用type（）定义类对象：为什么我不能使用要复制的属性的名称？

Question

由于某些原因，我不得不使用type()函数在Django中创建一个form类。

函数生成器（makeFurnitureForm()）的返回语句如下所示：

    return (type("FurnitureForm",
                 (forms.BaseForm, form_utils.forms.BetterBaseForm,),
                 {"_fieldsets" : dic, "base_fields" : fields,
                  "_fieldset_collection" : None, '_row_attrs' : {}}))

我创建的类对象继承了其他两个类。 下面是其中一个的构造函数：form_utils.forms.BetterBaseForm

    def __init__(self, *args, **kwargs):
        self._fieldsets = deepcopy(self.base_fieldsets)
        self._row_attrs = deepcopy(self.base_row_attrs)
        self._fieldset_collection = None
        super(BetterBaseForm, self).__init__(*args, **kwargs)

我的代码一切正常，但在以前的版本中，我使用了如下代码：

    return (type("FurnitureForm",
                 (forms.BaseForm, form_utils.forms.BetterBaseForm,),
                 {"base_fieldsets" : dic, "base_fields" : fields}))

由于_fieldsets和_fieldset_collection是通过深入复制base_fieldsets属性并分配None值在BetterBaseForm构造函数中显式定义和设置的，我不明白为什么在尝试访问实例的_fieldsets和_fieldset_collection属性时（例如呈现我的django页面时）会得到AttributeError。你知道吗

以下是我的问题：

1. type()是否调用基类的构造函数，这些构造函数是否正确地定义和设置了属性？

2. 为什么我必须自己显式地设置这些属性？

3. 为什么当构造函数试图 在type()期间访问属性（例如row_attrs） 打电话？（不过，我后来尝试访问它时得到了一个）。

我在django视图中使用makeFurnitureForm()：

return (render(request,'main/devis-bis.html', {"form" : (makeFurnitureForm())()}))

类对象直接传递给视图，然后传递给要呈现的模板。你知道吗

编辑：回溯和代码

此返回的示例：

    return (type("FurnitureForm",
                 (forms.BaseForm, form_utils.forms.BetterBaseForm,),
                 {"base_fieldsets" : dic, "base_fields" : fields,
                  "_fieldset_collection" : None, '_row_attrs' : {}}))

我还发现我直接将一个类对象传递给render（）函数，我不明白为什么它会工作，因为它需要一个实例。我在render()调用中更正了它。你知道吗

File "/nfs/2013/v/vmonteco/Envs/demenhouse/lib/python3.5/site-packages/django/forms/forms.py" in __getitem__
  141.             field = self.fields[name]

During handling of the above exception ('fieldsets'), another exception occurred:

File "/nfs/2013/v/vmonteco/Envs/demenhouse/lib/python3.5/site-packages/django/template/base.py" in _resolve_lookup
  883.                     current = current[bit]

File "/nfs/2013/v/vmonteco/Envs/demenhouse/lib/python3.5/site-packages/django/forms/forms.py" in __getitem__
  144.                 "Key %r not found in '%s'" % (name, self.__class__.__name__))

During handling of the above exception ("Key 'fieldsets' not found in 'FurnitureForm'"), another exception occurred:

File "/nfs/2013/v/vmonteco/Envs/demenhouse/lib/python3.5/site-packages/django/core/handlers/base.py" in get_response
  149.                     response = self.process_exception_by_middleware(e, request)

File "/nfs/2013/v/vmonteco/Envs/demenhouse/lib/python3.5/site-packages/django/core/handlers/base.py" in get_response
  147.                     response = wrapped_callback(request, *callback_args, **callback_kwargs)

File "/nfs/2013/v/vmonteco/Code/web/demenhouse/htdocs/main/views.py" in secondform
  69.         return (render(request,'main/devis-bis.html', {"form" : makeFurnitureForm()}))

File "/nfs/2013/v/vmonteco/Envs/demenhouse/lib/python3.5/site-packages/django/shortcuts.py" in render
  67.             template_name, context, request=request, using=using)

File "/nfs/2013/v/vmonteco/Envs/demenhouse/lib/python3.5/site-packages/django/template/loader.py" in render_to_string
  97.         return template.render(context, request)

File "/nfs/2013/v/vmonteco/Envs/demenhouse/lib/python3.5/site-packages/django/template/backends/django.py" in render
  95.             return self.template.render(context)

File "/nfs/2013/v/vmonteco/Envs/demenhouse/lib/python3.5/site-packages/django/template/base.py" in render
  206.                     return self._render(context)

File "/nfs/2013/v/vmonteco/Envs/demenhouse/lib/python3.5/site-packages/django/template/base.py" in _render
  197.         return self.nodelist.render(context)

File "/nfs/2013/v/vmonteco/Envs/demenhouse/lib/python3.5/site-packages/django/template/base.py" in render
  992.                 bit = node.render_annotated(context)

File "/nfs/2013/v/vmonteco/Envs/demenhouse/lib/python3.5/site-packages/django/template/base.py" in render_annotated
  959.             return self.render(context)

File "/nfs/2013/v/vmonteco/Envs/demenhouse/lib/python3.5/site-packages/django/template/loader_tags.py" in render
  173.         return compiled_parent._render(context)

File "/nfs/2013/v/vmonteco/Envs/demenhouse/lib/python3.5/site-packages/django/template/base.py" in _render
  197.         return self.nodelist.render(context)

File "/nfs/2013/v/vmonteco/Envs/demenhouse/lib/python3.5/site-packages/django/template/base.py" in render
  992.                 bit = node.render_annotated(context)

File "/nfs/2013/v/vmonteco/Envs/demenhouse/lib/python3.5/site-packages/django/template/base.py" in render_annotated
  959.             return self.render(context)

File "/nfs/2013/v/vmonteco/Envs/demenhouse/lib/python3.5/site-packages/django/template/loader_tags.py" in render
  69.                 result = block.nodelist.render(context)

File "/nfs/2013/v/vmonteco/Envs/demenhouse/lib/python3.5/site-packages/django/template/base.py" in render
  992.                 bit = node.render_annotated(context)

File "/nfs/2013/v/vmonteco/Envs/demenhouse/lib/python3.5/site-packages/django/template/base.py" in render_annotated
  959.             return self.render(context)

File "/nfs/2013/v/vmonteco/Envs/demenhouse/lib/python3.5/site-packages/django/template/defaulttags.py" in render
  161.                 values = self.sequence.resolve(context, True)

File "/nfs/2013/v/vmonteco/Envs/demenhouse/lib/python3.5/site-packages/django/template/base.py" in resolve
  709.                 obj = self.var.resolve(context)

File "/nfs/2013/v/vmonteco/Envs/demenhouse/lib/python3.5/site-packages/django/template/base.py" in resolve
  850.             value = self._resolve_lookup(context)

File "/nfs/2013/v/vmonteco/Envs/demenhouse/lib/python3.5/site-packages/django/template/base.py" in _resolve_lookup
  891.                         current = getattr(current, bit)

File "/nfs/2013/v/vmonteco/Envs/demenhouse/lib/python3.5/site-packages/form_utils/forms.py" in fieldsets
  253.                 self, self._fieldsets)

Exception Type: AttributeError at /devisbis/1679091c5a880faf6fb5e6087eb1b2dc/
Exception Value: 'FurnitureForm' object has no attribute '_fieldsets'

Answer 1

1. Doesn't type() calls base classes` constructors, and don't these constructors define and set attributes properly?

   您看到的是初始化器，而不是构造函数，因此命名为__init__。创建实例时会自动调用它们。type()不调用这些，因为您不是在生成实例，而是在生成一个类。你知道吗

   一旦调用结果类来创建实例，然后调用__init__方法。

2. Why did I have to explicitly set these attributes by myself?

   我不知道，你没有显示你是如何创建一个实例，或任何其他代码显示你的错误。如果您希望这些属性存在于类中，那么您的假设是错误的。

3. Why didn't I got an AttributeError when the constructor tries to access an attribute (row_attrs for instance) during the type() call? (I got one when trying to access it later though).

   同样，type()生成一个新类，而不是一个实例，此时不调用__init__。

如果__init__方法将这些属性添加到实例命名空间，则不需要将这些属性添加到类命名空间。你知道吗

我必须注意的是^{} method的设计不是为了与MRO后面的其他类协作；它从不调用super().__init__()。如果确实创建了新类的实例，则不会调用form_utils.forms.BetterBaseForm.__init__()方法，除非要交换两个基类的顺序：

return type(
    "FurnitureForm", (form_utils.forms.BetterBaseForm, forms.BaseForm,),
    {"base_fieldsets" : dic, "base_fields" : fields})

这是因为首先列出BetterBaseForm，它将调用它的__init__，然后该方法中的super(BetterBaseForm, self).__init__(*args, **kwargs)行将调用BaseForm.__init__()。你知道吗

阅读你的回溯我注意到你从来没有创建过一个实例。您应该始终创建一个表单类的实例，并可以选择传入request.POST，以预填充字段并验证表单输入。你知道吗

所以不是：

return (render(request,'main/devis-bis.html', {"form" : makeFurnitureForm()}))

使用

return render(request,'main/devis-bis.html', {"form" : makeFurnitureForm()()})

注意额外的()。但是，您可能需要检查表单在某个时刻是否有效，因此您可以使用以下方法：

form_class = makeFurnitureForm()
if request.method == 'POST':
    form = form_class(request.POST)
    if form.is_valid():
        # ...
        # process data, redirect somewhere
        return HttpResponseRedirect('/thanks/')

else:
    form = form_class()

return render(request,'main/devis-bis.html', {"form" : form})