django - 为什么request.POST对象不可变？ [英] django - why is the request.POST object immutable?

问题描述

正如标题所说，为什么Django的人决定实现request.POST对象与一个querydict（这当然反过来，使整个事情是不可变的？）

我知道您可以通过复制发布数据

  post = request.POST.copy（）
  

但为什么要这样做？当然这只是让事情变得更加简单呢？还是用于某些其他原因也可能导致问题？

解决方案

这有点神秘，不是它？几个表面上似乎合理的理论在调查中是错误的：

1. 所以 POST 对象不需要实现变异方法？否： POST 对象属于 django.http.QueryDict class ，它实现了一整套突变方法，包括 __ setitem __ ， __ delitem __ ， pop 和清除。当您调用其中一种突变方法时，通过检查标志来实现不变性。当您调用复制方法时，您将获得另一个 QueryDict 实例，并启用可变标志。

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2. 为了提高性能？否：当可变标志关闭时， QueryDict 类不会获得性能优势。




3. 可以将 POST 对象用作字典键？否： QueryDict 对象不可哈希。




4. 所以 / code>数据可以懒惰地构建（而不是承诺读取整个响应），如此声明我在代码中没有看到这一点：据我所知，整个响应总是被读取，直接，或通过 multipart 回复




5. 保护您免受编程错误的影响？我已经看到这个声明，但我从来没有看到这些错误是什么的好解释，以及不可变性如何保护你免受他们的伤害。

在任何情况下， POST 不总是不可变的：当响应是 multipart ，然后 POST 是可变的。这似乎把kibosh放在你可能会想到的大多数理论上。 （除非这个行为是一个疏忽）。

总而言之， 我可以看到没有明确的理由在Django中 POST 对象不可变为非 multipart 请求。

As the title asks, why did the Django guys decide to implement the request.POST object with a querydict (which, of course, in turn, makes the whole thing immutable?)

I know you can mutify it by making a copy of the post data

post = request.POST.copy()

but why do this? Surely it would be simpler just to allow the thing to be mutable anyway? Or is it being used for some other reason too which might cause issue?

解决方案

It's a bit of a mystery, isn't it? Several superficially plausible theories turn out to be wrong on investigation:

1. So that the POST object doesn't have to implement mutation methods? No: the POST object belongs to the django.http.QueryDict class, which implements a full set of mutation methods including __setitem__, __delitem__, pop and clear. It implements immutability by checking a flag when you call one of the mutation methods. And when you call the copy method you get another QueryDict instance with the mutable flag turned on.

2. For performance improvement? No: the QueryDict class gains no performance benefit when the mutable flag is turned off.

3. So that the POST object can be used as a dictionary key? No: QueryDict objects are not hashable.

4. So that the POST data can be built lazily (without committing to read the whole response), as claimed here? I see no evidence of this in the code: as far as I can tell, the whole of the response is always read, either directly, or via MultiPartParser for multipart responses.

5. To protect you against programming errors? I've seen this claimed, but I've never seen a good explanation of what these errors are, and how immutability protects you against them.

In any case, POST is not always immutable: when the response is multipart, then POST is mutable. This seems to put the kibosh on most theories you might think of. (Unless this behaviour is an oversight.)

In summary, I can see no clear rationale in Django for the POST object to be immutable for non-multipart requests.

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