[systoll]

The proposal: https://www.python.org/dev/peps/pep-0572/

Short version.

(x =: y) is an expression that:

1. assigns the value y to the variable x

2. has the value y.

So `print((x := 1) + 1)` prints '2', and sets x=1.

A ton of languages [eg: c, js] have '=' work this way. And a ton of style guides for those languages tell you to avoid using it like that, because it's confusing. So this is a bit controversial.