Python 检查字典键是否存在并在存在时处理其值的最有效方法
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Most efficient method to check if dictionary key exists and process its value if it does
提问by Lord_JABA
MyDict = {'key1':'value1', 'key2':'value2'}
I can do this few ways:
我可以通过以下几种方式做到这一点：
if 'key1' in MyDict:
var1 = MyDict['key1']
or
或者
if MyDict.has_key('key1'):
var1 = MyDict['key1']
or
或者
if MyDict['key1']:
var1=MyDict['key1']
if MyDict['key1']:
var1=MyDict['key1']
or
或者
try:
var1=MyDict['key1]
except KeyError, e:
pass
or I tried something like this but it does NOT WORK like this in python
或者我尝试过这样的事情，但它在 python 中不起作用
if v=MyDict.get('key1'):
var1=v
And we cold probably figure out lot more working ways to do that. Which one is most efficient in terms of computing speed?
我们很可能会想出更多可行的方法来做到这一点。哪一种在计算速度方面最有效？
采纳答案by pavel_form
A little benchmark for you (ipython):
给你的一个小基准（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
Conclusion: construction key in dictis generally fastest, outperformed only by try exceptin case of valid key, because it doesn't perform ifoperation.
结论：构造key in dict通常是最快的，只有try except在有效密钥的情况下才表现得更好，因为它不执行if操作。
回答by StackG
This one of your methods is very fast:
您的方法之一非常快：
if 'key1' in MyDict:
var1 = MyDict['key1']
Assuming certain conditions on the items in your dictionary and the hashing, this should be on average O[1]
假设你的字典中的项目和散列有一定的条件，这平均应该是 O[1]
回答by bruno desthuilliers
Assuming you dontwant var1to be defined only if MyDict["key1"]is set, the obvious solution is var1 = MyDict.get("key1", default=some_sentinel_or_default_value).
假设你不希望var1被仅定义MyDict["key1"]设置，显而易见的解决方案是var1 = MyDict.get("key1", default=some_sentinel_or_default_value)。
wrt/ performances, it mostly depends on whether you expect "key1" to be in your dict most of the times or not. If the first, a try/except block might be faster, else it will be slower (try/except blocks are cheap to setup but costly when there's an actual exception).
wrt/表演，这主要取决于您是否希望“key1”在大多数情况下都在您的字典中。如果是第一个，try/except 块可能会更快，否则它会更慢（try/except 块设置起来很便宜，但在出现实际异常时成本很高）。
If you really worry that much about performances, I suggest you test the various options on real-life datausing the timeitmodule.
如果您真的非常担心性能，我建议您使用该模块测试真实数据的各种选项timeit。
