What is a None value?
if-statement, python, variables
Solution
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
Assigning a value of `None` to a variable is one way to reset it to its original, empty state.
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:
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"
you put the sticker "F" on a string object `"fork"`. If you then write
F = None
you move the sticker to the `None` object.
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`.
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
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
like we do after we type `F = None` and put the sticker on `None`.
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 is pretending that all possible stickers you might want to write were already stuck to the `None` object.
Problem
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. I want to know what the `None` value is and what do you use it for? And also, I don't get this part of the book: Assigning a value of `None` to a variable is one way to reset it to its original, empty state. What does that mean? 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? Assigning a value of `None` to a variable is one way to reset it to its original, empty state. Final: 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.