I have a list:
replace_by=['a1','b1','c1','d1','e1']
main_list=[('a',[]),('b',[]),('c',[]),('d',[]),('e',[])]
Tuples cannot be modified in place. However you can easily create a new version,
replace_by=['a1','b1','c1','d1','e1']
main_list=[('a',[]),('b',[]),('c',[]),('d',[]),('e',[])]
new_list = [(replace_by[i],b)for (i,(a,b)) in enumerate(main_list)]
print(new_list)