https://blog.csdn.net/u011089523/article/details/72887163

在读取

dict

的

key

和

value

时，如果

key

不存在，就会触发

KeyError

错误，如：

Python

1. 

t = {




2. 

'a': '1',



3. 

'b': '2',



4. 

'c': '3',



5. 

}



6. 

print(t['d'])



就会出现：

<code class="language-plain hljs" style="font-family:Consolas,Monaco,"Andale Mono","Ubuntu Mono",monospace; font-size:13.6px; padding:0px; background:transparent; margin:0px; word-spacing:normal; word-break:normal; word-wrap:normal; line-height:inherit; border:0px; display:inline; max-width:initial; overflow:initial; position:relative">KeyError: 'd'
<span class="line-numbers-rows" style="position:absolute; top:0px; font-size:13.6px; left:-3.8em; width:3em; letter-spacing:-1px; border-right:1px solid rgb(153,153,153)"><span style="display:block; counter-increment:linenumber 1"></span></span></code>

第一种解决方法

首先测试key是否存在，然后才进行下一步操作，如：

Python

1. 

t = {




2. 

'a': '1',



3. 

'b': '2',



4. 

'c': '3',



5. 

}



6. 

if 'd' in t:



7. 

print(t['d'])



8. 

else:



9. 

print('not exist')



会出现：

<code class="language-plain hljs" style="font-family:Consolas,Monaco,"Andale Mono","Ubuntu Mono",monospace; font-size:13.6px; padding:0px; background:transparent; margin:0px; word-spacing:normal; word-break:normal; word-wrap:normal; line-height:inherit; border:0px; display:inline; max-width:initial; overflow:initial; position:relative">not exist
<span class="line-numbers-rows" style="position:absolute; top:0px; font-size:13.6px; left:-3.8em; width:3em; letter-spacing:-1px; border-right:1px solid rgb(153,153,153)"><span style="display:block; counter-increment:linenumber 1"></span></span></code>

第二种解决方法

利用

dict

内置的

get(key[,default])

方法，如果

key

存在，则返回其

value

,否则返回

default

;使用这个方法永远不会触发

KeyError

，如：

Python

1. 

t = {




2. 

'a': '1',



3. 

'b': '2',



4. 

'c': '3',



5. 

}



6. 

print(t.get('d'))



会出现：

<code class="language-plain hljs" style="font-family:Consolas,Monaco,"Andale Mono","Ubuntu Mono",monospace; font-size:13.6px; padding:0px; background:transparent; margin:0px; word-spacing:normal; word-break:normal; word-wrap:normal; line-height:inherit; border:0px; display:inline; max-width:initial; overflow:initial; position:relative">None
<span class="line-numbers-rows" style="position:absolute; top:0px; font-size:13.6px; left:-3.8em; width:3em; letter-spacing:-1px; border-right:1px solid rgb(153,153,153)"><span style="display:block; counter-increment:linenumber 1"></span></span></code>

加上

default

参数：

Python

1. 

t = {




2. 

'a': '1',



3. 

'b': '2',



4. 

'c': '3',



5. 

}



6. 

print(t.get('d', 'not exist'))



7. 

print(t)



会出现：

<code class="language-plain hljs" style="font-family:Consolas,Monaco,"Andale Mono","Ubuntu Mono",monospace; font-size:13.6px; padding:0px; background:transparent; margin:0px; word-spacing:normal; word-break:normal; word-wrap:normal; line-height:inherit; border:0px; display:inline; max-width:initial; overflow:initial; position:relative">not exist
{'a': '1', 'c': '3', 'b': '2'}
<span class="line-numbers-rows" style="position:absolute; top:0px; font-size:13.6px; left:-3.8em; width:3em; letter-spacing:-1px; border-right:1px solid rgb(153,153,153)"><span style="display:block; counter-increment:linenumber 1"></span><span style="display:block; counter-increment:linenumber 1"></span></span></code>

第三种解决方法

利用

dict

内置的

setdefault(key[,default])

方法，如果

key

存在，则返回其

value

;否则插入此

key

，其

value

为

default

，并返回

default

;使用这个方法也永远不会触发

KeyError

，如：

Python

1. 

t = {




2. 

'a': '1',



3. 

'b': '2',



4. 

'c': '3',



5. 

}



6. 

print(t.setdefault('d'))



7. 

print(t)



会出现：

<code class="language-plain hljs" style="font-family:Consolas,Monaco,"Andale Mono","Ubuntu Mono",monospace; font-size:13.6px; padding:0px; background:transparent; margin:0px; word-spacing:normal; word-break:normal; word-wrap:normal; line-height:inherit; border:0px; display:inline; max-width:initial; overflow:initial; position:relative">None
{'b': '2', 'd': None, 'a': '1', 'c': '3'}
<span class="line-numbers-rows" style="position:absolute; top:0px; font-size:13.6px; left:-3.8em; width:3em; letter-spacing:-1px; border-right:1px solid rgb(153,153,153)"><span style="display:block; counter-increment:linenumber 1"></span><span style="display:block; counter-increment:linenumber 1"></span></span></code>

加上

default

参数：

Python

1. 

t = {




2. 

'a': '1',



3. 

'b': '2',



4. 

'c': '3',



5. 

}



6. 

print(t.setdefault('d', 'not exist'))



7. 

print(t)



会出现：

<code class="language-plain hljs" style="font-family:Consolas,Monaco,"Andale Mono","Ubuntu Mono",monospace; font-size:13.6px; padding:0px; background:transparent; margin:0px; word-spacing:normal; word-break:normal; word-wrap:normal; line-height:inherit; border:0px; display:inline; max-width:initial; overflow:initial; position:relative">not exist
{'c': '3', 'd': 'not exist', 'a': '1', 'b': '2'}
<span class="line-numbers-rows" style="position:absolute; top:0px; font-size:13.6px; left:-3.8em; width:3em; letter-spacing:-1px; border-right:1px solid rgb(153,153,153)"><span style="display:block; counter-increment:linenumber 1"></span><span style="display:block; counter-increment:linenumber 1"></span></span></code>

第四种解决方法

向类

dict

增加

__missing__()

方法，当

key

不存在时，会转向

__missing__()

方法处理，而不触发

KeyError

,如：

Python

1. 

t = {




2. 

'a': '1',



3. 

'b': '2',



4. 

'c': '3',



5. 

}



8. 

class Counter(dict):



10. 

def __missing__(self, key):



11. 

return None



12. 

c = Counter(t)



13. 

print(c['d'])



会出现：

<code class="language-plain hljs" style="font-family:Consolas,Monaco,"Andale Mono","Ubuntu Mono",monospace; font-size:13.6px; padding:0px; background:transparent; margin:0px; word-spacing:normal; word-break:normal; word-wrap:normal; line-height:inherit; border:0px; display:inline; max-width:initial; overflow:initial; position:relative">None
<span class="line-numbers-rows" style="position:absolute; top:0px; font-size:13.6px; left:-3.8em; width:3em; letter-spacing:-1px; border-right:1px solid rgb(153,153,153)"><span style="display:block; counter-increment:linenumber 1"></span></span></code>

更改

return

值：

Python

1. 

t = {




2. 

'a': '1',



3. 

'b': '2',



4. 

'c': '3',



5. 

}



8. 

class Counter(dict):



10. 

def __missing__(self, key):



11. 

return key



12. 

c = Counter(t)



13. 

print(c['d'])



14. 

print(c)



会出现：

<code class="language-plain hljs" style="font-family:Consolas,Monaco,"Andale Mono","Ubuntu Mono",monospace; font-size:13.6px; padding:0px; background:transparent; margin:0px; word-spacing:normal; word-break:normal; word-wrap:normal; line-height:inherit; border:0px; display:inline; max-width:initial; overflow:initial; position:relative">d
{'c': '3', 'a': '1', 'b': '2'}
<span class="line-numbers-rows" style="position:absolute; top:0px; font-size:13.6px; left:-3.8em; width:3em; letter-spacing:-1px; border-right:1px solid rgb(153,153,153)"><span style="display:block; counter-increment:linenumber 1"></span><span style="display:block; counter-increment:linenumber 1"></span></span></code>

第五种解决方法

利用

collections.defaultdict([default_factory[,...]])

对象，实际上这个是继承自

dict

，而且实际也是用到的

__missing__()

方法，其

default_factory

参数就是向

__missing__()

方法传递的，不过使用起来更加顺手：

如果

default_factory

为

None

，则与

dict

无区别，会触发

KeyError

错误，如：

Python

1. 

import collections



2. 

t = {




3. 

'a': '1',



4. 

'b': '2',



5. 

'c': '3',



6. 

}



7. 

t = collections.defaultdict(None, t)



8. 

print(t['d'])



会出现：

<code class="language-plain hljs" style="font-family:Consolas,Monaco,"Andale Mono","Ubuntu Mono",monospace; font-size:13.6px; padding:0px; background:transparent; margin:0px; word-spacing:normal; word-break:normal; word-wrap:normal; line-height:inherit; border:0px; display:inline; max-width:initial; overflow:initial; position:relative">KeyError: 'd'
<span class="line-numbers-rows" style="position:absolute; top:0px; font-size:13.6px; left:-3.8em; width:3em; letter-spacing:-1px; border-right:1px solid rgb(153,153,153)"><span style="display:block; counter-increment:linenumber 1"></span></span></code>

但如果真的想返回

None

也不是没有办法：

Python

1. 

import collections



2. 

t = {




3. 

'a': '1',



4. 

'b': '2',



5. 

'c': '3',



6. 

}



8. 

def handle():



9. 

return None



10. 

t = collections.defaultdict(handle, t)



11. 

print(t['d'])



会出现：

<code class="language-plain hljs" style="font-family:Consolas,Monaco,"Andale Mono","Ubuntu Mono",monospace; font-size:13.6px; padding:0px; background:transparent; margin:0px; word-spacing:normal; word-break:normal; word-wrap:normal; line-height:inherit; border:0px; display:inline; max-width:initial; overflow:initial; position:relative">None
<span class="line-numbers-rows" style="position:absolute; top:0px; font-size:13.6px; left:-3.8em; width:3em; letter-spacing:-1px; border-right:1px solid rgb(153,153,153)"><span style="display:block; counter-increment:linenumber 1"></span></span></code>

如果

default_factory

参数是某种数据类型，则会返回其默认值，如：

Python

1. 

import collections



2. 

t = {




3. 

'a': '1',



4. 

'b': '2',



5. 

'c': '3',



6. 

}



7. 

t = collections.defaultdict(int, t)



8. 

print(t['d'])



会出现：

<code class="language-plain hljs" style="font-family:Consolas,Monaco,"Andale Mono","Ubuntu Mono",monospace; font-size:13.6px; padding:0px; background:transparent; margin:0px; word-spacing:normal; word-break:normal; word-wrap:normal; line-height:inherit; border:0px; display:inline; max-width:initial; overflow:initial; position:relative">0
<span class="line-numbers-rows" style="position:absolute; top:0px; font-size:13.6px; left:-3.8em; width:3em; letter-spacing:-1px; border-right:1px solid rgb(153,153,153)"><span style="display:block; counter-increment:linenumber 1"></span></span></code>

又如：

Python

1. 

import collections



2. 

t = {




3. 

'a': '1',



4. 

'b': '2',



5. 

'c': '3',



6. 

}



7. 

t = collections.defaultdict(list, t)



8. 

print(t['d'])



会出现：

<code class="language-plain hljs" style="font-family:Consolas,Monaco,"Andale Mono","Ubuntu Mono",monospace; font-size:13.6px; padding:0px; background:transparent; margin:0px; word-spacing:normal; word-break:normal; word-wrap:normal; line-height:inherit; border:0px; display:inline; max-width:initial; overflow:initial; position:relative">[]
<span class="line-numbers-rows" style="position:absolute; top:0px; font-size:13.6px; left:-3.8em; width:3em; letter-spacing:-1px; border-right:1px solid rgb(153,153,153)"><span style="display:block; counter-increment:linenumber 1"></span></span></code>

注意：

如果

dict

内又含有

dict

，

key

嵌套获取

value

时，如果中间某个

key

不存在，则上述方法均失效，一定会触发

KeyError

：

Python

1. 

import collections



2. 

t = {




3. 

'a': '1',



4. 

'b': '2',



5. 

'c': '3',



6. 

}



7. 

t = collections.defaultdict(dict, t)



8. 

print(t['d']['y'])



会出现：

<code class="language-plain hljs" style="font-family:Consolas,Monaco,"Andale Mono","Ubuntu Mono",monospace; font-size:13.6px; padding:0px; background:transparent; margin:0px; word-spacing:normal; word-break:normal; word-wrap:normal; line-height:inherit; border:0px; display:inline; max-width:initial; overflow:initial; position:relative">KeyError: 'y'</code>