Add parameter info to fixture assert_num_queries to display additiona…

…l message on failure.

docs/helpers.rst

@@ -286,8 +286,10 @@ Example
 This fixture allows to check for an expected number of DB queries.
 
 It wraps `django.test.utils.CaptureQueriesContext`.  A non-default DB
-connection can be passed in using the `connection` keyword argument, and it
-will yield the wrapped CaptureQueriesContext instance.
+connection can be passed in using the `connection` keyword argument, an
+additional info message which is displayed on fail can be passed in using
+the `info` keyword argument, and it will yield the wrapped
+CaptureQueriesContext instance.
 
 
 Example

pytest_django/fixtures.py

@@ -407,7 +407,7 @@ def _live_server_helper(request):
 
 
 @contextmanager
-def _assert_num_queries(config, num, exact=True, connection=None):
+def _assert_num_queries(config, num, exact=True, connection=None, info=None):
     from django.test.utils import CaptureQueriesContext
 
     if connection is None:
@@ -429,6 +429,8 @@ def _assert_num_queries(config, num, exact=True, connection=None):
                     num_performed == 1 and "1 was" or "%d were" % (num_performed,)
                 ),
             )
+            if info:
+                msg += "\n{}".format(info)
             if verbose:
                 sqls = (q["sql"] for q in context.captured_queries)
                 msg += "\n\nQueries:\n========\n\n%s" % "\n\n".join(sqls)

tests/test_fixtures.py

@@ -158,6 +158,32 @@ def test_django_assert_num_queries_db_connection(django_assert_num_queries):
             pass
 
 
+@pytest.mark.django_db
+def test_django_assert_num_queries_output_info(django_testdir):
+    django_testdir.create_test_module("""
+        from django.contrib.contenttypes.models import ContentType
+        import pytest
+
+        @pytest.mark.django_db
+        def test_queries(django_assert_num_queries):
+            with django_assert_num_queries(
+                num=2,
+                info="Expected: 1 for select all, 1 for count"
+            ):
+                list(ContentType.objects.all())
+                ContentType.objects.count()
+                ContentType.objects.first()  # additional wrong query
+    """)
+    result = django_testdir.runpytest_subprocess('--tb=short', '-v')
+    result.stdout.fnmatch_lines([
+        '*Expected to perform 2 queries but 3 were done*',
+        '*Expected: 1 for select all, 1 for count*',
+        '*Queries:*',
+        '*========*',
+    ])
+    assert result.ret == 1
+
+
 class TestSettings:
     """Tests for the settings fixture, order matters"""