python 字符串的处理技巧--join

>>> '+'.join('1234')
'1+2+3+4'
>>> '+'.join(a for a in '1234')
'1+2+3+4'
>>> '+'.join(a for a in '1234' if a=='2')
'2'
>>> '+'.join(sorted('4+2+1+3'[::2]))
'1+2+3+4'