升級到Django 1.9或1.11（從1.6） - 調用元類庫時出錯

Question

我升級到django 1.9（從1.6）

安裝了mysql-connector-python 2.1.6

當我嘗試運行我的應用程序時，我得到：

TypeError: Error when calling the metaclass bases
    'NoneType' object is not callable

有人有這個問題嗎？

堆棧跟蹤：

/Library/Python/2.7/site-packages/django/core/handlers/wsgi.pyc in __init__(self, *args, **kwargs)
    149     def __init__(self, *args, **kwargs):
    150         super(WSGIHandler, self).__init__(*args, **kwargs)
--> 151         self.load_middleware()
    152 
    153     def __call__(self, environ, start_response):

/Library/Python/2.7/site-packages/django/core/handlers/base.pyc in load_middleware(self)
     55             for middleware_path in settings.MIDDLEWARE_CLASSES:
     56                 print middleware_path
---> 57                 mw_class = import_string(middleware_path)
     58                 try:
     59                     mw_instance = mw_class()

/Library/Python/2.7/site-packages/django/utils/module_loading.pyc in import_string(dotted_path)
     18         six.reraise(ImportError, ImportError(msg), sys.exc_info()[2])
     19 
---> 20     module = import_module(module_path)
     21 
     22     try:

/System/Library/Frameworks/Python.framework/Versions/2.7/lib/python2.7/importlib/__init__.pyc in import_module(name, package)
     35             level += 1
     36         name = _resolve_name(name[level:], package, level)
---> 37     __import__(name)
     38     return sys.modules[name]

/Library/Python/2.7/site-packages/django/contrib/auth/middleware.py in <module>()
      2 from django.contrib import auth
      3 from django.contrib.auth import load_backend
----> 4 from django.contrib.auth.backends import RemoteUserBackend
      5 from django.core.exceptions import ImproperlyConfigured
      6 from django.utils.deprecation import MiddlewareMixin

/Library/Python/2.7/site-packages/django/contrib/auth/backends.py in <module>()
      2 
      3 from django.contrib.auth import get_user_model
----> 4 from django.contrib.auth.models import Permission
      5 
      6 UserModel = get_user_model()

/Library/Python/2.7/site-packages/django/contrib/auth/models.py in <module>()
      2 
      3 from django.contrib import auth
----> 4 from django.contrib.auth.base_user import AbstractBaseUser, BaseUserManager
      5 from django.contrib.auth.signals import user_logged_in
      6 from django.contrib.contenttypes.models import ContentType

/Library/Python/2.7/site-packages/django/contrib/auth/base_user.py in <module>()
     50 
     51 @python_2_unicode_compatible
---> 52 class AbstractBaseUser(models.Model):
     53     password = models.CharField(_('password'), max_length=128)
     54     last_login = models.DateTimeField(_('last login'), blank=True, null=True)

/Library/Python/2.7/site-packages/django/db/models/base.pyc in __new__(cls, name, bases, attrs)
    122                 app_label = app_config.label
    123 
--> 124         new_class.add_to_class('_meta', Options(meta, app_label))
    125         if not abstract:
    126             new_class.add_to_class(

/Library/Python/2.7/site-packages/django/db/models/base.pyc in add_to_class(cls, name, value)
    328         # We should call the contribute_to_class method only if it's bound
    329         if not inspect.isclass(value) and hasattr(value, 'contribute_to_class'):
--> 330             value.contribute_to_class(cls, name)
    331         else:
    332             setattr(cls, name, value)

/Library/Python/2.7/site-packages/django/db/models/options.pyc in contribute_to_class(self, cls, name)
    212         if not self.db_table:
    213             self.db_table = "%s_%s" % (self.app_label, self.model_name)
--> 214             self.db_table = truncate_name(self.db_table, connection.ops.max_name_length())
    215 
    216     def _prepare(self, model):

/Library/Python/2.7/site-packages/django/db/__init__.pyc in __getattr__(self, item)
     31     """
     32     def __getattr__(self, item):
---> 33         return getattr(connections[DEFAULT_DB_ALIAS], item)
     34 
     35     def __setattr__(self, name, value):

/Library/Python/2.7/site-packages/django/db/utils.pyc in __getitem__(self, alias)
    210         db = self.databases[alias]
    211         backend = load_backend(db['ENGINE'])
--> 212         conn = backend.DatabaseWrapper(db, alias)
    213         setattr(self._connections, alias, conn)
    214         return conn

/Library/Python/2.7/site-packages/mysql/connector/django/base.pyc in __init__(self, *args, **kwargs)
    334 
    335     def __init__(self, *args, **kwargs):
--> 336         super(DatabaseWrapper, self).__init__(*args, **kwargs)
    337 
    338         try:

/Library/Python/2.7/site-packages/django/db/backends/base/base.pyc in __init__(self, settings_dict, alias, allow_thread_sharing)
     94         self.run_commit_hooks_on_set_autocommit_on = False
     95 
---> 96         self.client = self.client_class(self)
     97         self.creation = self.creation_class(self)
     98         self.features = self.features_class(self)

TypeError: Error when calling the metaclass bases
    'NoneType' object is not callable

---

更新：我補充說

client_class = DatabaseClient 
creation_class = DatabaseCreation
features_class = DatabaseFeatures
introspection_class = DatabaseIntrospection
ops_class = DatabaseOperations
validation_class = DatabaseValidation 

在DatabaseWrapper的mysql / connecotr / django / base.py中（看起來像丟失了？ - 它在django1.11 / db / backends / mysql / base.py DatabaseWrapper上）。 但現在我得到錯誤：

 Model class django.contrib.contenttypes.models.ContentType doesn't declare an explicit app_label and isn't in an application in INSTALLED_APPS

所以我不知道這是否解決了問題或創建了新問題

Answer 1

模型類django.contrib.contenttypes.models.ContentType不聲明顯式的app_label，也不在INSTALLED_APPS的應用程序中

在完全導入設置文件之前（即在設置完成導入后發生的INSTALLED_APPS初始化之前）導入應用程序時，通常會發生此錯誤。 因此，請確保您在設置文件中沒有任何代碼，該文件從其他應用程序導入代碼。

還要確保您沒有在應用的__init__.py文件中導入模型或類似的應用代碼，以便在設置級別不會自動觸發導入。