如何判断Python字典中是否存在某个key

在Python中有各种数据结构，而字典是我们生产中经常会用到的数据结构，这里记录一下如果判断某个key是否存在于字典中的二种方法。

方法一:字典自带属性has_key

Python2下：

nock:work nock$ python2.7
Python 2.7.10 (default, Jul 14 2015, 19:46:27)
[GCC 4.2.1 Compatible Apple LLVM 6.0 (clang-600.0.39)] on darwin
Type "help", "copyright", "credits" or "license" for more information.
>>> user_info = {'name': 'nock', 'age': 18}
>>> user_info.has_key('job')
False
>>> user_info.has_key('age')
True
>>> user_info.has_key('name')
True

Python3下：

nock:work nock$ python3
Python 3.5.1 (default, Dec 26 2015, 18:08:53)
[GCC 4.2.1 Compatible Apple LLVM 7.0.2 (clang-700.1.81)] on darwin
Type "help", "copyright", "credits" or "license" for more information.
>>> user_info = {'name': 'nock', 'age': 18}
>>> user_info.has_key('job')
Traceback (most recent call last):
 File "<stdin>", line 1, in <module>
AttributeError: 'dict' object has no attribute 'has_key'
>>> user_info.has_key('age')
Traceback (most recent call last):
 File "<stdin>", line 1, in <module>
AttributeError: 'dict' object has no attribute 'has_key'

如上所示可知，字典的has_key方法只能在Python2中使用，在Python3中已经移除。

方法二: in关键字

一般我们刚开始学习认识Python的时候我们都会先字典列表对象的形式把字典所有键返回，再判断该key是否存在于键列表中：

nock:work nock$ python3
Python 3.5.1 (default, Dec 26 2015, 18:08:53)
[GCC 4.2.1 Compatible Apple LLVM 7.0.2 (clang-700.1.81)] on darwin
Type "help", "copyright", "credits" or "license" for more information.
>>> dict_info = {'name': 'nock', 'age': 18}
>>> keys = dict_info.keys()
>>> print(type(keys), keys)
<class 'dict_keys'> dict_keys(['name', 'age'])
>>> for k in keys:
...     if 'name' == k:
...         print("key in ok")
...         break
...
key in ok

其实这不是最好的方法，那还有更好的方法？ Python2下：

nock:work nock$ python2.7
Python 2.7.10 (default, Jul 14 2015, 19:46:27)
[GCC 4.2.1 Compatible Apple LLVM 6.0 (clang-600.0.39)] on darwin
Type "help", "copyright", "credits" or "license" for more information.
>>> user_info = {'name': 'nock', 'age': 18}
>>> if 'name' in user_info:
...     print('in')
...
in
>>> if 'job' not in user_info:
...     print('not in')
... else:
...     print('in')
...
not in

Python3下：

nock:work nock$ python3
Python 3.5.1 (default, Dec 26 2015, 18:08:53)
[GCC 4.2.1 Compatible Apple LLVM 7.0.2 (clang-700.1.81)] on darwin
Type "help", "copyright", "credits" or "license" for more information.
>>> user_info = {'name': 'nock', 'age': 18}
>>> if 'job' not in user_info:
...     print('in')
...
in
>>> if 'job' not in user_info:
...     print('not in')
... else:
...     print('in')
...
not in

如上可知in关键字在Python2和Python3下都适用。

总结

如上实例可知用in关键字是最nice的方法，同时在字典数据量较大的情况下in也是最快的方法，我这里就不实验了，有兴趣的同学可以实践一下。