Is there any scientific/empirical proof on this? I indent 4 spaces, no tabs, but I would like to see the real rationale there, not just the PEP-8 "rules".
http://www.cs.umd.edu/~ben/papers/Miara1983Program.pdf
I'd classify it as an attempt rather than something decisive but whatever floats your boat.
The level of indentation that seems to produce optimal
results in comprehension is between 2 and 4 spaces; as the
number of spaces increase, the comprehension level
decreases.
On the other hand, the sample program hard parts that were indented nine times (!) in places. It's an interesting read, but I'd not try to use it in a discussion.Personally, I don't care; just pick one (hopefully one that matches what most people are using with that language, eg. 2 spaces for Ruby), and get on with it.
Infinity spaces gives less space for code than zero.
Conclusion: use as wide an indentation as possible while still keeping enough space for the code itself.