Python - Como sumar dos listas

promVectorA = sumatoria_a/len(vec_a)

promVectorB = sumatoria_b/len(vec_b)

diferencia = promVectorA - promVectorB

s = raw_input("Indica los valores separados por espacios: ")

numbers = map(int, s.split())

print numbers

a=[1,2]

b=[3,4]

a.extend(b)

print a

a=[1,2]

b=[1,2,3]

suma=[]

if len(a)>len(b):

	for i in range(len(a)):

		if len(b)>i:

			suma.append(a[i]+b[i])

		else:

			suma.append(a[i])

else:

	for i in range(len(b)):

		if len(a)>i:

			suma.append(a[i]+b[i])

		else:

			suma.append(b[i])

print suma

def suma(lista):

	total=0

	for i in range(len(lista)):

		total+=lista[i]

	return total

media_a=suma(a)/float(len(a))

print media_a

media_b=suma(b)/float(len(b))

print media_b