strange syntax for returning sign of number in python

Question

So the below snippet of code returns the sign of x:

x = -15  
(1, -1)[x < 0]

I have a hard time understanding WHY this syntax works. In particular, how come you can write a tuple following a list of booleans and get one or the other value in the tuple from that....Someone please explain this syntax!

Answer 1

这是一种基于布尔值进行索引的方法。如下所示，True和False分别在整数的上下文中评估为1和0：

>>> int(True)
1
>>> int(False)
0

此外，(1, -1)是一个元组，一个可以编入索引的序列对象：

>>> (314159,271828)[0]
314159
>>> (314159,271828)[1]
271828

结合这两个事实，你应该能够辨别：

(1, -1)[x < 0]

当1为假时，

会给您x < 0，当{1}}为真时，{p}会给您-1。这基本上是x的标志。