You should use + or += for appending the string. This is because it's readable and fast. They are also just as fast, which one you choose is a matter of taste, the latter one is the most common. Here are timings with the timeit module:
a = a + b:
0.11338996887207031
a += b:
0.11040496826171875
To know more about this you can have a look at the following video tutorial:-