+            try:
+                assertXMLEqual(expected, actual)
+            except AssertionError as error:
+                assert "Set self.maxDiff to None" not in str(error)
+            else:
+                raise AssertionError("assertXMLEqual unexpectedly passed")
+        """
+    )
+
+    result = django_pytester.runpytest_subprocess()
+
+    result.assert_outcomes(passed=1)
+
+
+def test_django_asserts_max_diff_requires_an_integer_or_none(
+    django_pytester: pytest.Pytester,
+) -> None:
+    django_pytester.makeini(
+        """
+        [pytest]
+        django_asserts_max_diff = unlimited
+        """
+    )
+
+    result = django_pytester.runpytest_subprocess()
+
+    assert result.ret == pytest.ExitCode.USAGE_ERROR
+    result.stderr.fnmatch_lines(["*django_asserts_max_diff must be an integer or None*"])

tests/test_without_django_loaded.py

@@ -26,6 +26,30 @@ def test_cfg(pytestconfig):
     assert r.ret == 0
 
 
+def test_django_asserts_max_diff_does_not_import_django(
+    pytester: pytest.Pytester,
+) -> None:
+    pytester.makeini(
+        """
+        [pytest]
+        django_asserts_max_diff = None
+        """
+    )
+    pytester.makepyfile(
+        """
+        import sys
+
+
+        def test_django_is_not_imported():
+            assert "django" not in sys.modules
+        """
+    )
+
+    result = pytester.runpytest_subprocess()
+
+    result.assert_outcomes(passed=1)
+
+
 def test_database(pytester: pytest.Pytester) -> None:
     pytester.makepyfile(
         """