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>
 
  
 
 
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