Django 从 1.8 升级到 1.9 在 init 中的模型导入中断答案

【问题标题】：Django upgrade from 1.8 to 1.9 is breaking on model import in initDjango 从 1.8 升级到 1.9 在 init 中的模型导入中断
【发布时间】：2020-01-27 08:05:28
【问题描述】：

我遇到了@JohnnyQ 在here 上评论过的同样问题。
我的__init__.py 指的是多个导入，而这些导入又是指模型，这些是必需的。

代码在 django 1.8.x 上运行良好，但在 Django 升级到 1.9.x 时会中断。

我猜这是一些类加载顺序问题。在 1.9 w.r.t 中对触发 AppRegistryNotReady 异常的应用程序中的模型导入进行了哪些更改？

回溯在这里：

Traceback (most recent call last):
  File "/Applications/PyCharm CE.app/Contents/helpers/pydev/_pydev_bundle/pydev_monkey.py", line 729, in __call__
    ret = self.original_func(*self.args, **self.kwargs)
  File "/Users/vinod/virtualenvs/py2/lib/python2.7/site-packages/django/utils/autoreload.py", line 226, in wrapper
    fn(*args, **kwargs)
  File "/Users/vinod/virtualenvs/py2/lib/python2.7/site-packages/django/core/management/commands/runserver.py", line 113, in inner_run
    autoreload.raise_last_exception()
  File "/Users/vinod/virtualenvs/py2/lib/python2.7/site-packages/django/utils/autoreload.py", line 249, in raise_last_exception
    six.reraise(*_exception)
  File "/Users/vinod/virtualenvs/py2/lib/python2.7/site-packages/django/utils/autoreload.py", line 226, in wrapper
    fn(*args, **kwargs)
  File "/Users/vinod/virtualenvs/py2/lib/python2.7/site-packages/django/__init__.py", line 27, in setup
    apps.populate(settings.INSTALLED_APPS)
  File "/Users/vinod/virtualenvs/py2/lib/python2.7/site-packages/django/apps/registry.py", line 85, in populate
    app_config = AppConfig.create(entry)
  File "/Users/vinod/virtualenvs/py2/lib/python2.7/site-packages/django/apps/config.py", line 90, in create
    module = import_module(entry)
  File "/usr/local/Cellar/python@2/2.7.17/Frameworks/Python.framework/Versions/2.7/lib/python2.7/importlib/__init__.py", line 37, in import_module
    __import__(name)
  File "/Users/vinod/mycompany/SourceCodes/mycompany_server/mycompany/core/__init__.py", line 1, in <module>
    from mycompany.core import care_team
  File "/Users/vinod/mycompany/SourceCodes/mycompany_server/mycompany/core/care_team.py", line 8, in <module>
    from mycompany.core import models as core_models
  File "/Users/vinod/mycompany/SourceCodes/mycompany_server/mycompany/core/models.py", line 16, in <module>
    from django.contrib.auth.models import User
  File "/Users/vinod/virtualenvs/py2/lib/python2.7/site-packages/django/contrib/auth/models.py", line 4, in <module>
    from django.contrib.auth.base_user import AbstractBaseUser, BaseUserManager
  File "/Users/vinod/virtualenvs/py2/lib/python2.7/site-packages/django/contrib/auth/base_user.py", line 52, in <module>
    class AbstractBaseUser(models.Model):
  File "/Users/vinod/virtualenvs/py2/lib/python2.7/site-packages/django/db/models/base.py", line 105, in __new__
    app_config = apps.get_containing_app_config(module)
  File "/Users/vinod/virtualenvs/py2/lib/python2.7/site-packages/django/apps/registry.py", line 237, in get_containing_app_config
    self.check_apps_ready()
  File "/Users/vinod/virtualenvs/py2/lib/python2.7/site-packages/django/apps/registry.py", line 124, in check_apps_ready
    raise AppRegistryNotReady("Apps aren't loaded yet.")
django.core.exceptions.AppRegistryNotReady: Apps aren't loaded yet.

【问题讨论】：

标签： python python-2.7 django-1.9 django-contrib django-upgrade

【解决方案1】：

Django 1.9 引入了app registry，如文档中所述，它分三个步骤进行初始化。

首先，它会导入 INSTALLED_APPS 中的所有项目。在这个阶段，代码不应导入任何模型。
其次，Django 尝试导入每个应用程序的模型子模块（如果存在）。
第三，Django 在每个应用配置上运行.ready()。

在core/__init__.py 中，有care_team 导入，这反过来又导入了这个from django.contrib.auth.models import User。应在应用级别初始化时避免此模型导入。

这应该是延迟加载的。或者需要重构代码，以便在应用初始化期间不会导入任何模型。

【讨论】：