[brownbat]

Obvious depends on your constraints.

Suppose (1) some of your newlines are indented but some are aligned for readability, and (2) mixing whitespace has a tendency to create subtle bugs.

Under those constraints, the only possible choice is spaces. Tabs can indent, but only spaces can either indent or align. Process of elimination forces you to prefer spaces over tabs in that situation.

This might be why PEP 0008 urges, "Spaces are the preferred indentation method." https://www.python.org/dev/peps/pep-0008/ [Specifically, four spaces.]

I had heard Google urged two spaces for indentation of Python, per some old style guide, but not sure if that's still followed or not.

[arantius]

Google's current Python style guide absolutely calls for two spaces. All of the languages I'm familiar with (at Google) do this. Except golang ...