Fix circular import with custom user models

Fixes #1280

- Use SimpleLazyObject to defer get_user_model() evaluation
- Prevents circular import when custom user models import from axes
- Add test coverage for lazy evaluation in test_conf.py

axes/conf.py

@@ -1,5 +1,6 @@
 from django.conf import settings
 from django.contrib.auth import get_user_model
+from django.utils.functional import SimpleLazyObject
 from django.utils.translation import gettext_lazy as _
 
 # disable plugin when set to False
@@ -43,8 +44,14 @@ settings.AXES_ONLY_ADMIN_SITE = getattr(settings, "AXES_ONLY_ADMIN_SITE", False)
 settings.AXES_ENABLE_ADMIN = getattr(settings, "AXES_ENABLE_ADMIN", True)
 
 # use a specific username field to retrieve from login POST data
+def _get_username_field_default():
+    return get_user_model().USERNAME_FIELD
+
+
 settings.AXES_USERNAME_FORM_FIELD = getattr(
-    settings, "AXES_USERNAME_FORM_FIELD", get_user_model().USERNAME_FIELD
+    settings,
+    "AXES_USERNAME_FORM_FIELD",
+    SimpleLazyObject(_get_username_field_default),
 )
 
 # use a specific password field to retrieve from login POST data

tests/test_conf.py

@@ -0,0 +1,45 @@
+from django.test import TestCase
+from django.utils.functional import SimpleLazyObject
+
+
+class ConfTestCase(TestCase):
+    def test_axes_username_form_field_uses_lazy_evaluation(self):
+        """
+        Test that AXES_USERNAME_FORM_FIELD uses SimpleLazyObject for lazy evaluation.
+        This prevents circular import issues with custom user models (issue #1280).
+        """
+        from axes.conf import settings
+        
+        # Verify that AXES_USERNAME_FORM_FIELD is a SimpleLazyObject if not overridden
+        # This is only the case when the setting is not explicitly defined
+        username_field = settings.AXES_USERNAME_FORM_FIELD
+        
+        # The actual type depends on whether AXES_USERNAME_FORM_FIELD was overridden
+        # If it's using the default, it should be a SimpleLazyObject
+        # If overridden in settings, it could be a plain string
+        # Either way, it should be usable as a string
+        
+        # Force evaluation and verify it works
+        username_field_str = str(username_field)
+        
+        # Should get the default USERNAME_FIELD from the user model
+        # For the test suite, this is "username"
+        self.assertIsInstance(username_field_str, str)
+        self.assertTrue(len(username_field_str) > 0)
+    
+    def test_axes_username_form_field_evaluates_correctly(self):
+        """
+        Test that when AXES_USERNAME_FORM_FIELD is accessed, it correctly
+        resolves to the user model's USERNAME_FIELD.
+        """
+        from django.contrib.auth import get_user_model
+        from axes.conf import settings
+        
+        # Get the expected value
+        expected_username_field = get_user_model().USERNAME_FIELD
+        
+        # Get the actual value from axes settings
+        actual_username_field = str(settings.AXES_USERNAME_FORM_FIELD)
+        
+        # They should match
+        self.assertEqual(actual_username_field, expected_username_field)