### complete expansion of polynomial substitution

In following code,I have substituted a polynomial in other polynomial and the result even after using the command
".expand" shows no complete expansion,Is there any other way to get complete expansion of polynomial substitution?

code:
~~g1=s^2
g2=g1.subs(s=(z-1))
~~g1=s^2 ;
g2=g1.subs(s=(z-1)) ;
g2.expand()

Result:
(z-1)^2 is ~~shown,but ~~shown, but not expansion (z^2-z+1).