class List:
def __init__(self):
self.m = []
def set(self, /, *i, a='', b=''):
self.m.append((i, a, b))
def to_string(self, x):
f = lambda: a + x[i] + b
for i, a, b in self.m:
for i in i: x[i] = f()
return ''.join(x)
x = List()
match n := len(a):
case 1: ...
case 2: x.set(0, b=' and ')
case _:
x.set(*range(n - 1), b=', ')
x.set(-1, a='and ')
s = x.to_string(a)