0

我正在尝试覆盖模型实例__nonzero__，因此它评估为False

我首先在一个简单的 Python 类上进行了测试：

>>> class A():
...     pass
... 
>>> a = A()
>>> a.__nonzero__ = lambda: False
>>> b = A()
>>> bool(a)
False
>>> bool(b)
True
>>> type(a).__nonzero__(a)
False
>>> type(b).__nonzero__(b)
True

这表明__nonzero__被覆盖成功地改变了对象的bool()评估。

然后我尝试将相同的东西应用于 django 模型对象（feincms.module.page.Page它本身的实例继承自`django.db.models.Manager）。

from django.contrib import admin
from django.utils.translation import ugettext_lazy as _
from feincms.module.page.modeladmins import PageAdmin
from feincms.module.page.models import Page
from guardian.admin import GuardedModelAdmin
from reversion import VersionAdmin


class PageAdmin(PageAdmin, GuardedModelAdmin, VersionAdmin):

    # some stuff
    # ...
    #

    def render_revision_form(self, *args, **kwargs): 
        """Override render_revision_form so we can make ``obj`` evaluate to False
        so ``feincms.module.page.forms.PageAdminForm.clean()`` does not try
        to query ``self.page_manager.get(pk=current_id)`` where current_id
        is ``obj.pk`` in the below function.
        This is needed because ``self.page_manager.get(pk=current_id)`` is
        queried on feincms Page objects rather than reversion Version objects.
        """
        request, obj, version, context = args
        obj.__nonzero__ = lambda: False
        obj.hello = 'a'
        import pdb
        pdb.set_trace()
        return super(PageAdmin, self).render_revision_form(request, obj, version, context, **kwargs)

admin.site.register(Page, PageAdmin)

但这评估为True：

(Pdb) obj
<Page: test>
(Pdb) bool(obj)
True

有什么我不知道的原因可能导致这种行为吗？

4

1 回答 1

0

在此示例中，您使用的是旧式类。模型实例是一个新型类。

class B(object): pass
b = B()
b.__nonzero__ = lambda: False
bool(b) # True

显然，您不能覆盖实例的特殊方法，只能覆盖类型。

B.__nonzero__ = lambda self: False
bool(b) # False

阅读以下 SO 问题以获取示例和更详细的解释：“overriding-special-methods-on-an-instance”

于 2013-10-09T15:00:17.880 回答