Python Bool 和 int 比较和索引列表与布尔值 [英] Python Bool and int comparison and indexing on list with boolean values
问题描述
使用布尔值对列表进行索引工作正常.虽然索引应该是一个整数.
以下是我在控制台中尝试的:
<预><代码>>>>l = [1,2,3,4,5,6]>>>>>>l[假]1>>>l[真]2>>>l[假+真]2>>>l[假+2*真]3>>>>>>l['0']回溯(最近一次调用最后一次):文件<stdin>",第 1 行,在 <module> 中类型错误:列表索引必须是整数,而不是 str>>>类型(真)<输入'bool'>当我尝试 l['0']
时,它打印了索引中预期的 int 类型的错误,这很明显.然后,即使 'True'
和 'False'
的类型是 Bool
,列表上的索引也能正常工作并自动将其转换为 int 类型并执行操作.
请解释一下内部发生了什么.第一次发帖,如有错误请见谅.
发生的事情是布尔值实际上是整数.True 为 1,False 为 0.Bool 是 int 的子类型.
<预><代码>>>>isinstance(真,int)真的>>>issubclass(bool, int)真的因此它不会将它们转换为整数,而只是将它们用作整数.
(由于历史原因,Bool 是整数.在 Python 中存在 bool 类型之前,人们使用整数 0 表示 false,1 表示 true.因此,当他们添加 bool 类型时,他们将布尔值设为整数,以便保持与使用这些整数值的旧代码的向后兼容性.参见例如 http://www.peterbe.com/plog/bool-is-int .)
<预><代码>>>>帮助(真)bool 对象的帮助:类布尔(整数)|bool(x) ->布尔值||当参数 x 为真时返回 True,否则返回 False.|内置函数 True 和 False 是 bool 类仅有的两个实例.|类 bool 是类 int 的子类,不能被子类化.Indexing on list with boolean values works fine. Though the index should be an integer.
Following is what I tried in console:
>>> l = [1,2,3,4,5,6]
>>>
>>> l[False]
1
>>> l[True]
2
>>> l[False + True]
2
>>> l[False + 2*True]
3
>>>
>>> l['0']
Traceback (most recent call last):
File "<stdin>", line 1, in <module>
TypeError: list indices must be integers, not str
>>> type(True)
<type 'bool'>
When I tried l['0']
it printed error that int type expected in indices and that is obvious.
Then, even the type of 'True'
and 'False'
being Bool
, indexing on the list works fine and automatically converts it to int type and performs the operation.
Please explain what is going on internally. I am posting question for the first time, so please forgive me for any mistake.
What's going on is that booleans actually are integers. True is 1 and False is 0. Bool is a subtype of int.
>>> isinstance(True, int)
True
>>> issubclass(bool, int)
True
So it's not converting them to integers, it's just using them as integers.
(Bools are ints for historical reasons. Before a bool type existed in Python, people used the integer 0 to mean false and 1 to mean true. So when they added a bool type, they made the boolean values integers in order to maintain backward compatibility with old code that used these integer values. See for instance http://www.peterbe.com/plog/bool-is-int .)
>>> help(True)
Help on bool object:
class bool(int)
| bool(x) -> bool
|
| Returns True when the argument x is true, False otherwise.
| The builtins True and False are the only two instances of the class bool.
| The class bool is a subclass of the class int, and cannot be subclassed.
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