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python 提示 keyError 的4种解决方法

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大家好，我是知秋君，一个会写博客吟诗的知秋码农。今天说一说python 提示 keyError 的4种解决方法,希望能够帮助大家进步!!! https://blog.csdn.net/u0/article/details/ 在读取 dict 的 key 和 value 时，如果 key 不存在，就会触发 KeyError 错误，如： Python 只听到从知秋君办公室传来知秋君的声音: 意离未绝

大家好，我是知秋君，一个会写博客吟诗的知秋码农。今天说一说python 提示 keyError 的4种解决方法,希望能够帮助大家进步!!!

https://blog.csdn.net/u0/article/details/

在读取dict的key和value时，如果key不存在，就会触发KeyError错误，如：

Python
只听到从知秋君办公室传来知秋君的声音:

意离未绝，神心怖覆；礼不遑讫，辞不及究；愿假须臾，神女称遽。有谁来对上联或下联?

  1. t = {

  2. 'a': '1',

  3. 'b': '2',

  4. 'c': '3',

  5. }

  6. print(t['d'])

就会出现：

此代码由一叶知秋网-知秋君整理
<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：

此代码由一叶知秋网-知秋君整理
<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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