在Python中字符串是不可变变量，对其进行切片及其中的元素复制都会报错
>>> str = "This is Python"
>>> str[0:3] = "abc"
Traceback (most recent call last):
File "<stdin>", line 1, in <module>
TypeError: 'str' object does not support item assignment
其常用的方法：
split
用法：str.split(sep=None, maxsplit=-1)
将字符串划分为序列
>>> env = "/usr/bin/python"
>>> tmp_env = env.split("/")
>>> tmp_env
['', 'usr', 'bin', 'python']
replace
用法：str.replace(old,new[,max])
将指定子串替换为另一个子串，并返回替换后的结果,但不会改变原String的内容
>>> str = "This are Python"
>>> rep_str = str.replace("are","is")
>>> rep_str
'This is Python'
>>> str
'This are Python'
>>> str = "This is Python,That is Great"
>>> rep_str = str.replace("is","was",2)
>>> rep_str
'Thwas was Python,That is Great'
find
用法：str.find(sub[, start[, end]])
在字符串中查找字串，若找到，则返回字串的第一个字符的索引，否则，返回-1
>>> str = "This is Python,Hello ,中国，您好"
>>> fd_str = str.find("is")
>>> fd_str
2
>>> str[fd_str]
'i'
>>> fd_str2 = str.find("中")
>>> str[fd_str2]
'中'
>>> str1 = "That is dog".find("G")
>>> str1
-1
join
用法：str.join(iterable)
合并序列的元素
>>> dirs = ['','usr','bin','python']
>>> '/'.join(dirs)
'/usr/bin/python'
>>> num = [1,2,3,4] #合并数字列表，报错
>>> seq = '+'
>>> seq.join(num)
Traceback (most recent call last):
File "<stdin>", line 1, in <module>
TypeError: sequence item 0: expected str instance, int found
>>> str1 = ["A","B","C"] #合并字符列表，OK
>>> seq = "-"
>>> seq.join(str1)
'A-B-C'
>>> str1 = "ABCDE"
>>> seq = "abcde"
>>> seq.join(str1)
'AabcdeBabcdeCabcdeDabcdeE'
strip
用法：str.strip([chars])
将字符串开头和末尾的空白（不包括中间的空白）删除，并返回结果
>>> " Hello,World!".strip()
'Hello,World!'
>>> "Hello,World ".strip()
'Hello,World'
>>> " Hello,World ".strip()
'Hello,World'
>>> "Hello, World! !".strip()
'Hello, World! !'
>>> "www.linuxidc.com".strip("com")
'www.linuxidc.'
>>> "www.linuxidc.com".strip(".com")
'www.linuxid'
center
用法：str.center(width[, fillchar])
通过在两边填充字符（默认为空格）让string居中
>>> "0123456789".center(1)
'0123456789'
>>> "0123456789".center(-1)
'0123456789'
>>> "0123456789".center(10)
'0123456789'
>>> "0123456789".center(11)
' 0123456789'
>>> "0123456789".center(11,"*")
'*0123456789'
>>> "0123456789".center(12,"*")
'*0123456789*'
>>> "0123456789".center(20,"*")
'*****0123456789*****'
