今天写replace方法的时候的代码如下：

1 message = "I really like dogs"
2 message.replace('dog','cat')
3 print(message)

本以为运行结果会是：I really like cats

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出乎意料结果却是原字符串

 

查了一下才得知python中string是不可变的

>>> help(str.replace)
Help on method_descriptor:
replace(...)
    S.replace(old, new[, count]) -> string
    
    Return a copy of string S with all occurrences of substring
    old replaced by new.  If the optional argument count is
    given, only the first count occurrences are replaced.
>>> s='hello python,hello world,hello c++,hello java!'
>>> s.replace('hello','Hello')#将字符串s中的所有'hello'子串，替换成'Hello',返回替换后的字符串，原字符串s不变
'Hello python,Hello world,Hello c++,Hello java!'
>>> s
'hello python,hello world,hello c++,hello java!'
>>> s.replace('hello','Hello',2)#将字符串s中的前2个'hello'子串，替换成'Hello'
'Hello python,Hello world,hello c++,hello java!'
>>> s
'hello python,hello world,hello c++,hello java!'
>>> s.replace('wahaha','haha')#要替换的'wahaha'子串不存在，直接返回原字符串
'hello python,hello world,hello c++,hello java!'

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