Having a look at the source, you get this for free in python. If you don't use any threads the overhead is just the checking of one (non-atomic) variable in the main eval loop, so that is at least as cheap as GOMAXPROCS=1 in Go. There is a small overhead of dropping/taking the gil around I/O functions, but it isn't worth optimizing the few nanoseconds that takes for the much slower i/o functions.
But if even this overhead is too much you can compile python without treading support.
But if even this overhead is too much you can compile python without treading support.