Simple Python code with weird list manipulation/comprehension technique by which one element of the list gets assigned to a variable

Question

Today I was browsing through a coding website and found a simple code in Python which I was having a hard time to comprehend.

Below is the exact code (with extra details added as comments):

x = -23 

sign = [1,-1][x < 0] # -1 if x is negative and 1 if x is positive

print(sign) #outputs -1 since x=-23 is negative

Can anyone please help me understand how this code is working and what this technique is called (I presume it is some kind of list comprehension/manipulation)?

Answer 1

This is playing with the fact that True is equivalent to 1 and False to 0.

If x<0 returns True, then [1,-1][x < 0] is equivalent to [1,-1][1], and thus -1.

The logic is the same when x<0 returns False: [1,-1][0] -> 1

Answer 2

sign = [1,-1][x < 0] is just a fancy way to write

if x < 0:
    sign = -1
else:
   sign = 1

x < 0 is either True or False. Since True == 1 and False == 0 you can use booleans to index into the list [-1, 1].

Answer 3

x = -23
x2 = 23
sign = ["false","true"][x > 0]
sign2 = ["false","true"][x2 > 0]
print("sign:",sign)
print("sign2:",sign2)

the result is sign:false,sign2:true