Funny. But the output is not random. You can generate the sequence of letters by the following simple loop:

s = ''
for i in range(n):
    s = f'a{s}b{s}'

The length of this sequence is 2*(2**n-1). Finally, your code will raise a non-silenced RecursiveError, but it will just take some time (for printing around 2**1000 letters).