How to substitute a random sample of variables in Boolean Function

edit

I want to make a function which generates randomly a Boolean function with n variables and then substitutes k of them with zero. I tried to get the list of terms of the function and then do the substitution which is certainly wrong.

 def Pol_sub(n,k):

    if k>n:
        print("substitution can't be done")

    else:
        B=BooleanPolynomialRing(n,'x')
        list=sorted((sample(xrange(0,n),k)))  
        f=B.random_element()
        ls=f.terms()
        print(f)
        zero_vec=[0 for x in xrange(0,k)]


    return(ls.subs({ls[list[i]]:zero_vec[i] for i in xrange(0,k)}))