Python Immutable Strings, shouldn't all addresses be different?

a = ['s','j','d','f']
string_complete  = ''
for s in a:
    string_complete += s
    print(string_complete)
    print(id(string_complete))

This code outputs

S 4528461808 Sj 4578482160 Sjd 4578484144 Sjdf 4578483056

which is expected as everytime string_complete is updated it goes to new address But when I comment out print line, results are unexpected

string_complete  = ''
for s in a:
    string_complete += s
    print(id(string_complete))
    #print(string_complete)

4528461808 4590072304 4590072304 4590072304

Is this coincidence? I have tried multiple times still getting the last three addresses same? What is going on here ?