如何使用Python使用backet实现if语句？

Question

I saw this usage of Python, which is very clean, but I don't really understand this usage, and can't search any useful explanation about it either.

1. This is the normal method:

    if a < 0: b = 2 * a else: b = 3 * a 

2. And this realizes the same purpose:

    b = a * (3,2) [a<0] 

Can someone tell me Where does the official documentation say something about the above ?

Answer 1

b = a * (3,2) [a<0]

a<0 is a condition returning true or false ie 1/0

(3,2) is a tuple with 3 and 2 as elements

if the condition is true statement will be indexing the element at index 1 else it will indexing to element 0

like

In [33]: (3,2)[0]
Out[33]: 3

In [34]: (3,2)[1]
Out[34]: 2

The condition results in 0 or 1 because In python True means 1 and False means 0

In [35]: True == 1
Out[35]: True

In [36]: False == 0
Out[36]: True

Answer 2

Use the second option for golfing purposes only. The docs really don't say much about this syntax, but The Zen of Python does state that 'Simple is better than complex', and that 'Readability counts'. You should use the first option.

You could also use Python's ternary operator :

b = a*(2 if a < 0 else 3)