python使用字符串（第三章）
1）字符串---字符串是不可变的。
>>> website='http://www.python.org'
>>> website[-3:]='com'
Traceback (most recent call last):
File "<stdin>", line 1, in <module>
TypeError: 'str' object does not support item assignment
>>> website[-3:]='com'
Traceback (most recent call last):
File "<stdin>", line 1, in <module>
TypeError: 'str' object does not support item assignment
