Optimal tab size for code readability [closed]

Question

Personal preferences aside, is there an optimal tab size (2 spaces? 3 spaces? 8 spaces?) for code readability? In the different projects I've worked on, people seem to have vastly different standards. I can't seem to read 2 space indents, but companies like Google use it as a standard.

Can anyone point to documentation, studies, or well-reasoned arguments for the optimal size of a tab?

If we want to get specific, I work mostly in python. The goal of this question is to pick a standard for the team I work on.

Answer 1

Four spaces and no hard tabs, if you're a Pythonista.

Answer 2

I like 8 spaces (I know, right?). It makes the start/ end of blocks really obvious.

As to your question, a formal usability study would be required. Let's look at limits though:

0 spaces

function test(){
var x = 1;
for (i=0; i<=5; i++){
doSomething();
}
}

No indentation is obviously bad. You can't tell where anything begins or ends.

19 Spaces

function test(){
                   var x = 1;
                   for (i=0; i<=5; i++){
                                      doSomething();
                   }
}

Loads of indentation is obviously bad too, because you can't visually link code to its parent function or loop (or what have you) as your peripheral vision doesn't extend that far. Your eyes have to flick too far back and forth to facilitate reading.

8 spaces

function test(){
        var x = 1;
        for (i=0; i<=5; i++){
                doSomething();
        }
}

I think I decided on 8 spaces because the word 'function' is 8 characters long. But it just seems so useful for readability. All the code is in my peripheral vision, and there's no way I can miss the start of a new block of code if I'm quickly scanning.