检查字典密钥是否存在并处理其值的最有效方法 [英] Most efficient method to check if dictionary key exists and process its value if it does

问题描述

MyDict = {'key1':'value1', 'key2':'value2'}

我可以通过以下几种方法：

I can do this few ways:

if 'key1' in MyDict:
       var1 = MyDict['key1']

或

if MyDict.has_key('key1'):
       var1 = MyDict['key1']

或

or

if MyDict['key1']:
    var1=MyDict['key1']

或

or

try: 
   var1=MyDict['key1]
except KeyError, e:
   pass

或者我尝试过这样的东西，但它在python中不会这样工作如果v = MyDict.get（'key1'）：
var1 = v

or I tried something like this but it does NOT WORK like this in python

if v=MyDict.get('key1'):
       var1=v

我们很冷，可能会找出更多的工作方法来做到这一点。
哪一个在计算速度方面最有效？

And we cold probably figure out lot more working ways to do that. Which one is most efficient in terms of computing speed?

推荐答案

一个小的基准（ipython）：

A little benchmark for you (ipython):

In [1]: def test_1(d, k):
   ...:     if k in d:
   ...:         var1 = d[k]
   ...:         

In [2]: def test_2(d, k):
   ...:     if d.has_key(k):
   ...:         var1 = d[k]
   ...:         

In [3]: def test_3(d, k):
   ...:     try:
   ...:         var1 = d[k]
   ...:     except KeyError as e:
   ...:         pass
   ...:     

In [4]: def test_4(d, k):
   ...:     if d.get(k):
   ...:         var1 = d[k]
   ...:         

In [5]: my_dict = {'key{}'.format(i): 'value{}'.format(i) for i in range(1000)}

In [6]: key_valid = "key5"

In [7]: key_non_valid = "key"

In [8]: %timeit test_1(my_dict, key_valid)
10000000 loops, best of 3: 172 ns per loop

In [9]: %timeit test_1(my_dict, key_non_valid)
10000000 loops, best of 3: 132 ns per loop

In [10]: %timeit test_2(my_dict, key_valid)
1000000 loops, best of 3: 211 ns per loop

In [11]: %timeit test_2(my_dict, key_non_valid)
10000000 loops, best of 3: 171 ns per loop

In [12]: %timeit test_3(my_dict, key_valid)
10000000 loops, best of 3: 151 ns per loop

In [13]: %timeit test_3(my_dict, key_non_valid)
1000000 loops, best of 3: 1.07 µs per loop

In [14]: %timeit test_4(my_dict, key_valid)
1000000 loops, best of 3: 246 ns per loop

In [15]: %timeit test_4(my_dict, key_non_valid)
10000000 loops, best of 3: 189 ns per loop

结论：dict中的构造键在中通常最快，只有尝试除之外的有效键，因为它不执行如果操作。

Conclusion: construction key in dict is generally fastest, outperformed only by try except in case of valid key, because it doesn't perform if operation.

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