str.join(iterable)
Return a string which is concatenation the of the strings in the iterable iterable. The separator between elements is the string providing this method.
返回一个以str为间隔，元素是iterable里元素。
>>> a=['a','b','c']
>>> a
['a', 'b', 'c']
>>> ''.join(a)
'abc'
>>> ' '.join(a)
'a b c'
>>> 's'.join(a)
'asbsc'