python 提示 keyError 的4种解决方法

python 提示 keyError 的4种解决方法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 t={ 'a':'1', 'b':'2', 'c':'3', } print(t['d']) 就会出现:<codeclass="language-plainhljs"style="f

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

在读取dictkeyvalue时,如果key不存在,就会触发KeyError错误,如:

Python


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  1. t = {

  2. 'a': '1',

  3. 'b': '2',

  4. 'c': '3',

  5. }

  6. print(t['d'])

就会出现:

此代码由Java架构师必看网-架构君整理
<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Java架构师必看网-架构君整理
<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valuedefault,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. }

  6. class Counter(dict):

  7. def __missing__(self, key):

  8. return None

  9. c = Counter(t)

  10. 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. }

  6. class Counter(dict):

  7. def __missing__(self, key):

  8. return key

  9. c = Counter(t)

  10. print(c['d'])

  11. 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_factoryNone,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. }

  7. def handle():

  8. return None

  9. t = collections.defaultdict(handle, t)

  10. 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dictkey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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