append string in python 3.7 code example

Example 1: python string concatenation with separator

>>> strings = ['do', 're', 'mi']
>>> ','.join(strings)
'do,re,mi'

Example 2: python join string with common string

def merge(s1, s2):
    i = 0
    while not s2.startswith(s1[i:]):
        i += 1
    return s1[:i] + s2