[1.7.x] Fixed #22195 -- Used constants to define built-in tags for check framework.

Thanks Elvard for the patch.

Backport of b513fa5fc6 from master

django/contrib/admin/apps.py

@@ -11,7 +11,7 @@ class SimpleAdminConfig(AppConfig):
     verbose_name = _("Administration")
 
     def ready(self):
-        checks.register('admin')(check_admin_app)
+        checks.register(checks.Tags.admin)(check_admin_app)
 
 
 class AdminConfig(SimpleAdminConfig):

django/contrib/auth/apps.py

@@ -10,4 +10,4 @@ class AuthConfig(AppConfig):
     verbose_name = _("Authentication and Authorization")
 
     def ready(self):
-        checks.register('models')(check_user_model)
+        checks.register(checks.Tags.models)(check_user_model)

django/contrib/contenttypes/apps.py

@@ -9,4 +9,4 @@ class ContentTypesConfig(AppConfig):
     verbose_name = _("Content Types")
 
     def ready(self):
-        checks.register('models')(check_generic_foreign_keys)
+        checks.register(checks.Tags.models)(check_generic_foreign_keys)

django/core/checks/__init__.py

@@ -4,7 +4,7 @@ from __future__ import unicode_literals
 from .messages import (CheckMessage,
         Debug, Info, Warning, Error, Critical,
         DEBUG, INFO, WARNING, ERROR, CRITICAL)
-from .registry import register, run_checks, tag_exists
+from .registry import register, run_checks, tag_exists, Tags
 
 # Import these to force registration of checks
 import django.core.checks.compatibility.django_1_6_0  # NOQA
@@ -14,5 +14,5 @@ __all__ = [
     'CheckMessage',
     'Debug', 'Info', 'Warning', 'Error', 'Critical',
     'DEBUG', 'INFO', 'WARNING', 'ERROR', 'CRITICAL',
-    'register', 'run_checks', 'tag_exists',
+    'register', 'run_checks', 'tag_exists', 'Tags',
 ]

django/core/checks/compatibility/django_1_6_0.py

@@ -3,10 +3,10 @@ from __future__ import unicode_literals
 
 from django.apps import apps
 
-from .. import Warning, register
+from .. import Warning, register, Tags
 
 
-@register('compatibility')
+@register(Tags.compatibility)
 def check_1_6_compatibility(**kwargs):
     errors = []
     errors.extend(_check_test_runner(**kwargs))

django/core/checks/model_checks.py

@@ -6,10 +6,10 @@ import types
 
 from django.apps import apps
 
-from . import Error, register
+from . import Error, Tags, register
 
 
-@register('models')
+@register(Tags.models)
 def check_all_models(app_configs=None, **kwargs):
     errors = [model.check(**kwargs)
         for model in apps.get_models()
@@ -17,7 +17,7 @@ def check_all_models(app_configs=None, **kwargs):
     return list(chain(*errors))
 
 
-@register('models', 'signals')
+@register(Tags.models, Tags.signals)
 def check_model_signals(app_configs=None, **kwargs):
     """Ensure lazily referenced model signals senders are installed."""
     from django.db import models

django/core/checks/registry.py

@@ -6,6 +6,16 @@ from itertools import chain
 from django.utils.itercompat import is_iterable
 
 
+class Tags(object):
+    """
+    Built-in tags for internal checks.
+    """
+    admin = 'admin'
+    compatibility = 'compatibility'
+    models = 'models'
+    signals = 'signals'
+
+
 class CheckRegistry(object):
 
     def __init__(self):