Editing Dictionaries

[Astrikor]

I have 3 dictionaries in which I am trying to update the values of the keys.
The keys of each dictionary are identical, so I thought I could simply cycle round the dictionaries using a 'For' condition.

A = {'Bi' : 0,'Of' : 0,'Spd' : 0}
E = {'Bi' : 0,'Of' : 0,'Spd' : 0}
U = {'Bi' : 0,'Of' : 0,'Spd' : 0}

print(A)
Ab = 24
Ao = 41
Eb = 67
Eo = 80
Ub = 15
Uo = 30


for i in range(3): 
        if i == 0:
                Pair = 'A'
        elif i == 1:
                Pair = 'E'
        elif i == 2:
                Pair = 'U'
        
        Pair['Bi'] = Pair + 'b'
        Pair['Of'] = Pair + 'o'
        Pair['Spd'] = Pair + 'o' - Pair + 'b'
        print(Pair)

However, when run I get only the first printout followed by an error message as follows:

"{'Bi': 0, 'Of': 0, 'Spd': 0}
Traceback (most recent call last):
File "C:\Users\Astrikor\Desktop\CyclePairs.py", line 23, in <module>
Pair['Bi'] = Pair + 'b'
TypeError: 'str' object does not support item assignment"

As I am still a (very) newbie, I am sure I have a misunderstanding somewhere here (maybe I should be using a Function ?)

Any help would be welcome

Many thanks
Astrikor