Python function is changing the value of my input, and I can't figure out why

Inside your function, z and spins refer to the same list, which is also known by the global name of spin. If you modify one, those changes are visible through the other names as well. The variable z is superfluous.

If you want z to be a copy of spins then just do:

z = spins[:]

or:

z = list(spins)