join用來連接字符串，split恰好相反，拆分字符串的。
不用多解釋，看完代碼，其意自現了。

其join和split的英文版解釋如下：

join(...)
S.join(sequence) -> string

Return a string which is the concatenation of the strings in the
sequence.  The separator between elements is S.

split(...)
S.split([sep [,maxsplit]]) -> list of strings

Return a list of the words in the string S, using sep as the
delimiter string.  If maxsplit is given, at most maxsplit
splits are done. If sep is not specified or is None, any
whitespace string is a separator and empty strings are removed
from the result.