Python counter can be used to calculate the frequency in a string because the string is passed as input. it returns the output as a dictionary having keys that are the unique elements of the list and values are the corresponding frequencies of the elements.
Python Counter string
Simple example code.
from collections import Counter
s = 'ABCAB'
d = Counter(s)
# print Counter value
print(d)
# access the values corresponds to each keys of the returned dictionary
print(d.values())
# get the keys and values both of dictionary
print(d.items())
# get only the keys
print(d.keys())
# sort the values of dictionary
print(sorted(d))
Output:

Update Counter collection in python with string, not letter
You can update it with a dictionary since adding another string is the same as updating the key with the count.
from collections import Counter
c = Counter(['black', 'blue'])
c.update({"red": 1})
print(c)
Output: Counter({‘black’: 1, ‘blue’: 1, ‘red’: 1})
If the key already exists, the count will increase by one:
c.update({"red": 1})
# Counter({'black': 1, 'blue': 1, 'red': 2})
Do comment if you have any doubts or suggestions on this Python counter topic.
Note: IDE: PyCharm 2021.3.3 (Community Edition)
Windows 10
Python 3.10.1
All Python Examples are in Python 3, so Maybe its different from python 2 or upgraded versions.