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python 提示 keyError 的4种解决方法

老牧童 • • python, 未分类

python 提示 keyError 的4种解决方法本文介绍了五种处理 Python 字典中键不存在时避免 KeyError 的方法 包括 先检查键是否存在再操作 使用 get 方法 setdefault 方法 自定义 missing 方法和

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python 提示 keyError 的4种解决方法

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. 

  2. 'a': '1', 
    3. 

  3. 'b': '2', 
    4. 

  4. 'c': '3', 
    5. 

  5. } 
    6. 

  6. print(t['d']) 
    7. 

  7. 



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. 

  2. 'a': '1', 
    3. 

  3. 'b': '2', 
    4. 

  4. 'c': '3', 
    5. 

  5. } 
    6. 

  6. if 'd' in t: 
    7. 

  7. print(t['d']) 
    8. 

  8. else: 
    9. 

  9. print('not exist') 
    10. 

  10. 



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. 

  2. 'a': '1', 
    3. 

  3. 'b': '2', 
    4. 

  4. 'c': '3', 
    5. 

  5. } 
    6. 

  6. print(t.get('d')) 
    7. 

  7. 



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. 

  2. 'a': '1', 
    3. 

  3. 'b': '2', 
    4. 

  4. 'c': '3', 
    5. 

  5. } 
    6. 

  6. print(t.get('d', 'not exist')) 
    7. 

  7. print(t) 
    8. 

  8. 



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. 

  2. 'a': '1', 
    3. 

  3. 'b': '2', 
    4. 

  4. 'c': '3', 
    5. 

  5. } 
    6. 

  6. print(t.setdefault('d')) 
    7. 

  7. print(t) 
    8. 

  8. 



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. 

  2. 'a': '1', 
    3. 

  3. 'b': '2', 
    4. 

  4. 'c': '3', 
    5. 

  5. } 
    6. 

  6. print(t.setdefault('d', 'not exist')) 
    7. 

  7. print(t) 
    8. 

  8. 



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. 

  2. 'a': '1', 
    3. 

  3. 'b': '2', 
    4. 

  4. 'c': '3', 
    5. 

  5. } 
    6. 

  6. 

  7. 

  8. class Counter(dict): 
    9. 

  9. 

  10. def __missing__(self, key): 
    11. 

  11. return None 
    12. 

  12. c = Counter(t) 
    13. 

  13. print(c['d']) 
    14. 

  14. 



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. 

  2. 'a': '1', 
    3. 

  3. 'b': '2', 
    4. 

  4. 'c': '3', 
    5. 

  5. } 
    6. 

  6. 

  7. 

  8. class Counter(dict): 
    9. 

  9. 

  10. def __missing__(self, key): 
    11. 

  11. return key 
    12. 

  12. c = Counter(t) 
    13. 

  13. print(c['d']) 
    14. 

  14. print(c) 
    15. 

  15. 



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. 

  2. t = {
    
         
    3. 

  3. 'a': '1', 
    4. 

  4. 'b': '2', 
    5. 

  5. 'c': '3', 
    6. 

  6. } 
    7. 

  7. t = collections.defaultdict(None, t) 
    8. 

  8. print(t['d']) 
    9. 

  9. 



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. 

  2. t = {
    
         
    3. 

  3. 'a': '1', 
    4. 

  4. 'b': '2', 
    5. 

  5. 'c': '3', 
    6. 

  6. } 
    7. 

  7. 

  8. def handle(): 
    9. 

  9. return None 
    10. 

  10. t = collections.defaultdict(handle, t) 
    11. 

  11. print(t['d']) 
    12. 

  12. 



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. 

  2. t = {
    
         
    3. 

  3. 'a': '1', 
    4. 

  4. 'b': '2', 
    5. 

  5. 'c': '3', 
    6. 

  6. } 
    7. 

  7. t = collections.defaultdict(int, t) 
    8. 

  8. print(t['d']) 
    9. 

  9. 



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. 

  2. t = {
    
         
    3. 

  3. 'a': '1', 
    4. 

  4. 'b': '2', 
    5. 

  5. 'c': '3', 
    6. 

  6. } 
    7. 

  7. t = collections.defaultdict(list, t) 
    8. 

  8. print(t['d']) 
    9. 

  9. 



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>




Python




    

  1. import collections 
    2. 

  2. t = {
    
          
    3. 

  3. 'a': '1', 
    4. 

  4. 'b': '2', 
    5. 

  5. 'c': '3', 
    6. 

  6. } 
    7. 

  7. t = collections.defaultdict(dict, t) 
    8. 

  8. print(t['d']['y']) 
    9. 

  9. 



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>


                                                        
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