什么是常量和文字常量? [英] What are Constants and Literal constants?

问题描述

我正在学习Python，只是对Constants和Literal常量感到困惑，它们是什么?我们出于什么目的使用它们?与普通变量有什么区别?

I'm learning Python and i am just confused with the Constants and Literal constants.What are they?For what kind of purpose do we use them ? What is their difference from the normal variable?

非常感谢您.

我是一个真正的初学者.就像我可以说的初学者一样，我对编程世界一无所知.就像我不知道什么是表达式，反之亦然.

I'm a true beginner.As beginner as i can say i know nothing about the programming world.Like i don't know what an expression is and vice versa.

我一直在使用"A byte of python"这本书来学习python语言，在书中的某个地方，我遇到了一个讨论字面量和常量的部分.我在那儿分享该部分:

I have been learning the python language using the " A byte of python " book and somewhere in the book i came across a section which talks about literals and constants.I share the section there:

5.2.文字常量

5.2. Literal Constants

文字常量的一个示例是数字，例如5，1.23或a 像这是一个字符串"或这是一个字符串！"之类的字符串

An example of a literal constant is a number like 5 , 1.23 , or a string like 'This is a string' or "It's a string!" .

之所以称其为文字，是因为它是文字-您可以使用其值 字面上地.数字2总是代表自己，别无其他-它 是一个常量，因为它的值无法更改.因此，所有这些 被称为文字常量.

It is called a literal because it is literal - you use its value literally. The number 2 always represents itself and nothing else - it is a constant because its value cannot be changed. Hence, all these are referred to as literal constants.

上面写着因为它是字面上的，所以它被称为字面意思，因为您按字面意义使用它的值"，我只是不明白这一点.这本书试图说我们按字面意义使用值是什么?另一本书不清楚的是数字2是一个常量，因为它的值无法更改.怎么可能?我们可以更改它，例如:

Where it says,"it is called literal because it is literal-you use the it's value literally",i just don't get this part.What is the book trying to say that we use the value literally?the another vague point is that the number 2 is a constant because it's value cannot be changed.How is it possible?we can change it,like:

stack = 2
stack = 3

首先，我将数字2分配给堆栈，然后更改了堆栈的值(书中声称该数字2是一个常数，因此无法更改)，并为其分配了数字3.所以我很容易地更改了数字2的值.我真的很困惑，如果您不明白我的意思，请告诉我，以便我举更多的例子.谢谢您的陪伴.

First, I assigned the number 2 to the Stack then I changed the value of the Stack(Which is that number 2 that the book is claiming it is a constant so it cannot be changed)and assigned the number 3 to it.So i easily changed the value of the number 2.I am really confused,if you didn't get my point,please tell me so i can give more examples.Thank you for giving your time guys.

推荐答案

OP编辑后的答案

文字常量是实际的文字值；我知道字面量这个词会让您感到困惑，但是举个例子可能会使它更清楚.如果您在REPL中输入以下内容:

A literal constant is an actual literal value; I know the word literal confuses you but an example might make it clearer. If you type the following in the REPL:

>>> 2
2
>>> 'hello'
'hello'

2和hello是实际的文字常量，与您所认为的相反，您无法更改它们的值(嗯，作为初学者，最好不知道这一点).如果有:

2 and hello are actual literal constants and contrary to what you think, you can't change their value (well, you can, as a beginner, it's best to not know about that). When you have:

stack = 2
stack = 3

您首先要为stack分配常量文字(尽管，老实说，不用担心它叫什么，它是数字2).因此，名称stack指向值2 .然后，说出stack = 3，表示您正在 不是 更改值2； 您现在将名称stack指向另一个值3 .

You are first assigning the constant literal (though, honestly, don't worry about what it's called, it's the number 2) to stack. So, the name stack is pointing to the value 2. Then, by saying stack = 3, you are not changing the value 2; you are now making the name stack to point to another value, 3.

就其价值而言，常量字面量"听起来很复杂；只需将2或'John'等值视为实际值即可.关于实际常量(在编程常量中，常量指的是赋值后不能更改的变量)，该概念实际上在Python中并不存在.例如，常量是指您说stack = 2的时间，但随后您将永远无法更改stack指向的内容，否则会出现错误.在Python中，这个概念不存在.

For what it's worth, "constant literal" sounds complicated; just think of values like 2 or 'John' etc. as what they are. And with regards to actual constants (in programming constants are referred to variables that cannot be changed after assignment), that concept doesn't really exist in Python. A constant is when, for instance, you say stack = 2 but then you cannot ever change what stack is pointing to or you'll get an error. In Python, this concept does not exist.

原始答案:

对于初学者，我建议您阅读:

For starters, I recommend you read The story of None, True and False (and an explanation of literals, keywords and builtins thrown in) by Guido:

另一方面，文字是描述常量值的表达式的元素.文字的示例是数字(例如42、3.14或1.6e-10)和字符串(例如"Hello，world").解析器可以识别文字，并且解析文字的确切规则通常非常微妙.

A literal, on the other hand, is an element of an expression that describes a constant value. Examples of literals are numbers (e.g. 42, 3.14, or 1.6e-10) and strings (e.g. "Hello, world"). Literals are recognized by the parser, and the exact rules for how literals are parsed are often quite subtle.

对于常量"，您不能在Python中将变量声明为真常量".有内置常量，如True和False和None在Python中，但即使在Python 2.X中它们也不是真常量"，因为可以将它们分配为指向另一个值:

As for "constants", you cannot declare a variables as "true constants" in Python. There are a Built-in Constants like True and False and None in Python but even they are not"true constants" in Python 2.X as they can be assigned to point to another value:

True = False
if True:
    print 'Hey' 
else:
    print 'WAAAT!'

我希望这会有所帮助.如果不是，请编辑您的问题，并举例说明常量和文字常量的确切含义.

I hope this helps. If not, please edit your questions and give an example of what you mean exactly by Constants and Literal Constants.

注意:True和False等是Python 3.x中的关键字，因此，如果您说True = False，则解释器将引发SyntaxError: assignment to keyword.

Note: True and False and the like are keywords in Python 3.x, so if you say True = False, the interpreter will raise SyntaxError: assignment to keyword.

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