使用 True/False 作为键 - 如何/为什么这样做? [英] Using True/False as keys - how/why does this work?

问题描述

我很乐意使用这种简单的语法来初始化 dictionary

d = {'a':'Apple','b':'Bat'};

今天在看一个页面的时候，遇到了这段奇怪的代码

{True:0, False:1}[True];

我想知道为什么/如何工作?True 和 False 是保留关键字，所以，这种疯狂的语法应该毫无意义(对于编译器)，但它是不是.

<预><代码>>>>d = {真:0，假:1}；>>>d{假:1，真:0}

这变得更疯狂了

<预><代码>>>>d = dict(真 = 0, 假 = 1);语法错误:分配给关键字>>>d = dict(_True = 0, _False = 1);>>>d{'_假':1，'_真':0}

在dict() 构造函数中，不允许使用True 关键字！但是……

<小时>更新

Python 3.3.2 (v3.3.2:d047928ae3f6, May 16 2013, 00:03:43) [MSC v.1600 32 bit (Intel)] on win32输入copyright"、credits"或license()"以获取更多信息.>>>导入关键字>>>关键字.iskeyword('打印');错误的>>>关键字.iskeyword('else');真的>>>关键字.iskeyword('真');真的>>>关键字.iskeyword('假');真的

解决方案

虽然它们是关键字(在 Python 3 中)，True 和 False 仍然是对象的名称(它们分别是 bool(1) 和 bool(0)).

所以你可以在任何有意义的地方使用它们.由于它们是可散列的，这包括将它们用作字典键.你可以这样做:

d = {}d[真] = "真"d[假] = "假"

你可以用大括号直接得到同一个字典(d = {True: "True", False: "False"})，但不能使用dict带有关键字参数的构造函数.构造函数的关键字版本仅构建具有字符串键的字典，并且这些字符串必须是合法的 Python 标识符.True 和 False 不是字符串，也不能(因为它们是关键字)用作标识符.

您当然可以使用键/值元组构造函数的序列，如果有某种原因您需要调用 dict 而不是使用括号:

d = dict([(True, "True"), (False, "False")])

我注意到您展示的代码片段有点傻.Python 中的 bool 类型是 int 的子类，所以你可以对 True 或 False 进行数学运算，如果你要.字典索引代码等价于1-True，即0.

I am comfortable using this simple syntax for initializing a dictionary

d = {'a':'Apple','b':'Bat'};

Today, while reading a page, I encountered this weird piece of code

{True:0, False:1}[True];

I was wondering why/how that could work? True and False are reserved keywords, and so, that crazy syntax should be meaningless (for the compiler), but it is not.

>>> d = {True:0, False:1};
>>> d
{False: 1, True: 0}

And this gets crazier

>>> d = dict(True = 0, False = 1);
SyntaxError: assignment to keyword
>>> d = dict(_True = 0, _False = 1);
>>> d
{'_False': 1, '_True': 0}

In dict() constructor, True keyword is not allowed! But...

---

Update

Python 3.3.2 (v3.3.2:d047928ae3f6, May 16 2013, 00:03:43) [MSC v.1600 32 bit (Intel)] on win32
Type "copyright", "credits" or "license()" for more information.
>>> import keyword
>>> keyword.iskeyword('print');
False
>>> keyword.iskeyword('else');
True
>>> keyword.iskeyword('True');
True
>>> keyword.iskeyword('False');
True

解决方案

While they are keywords (in Python 3), True and False are still names of objects (they are, respectively bool(1) and bool(0)).

So you can use them anywhere a value makes sense. Since they're hashable, that includes using them as dictionary keys. You can do:

d = {}
d[True] = "True"
d[False] = "False"

You can get the same dictionary directly with curly brackets (d = {True: "True", False: "False"}), but not using the dict constructor with keyword arguments. The keyword version of the constructor only builds dicts with keys that are strings, and those strings must be legal Python identifiers. True and False are not strings, nor (since they're keywords) can they be used as identifiers.

You can of course use the sequence of key/value tuples constructor, if there's some reason you need to be calling dict rather than using brackets:

d = dict([(True, "True"), (False, "False")])

I would note that the code fragment you show is a bit silly. The bool type in Python is a subclass of int, so you can do mathematical operations on True or False if you want. The dictionary indexing code is equivalent to 1-True, which will be 0.

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