Python 中的元类基础是什么？

Question

我正在尝试扩展 django-allauth 以针对我的项目做一些特定的事情。我基本上是在尝试在 django-allauth 之上编写自己的包装器，并希望安装、配置和其他东西与 allauth 非常相似。

为此，我从 allauth/accounts/app_settings.py 扩展 AppSettings 类开始。我创建了自己的app_settings.py 做了这样的事情：

from allauth.account import app_settings as AllAuthAppSettings
class MyAppSettings (AllAuthAppSettings):
     def __init__(self, prefix):
         # do something

另外，在 app_settings.py 的末尾，我简单地放了以下内容（从 django-allauth 本身复制）：

import sys
my_app_settings = MyAppSettings('MY_PREFIX_')
my_app_settings.__name__ = __name__
sys.modules[__name__] = my_app_settings

现在，当我开始我的项目时，它给了我以下错误：

TypeError: Error when calling the metaclass bases
    __init__() takes exactly 2 arguments (4 given)

老实说，我对 Python-Django 世界还很陌生，并不真正了解最后四行发生了什么。

什么是元类基础？传递给它的四个参数是什么？如何使这个流程发挥作用？

这是堆栈跟踪：

Unhandled exception in thread started by <function wrapper at 0x104146578>
Traceback (most recent call last):
  File "/Users/user/anaconda/lib/python2.7/site-packages/django/utils/autoreload.py", line 226, in wrapper
    fn(*args, **kwargs)
  File "/Users/user/anaconda/lib/python2.7/site-packages/django/core/management/commands/runserver.py", line 109, in inner_run
    autoreload.raise_last_exception()
  File "/Users/user/anaconda/lib/python2.7/site-packages/django/utils/autoreload.py", line 249, in raise_last_exception
    six.reraise(*_exception)
  File "/Users/user/anaconda/lib/python2.7/site-packages/django/utils/autoreload.py", line 226, in wrapper
    fn(*args, **kwargs)
  File "/Users/user/anaconda/lib/python2.7/site-packages/django/__init__.py", line 18, in setup
    apps.populate(settings.INSTALLED_APPS)
  File "/Users/user/anaconda/lib/python2.7/site-packages/django/apps/registry.py", line 108, in populate
    app_config.import_models(all_models)
  File "/Users/user/anaconda/lib/python2.7/site-packages/django/apps/config.py", line 202, in import_models
    self.models_module = import_module(models_module_name)
  File "/Users/user/anaconda/lib/python2.7/importlib/__init__.py", line 37, in import_module
    __import__(name)
  File "/Users/user/myproject/my_app/models.py", line 18, in <module>
    from .model_managers import *
  File "/Users/user/myproject/my_app/model_managers.py", line 89, in <module>
    from . import app_settings
  File "/Users/user/myproject/my_app/app_settings.py", line 9, in <module>
TypeError: Error when calling the metaclass bases
    __init__() takes exactly 2 arguments (4 given)

Answer 1

看起来您不应该能够从 AllAuthAppSettings 继承

django-allauth 包正在做一些非常丑陋的 python 魔术

import sys  # noqa
app_settings = AppSettings('ACCOUNT_')
app_settings.__name__ = __name__
sys.modules[__name__] = app_settings

基本上，当您导入app_settings 模块时，它会创建AppSettings 类的实例，将其重命名为app_settings 模块的名称，然后将导入的模块替换为班级！

您不能从类实例继承。我猜你想继承非实例化的AppSettings 类。为此，您需要从app_settings 的class 继承，而不是直接从app_settings 继承

from allauth.account import app_settings as AllAuthAppSettings


class MyAppSettings(AllAuthAppSettings.__class__):
    ...

我认为您不需要从 app_settings 模块的末尾复制这些行来将您的模块破解为一个类。