Variation on python if statement syntax

Question

I ran across some python code syntax that I have never seen before. Here is an example:

i = 0
for spam in range(10):
    i += [1, 3][i > 5]
    print(i)

The result is the sequence of 1,2,3,4,5,6,9,12,15,18. So, it increments by 1 until i > 5, then increments by 3 thereafter.

Previously, I would have written the line as:

if i > 5:
    i += 3
else:
    i += 1

So what is the line: i += [1, 3][i > 5]?

• What do you call that syntax structure?
• Is it some form of list comprehension or something else entirely?

The syntax is interesting and I wanted to read more about it, but don't know where to look.

Edit: Thank you Darkstarone. I had never thought of using an expression to return a list index. That is very cool. This means you could also do things like: spam = ["Even", "Odd"][eggs % 2] to return an even or odd string or foo = ["A", "B", "C"][zot % 3] to cycle through three choices when looping through values of zot.

Probably won't make a habit of using this construct since other expressions are easier to understand. But, I'll take this into the bag o' tricks for that perfect situation.

Answer 1

So what I believe is happening here is the list ([1,3]) can either be:

[1,3][0] # 1

Or

[1,3][1] # 3

It's taking advantage of the fact that 0 == False and 1 == True. It's rather neat, although a little opaque. I probably would have written it like so:

i = 0
for _ in range(10):
    i += 3 if i > 5 else 1
    print(i)