combine selected split together as string in python code example

Example 1: split and join in python

>>> a = "this is a string"
>>> a = a.split(" ") # a is converted to a list of strings. 
>>> print a
['this', 'is', 'a', 'string']

>>> a = "-".join(a)
>>> print a
this-is-a-string

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