result = []
text = "aaabbbbcca"
prev = ''
for c in text:
  if c == prev:
   	result.append(result[-1] + c)
  else:
    result.append(c)
    prev = c
 
print(result)  		
# => ['a', 'aa', 'aaa', 'b', 'bb', 'bbb', 'bbbb', 'c', 'cc', 'a']