text = 'ABAABA'

d = {}

for b in text:
    d[b] = d.get(b, 0) + 1

print(d)
{'A': 4, 'B': 2}

a = ["A","A","A","B","A","B"]
b = set(a)
c = {i:0 for i in b}
for i in b:
    c[i] = a.count(i)
print(c)