什么是无价值？ [英] What is a None value?
问题描述
我一直在研究Python，我读了一章描述无的值，但遗憾的是这本书在某些方面并不是很清楚。如果我在那里分享，我想我会找到问题的答案。
I have been studying Python, and I read a chapter which describes the None value, but unfortunately this book isn't very clear at some points. I thought that I would find the answer to my question, if I share it there.
我想知道无 value 是，你用它做什么？
I want to know what the None value is and what do you use it for?
而且，我还没有得到这本书的这一部分： / p>
And also, I don't get this part of the book:
为变量赋值
无是将其重置为$的一种方法b $ b原始的空状态。
Assigning a value of
Noneto a variable is one way to reset it to its original, empty state.
这是什么意思？
答案很棒，虽然由于我对计算机世界的了解不足（我还没有学过课程，对象等），所以我不理解大多数答案。这句话是什么意思？
The answers were great, although I didn't understand most of answers due to my low knowledge of the computer world (I haven't learned about classes, objects, etc.). What does this sentence mean?
为变量分配
无的值是一种方法可以将
重置为原来的空状态。
Assigning a value of
Noneto a variable is one way to reset it to its original, empty state.
最终：
最后，我通过寻找不同的答案得到了答案。我必须感谢那些把时间用来帮助我的人（特别是Martijn Pieters和DSM），我希望我能选择所有答案作为最佳答案，但选择仅限于一个。所有答案都很棒。
Finally I've got my answer from looking to different answers. I must appreciate all the people who put their times to help me (especially Martijn Pieters and DSM), and I wish that I could choose all answers as the best, but the selection is limited to one. All of the answers were great.
推荐答案
Martijn的答案解释了无是在Python中，并正确地指出该书具有误导性。由于Python程序员通常不会说
Martijn's answer explains what None is in Python, and correctly states that the book is misleading. Since Python programmers as a rule would never say
分配值
无到变量是将其重置为原始空状态
的一种方法。
Assigning a value of
Noneto a variable is one way to reset it to its original, empty state.
很难解释Briggs的含义一种有意义的方式，并解释了为什么这里没有人对它感到满意。一个可能有用的类比：
it's hard to explain what Briggs means in a way which makes sense and explains why no one here seems happy with it. One analogy which may help:
在Python中，变量名称就像贴在物体上的贴纸一样。每个贴纸都有一个独特的名字写在上面，它一次只能放在一个物体上，但如果你愿意，你可以在同一个物体上贴上不止一个贴纸。写的时候
In Python, variable names are like stickers put on objects. Every sticker has a unique name written on it, and it can only be on one object at a time, but you could put more than one sticker on the same object, if you wanted to. When you write
F = "fork"
您将标签F放在字符串对象fork上。如果你那么写
you put the sticker "F" on a string object "fork". If you then write
F = None
您将贴纸移动到无对象。
Briggs是什么要求你想象的是你没有写贴纸F，已经 F 无上的贴纸，你所做的就是移动，来自无到fork。因此，当您键入 F =无时，如果我们决定处理无，则将其重置为原始的空状态 含义空状态。
What Briggs is asking you to imagine is that you didn't write the sticker "F", there was already an F sticker on the None, and all you did was move it, from None to "fork". So when you type F = None, you're "reset[ting] it to its original, empty state", if we decided to treat None as meaning empty state.
我可以看到他得到了什么，但那是看待它的坏方法。如果你启动Python并输入 print（F），你会看到
I can see what he's getting at, but that's a bad way to look at it. If you start Python and type print(F), you see
>>> print(F)
Traceback (most recent call last):
File "<stdin>", line 1, in <module>
NameError: name 'F' is not defined
和 NameError 表示Python无法识别名称 F ，因为没有这样的贴纸。如果Briggs是正确的并且 F =无将 F 重置为其原始状态，那么它现在应该在那里，我们应该看
and that NameError means Python doesn't recognize the name F, because there is no such sticker. If Briggs were right and F = None resets F to its original state, then it should be there now, and we should see
>>> print(F)
None
就像我们输入后一样F =无并将贴纸贴在无。
like we do after we type F = None and put the sticker on None.
这就是所有这一切。实际上，Python附带了一些贴在对象上的贴纸（内置名称），但是其他的你需要用 F =fork和 A = 2 和 c17 = 3.14 ，然后您可以将它们粘贴在其他对象上（例如 F = 10 或 F =无;它们都是一样的。）
So that's all that's going on. In reality, Python comes with some stickers already attached to objects (built-in names), but others you have to write yourself with lines like F = "fork" and A = 2 and c17 = 3.14, and then you can stick them on other objects later (like F = 10 or F = None; it's all the same.)
Briggs假装您可能想写的所有可能的贴纸已经卡在无对象中。
Briggs is pretending that all possible stickers you might want to write were already stuck to the None object.
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