问题描述

使用布尔值对列表进行索引可以正常工作.虽然索引应该是一个整数.

Indexing on list with boolean values works fine. Though the index should be an integer.

以下是我在控制台中尝试过的:

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'>

当我尝试 l['0'] 时，它会打印出索引中预期的 int 类型的错误，这很明显.然后，即使 'True' 和 'False' 的类型是 Bool，列表上的索引也可以正常工作并自动将其转换为 int 类型并执行操作.

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.

请解释内部发生的情况.我是第一次发问题，如有错误请见谅.

Please explain what is going on internally. I am posting question for the first time, so please forgive me for any mistake.

推荐答案

布尔值实际上是 整数.True 为 1，False 为 0.Bool 是 int 的子类型.

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

所以它没有将它们转换为整数，它只是将它们用作整数.

So it's not converting them to integers, it's just using them as integers.

(由于历史原因，Bools 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.例如，参见 http://www.peterbe.com/plog/bool-is-int .)

(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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