blocks|key|430848|text|如下所示：|type|unstyled|depth|inlineStyleRanges|entityRanges|data|430849|host+=+connectionDetails.get('host',+someDefaultValue)|code-block|syntax|javascript|430850|entityMap^0|0|0^^$0|@$1|2|3|4|5|6|7|I|8|@]|9|@]|A|$]]|$1|B|3|C|5|D|7|J|8|@]|9|@]|A|$E|F]]|$1|G|3|-4|5|6|7|K|8|@]|9|@]|A|$]]]|H|$]]

Like this:

<pre><code>host = connectionDetails.get('host', someDefaultValue)
</code></pre>

blocks|key|431428|text|您也可以像这样使用defaultdict：|type|unstyled|depth|inlineStyleRanges|offset|length|style|CODE|entityRanges|data|431429|from+collections+import+defaultdict
a+=+defaultdict(lambda:+"default",+key="some_value")
a["blabla"]+=>+"default"
a["key"]+=>+"some_value"|code-block|syntax|javascript|431430|您可以传递任何普通函数，而不是lambda：|431431|from+collections+import+defaultdict
def+a():
++return+4

b+=+defaultdict(a,+key="some_value")
b['absent']+=>+4
b['key']+=>+"some_value"|431432|entityMap|0|LINK|mutability|MUTABLE|url|https://docs.python.org/2/library/collections.html#collections.defaultdict^0|9|B|9|B|0|0|0|0|0^^$0|@$1|2|3|4|5|6|7|W|8|@$9|X|A|Y|B|C]]|D|@$9|Z|A|10|1|11]]|E|$]]|$1|F|3|G|5|H|7|12|8|@]|D|@]|E|$I|J]]|$1|K|3|L|5|6|7|13|8|@]|D|@]|E|$]]|$1|M|3|N|5|H|7|14|8|@]|D|@]|E|$I|J]]|$1|O|3|-4|5|6|7|15|8|@]|D|@]|E|$]]]|P|$Q|$5|R|S|T|E|$U|V]]]]

You can also use the <a href="https://docs.python.org/2/library/collections.html#collections.defaultdict" rel="noreferrer"><code>defaultdict</code></a> like so:

<pre><code>from collections import defaultdict
a = defaultdict(lambda: "default", key="some_value")
a["blabla"] =&gt; "default"
a["key"] =&gt; "some_value"
</code></pre>

You can pass any ordinary function instead of lambda:

<pre><code>from collections import defaultdict
def a():
 return 4

b = defaultdict(a, key="some_value")
b['absent'] =&gt; 4
b['key'] =&gt; "some_value"
</code></pre>

blocks|key|645629|text|虽然.get()是一个很好的习惯用法，但它比if/else慢(如果在大多数情况下可以预料到字典中存在键，那么它比try/except还慢)：|type|unstyled|depth|inlineStyleRanges|offset|length|style|CODE|entityRanges|data|645630|>>>+timeit.timeit(setup="d={1:2,+3:4,+5:6,+7:8,+9:0}",+
...+stmt="try:\n+a=d[1]\nexcept+KeyError:\n+a=10")
0.07691968797894333
>>>+timeit.timeit(setup="d={1:2,+3:4,+5:6,+7:8,+9:0}",+
...+stmt="try:\n+a=d[2]\nexcept+KeyError:\n+a=10")
0.4583777282275605
>>>+timeit.timeit(setup="d={1:2,+3:4,+5:6,+7:8,+9:0}",+
...+stmt="a=d.get(1,+10)")
0.17784020746671558
>>>+timeit.timeit(setup="d={1:2,+3:4,+5:6,+7:8,+9:0}",+
...+stmt="a=d.get(2,+10)")
0.17952161730158878
>>>+timeit.timeit(setup="d={1:2,+3:4,+5:6,+7:8,+9:0}",+
...+stmt="if+1+in+d:\n+a=d[1]\nelse:\n+a=10")
0.10071221458065338
>>>+timeit.timeit(setup="d={1:2,+3:4,+5:6,+7:8,+9:0}",+
...+stmt="if+2+in+d:\n+a=d[2]\nelse:\n+a=10")
0.06966537335119938|code-block|syntax|javascript|645631|entityMap^0|2|6|M|7|1K|A|0|0^^$0|@$1|2|3|4|5|6|7|M|8|@$9|N|A|O|B|C]|$9|P|A|Q|B|C]|$9|R|A|S|B|C]]|D|@]|E|$]]|$1|F|3|G|5|H|7|T|8|@]|D|@]|E|$I|J]]|$1|K|3|-4|5|6|7|U|8|@]|D|@]|E|$]]]|L|$]]

While <code>.get()</code> is a nice idiom, it's slower than <code>if/else</code> (and slower than <code>try/except</code> if presence of the key in the dictionary can be expected most of the time):

<pre><code>&gt;&gt;&gt; timeit.timeit(setup="d={1:2, 3:4, 5:6, 7:8, 9:0}", 
... stmt="try:\n a=d[1]\nexcept KeyError:\n a=10")
0.07691968797894333
&gt;&gt;&gt; timeit.timeit(setup="d={1:2, 3:4, 5:6, 7:8, 9:0}", 
... stmt="try:\n a=d[2]\nexcept KeyError:\n a=10")
0.4583777282275605
&gt;&gt;&gt; timeit.timeit(setup="d={1:2, 3:4, 5:6, 7:8, 9:0}", 
... stmt="a=d.get(1, 10)")
0.17784020746671558
&gt;&gt;&gt; timeit.timeit(setup="d={1:2, 3:4, 5:6, 7:8, 9:0}", 
... stmt="a=d.get(2, 10)")
0.17952161730158878
&gt;&gt;&gt; timeit.timeit(setup="d={1:2, 3:4, 5:6, 7:8, 9:0}", 
... stmt="if 1 in d:\n a=d[1]\nelse:\n a=10")
0.10071221458065338
&gt;&gt;&gt; timeit.timeit(setup="d={1:2, 3:4, 5:6, 7:8, 9:0}", 
... stmt="if 2 in d:\n a=d[2]\nelse:\n a=10")
0.06966537335119938
</code></pre>

blocks|key|645538|text|对于多个不同的默认值，请尝试执行以下操作：|type|unstyled|depth|inlineStyleRanges|entityRanges|data|645539|connectionDetails+=+{+"host":+"www.example.com"+}
defaults+=+{+"host":+"127.0.0.1",+"port":+8080+}

completeDetails+=+{}
completeDetails.update(defaults)
completeDetails.update(connectionDetails)
completeDetails["host"]++#+==>+"www.example.com"
completeDetails["port"]++#+==>+8080|code-block|syntax|javascript|645540|entityMap^0|0|0^^$0|@$1|2|3|4|5|6|7|I|8|@]|9|@]|A|$]]|$1|B|3|C|5|D|7|J|8|@]|9|@]|A|$E|F]]|$1|G|3|-4|5|6|7|K|8|@]|9|@]|A|$]]]|H|$]]

For multiple different defaults try this:

<pre><code>connectionDetails = { "host": "www.example.com" }
defaults = { "host": "127.0.0.1", "port": 8080 }

completeDetails = {}
completeDetails.update(defaults)
completeDetails.update(connectionDetails)
completeDetails["host"] # ==&gt; "www.example.com"
completeDetails["port"] # ==&gt; 8080
</code></pre>

blocks|key|431475|text|这并不是所要求的问题，但是在python字典中有一个方法：dict.setdefault|type|unstyled|depth|inlineStyleRanges|offset|length|style|CODE|entityRanges|data|431476|++++host+=+connectionDetails.setdefault('host',someDefaultValue)|code-block|syntax|javascript|431477|但是，如果没有定义key+host，则此方法会将connectionDetails['host']的值设为someDefaultValue，这与问题所问的不同。|431478|entityMap^0|T|F|0|0|D|4|O|P|1H|G|0^^$0|@$1|2|3|4|5|6|7|O|8|@$9|P|A|Q|B|C]]|D|@]|E|$]]|$1|F|3|G|5|H|7|R|8|@]|D|@]|E|$I|J]]|$1|K|3|L|5|6|7|S|8|@$9|T|A|U|B|C]|$9|V|A|W|B|C]|$9|X|A|Y|B|C]]|D|@]|E|$]]|$1|M|3|-4|5|6|7|Z|8|@]|D|@]|E|$]]]|N|$]]

This is not exactly the question asked for but there is a method in python dictionaries: <code>dict.setdefault</code>
<pre class="lang-py prettyprint-override"><code> host = connectionDetails.setdefault('host',someDefaultValue)
</code></pre>
However this method sets the value of <code>connectionDetails['host']</code> to <code>someDefaultValue</code> if key <code>host</code> is not already defined, unlike what the question asked.

blocks|key|431543|text|(这是一个迟来的答案)|type|unstyled|depth|inlineStyleRanges|entityRanges|data|431544|另一种方法是创建dict类的子类并实现__missing__()方法，如下所示：|offset|length|style|CODE|431545|class+ConnectionDetails(dict):
++++def+__missing__(self,+key):
++++++++if+key+==+'host':
++++++++++++return+"localhost"
++++++++raise+KeyError(key)|code-block|syntax|javascript|431546|示例：|431547|>>>+connection_details+=+ConnectionDetails(port=80)

>>>+connection_details['host']
'localhost'

>>>+connection_details['port']
80

>>>+connection_details['password']
Traceback+(most+recent+call+last):
++File+"python",+line+1,+in+<module>
++File+"python",+line+6,+in+__missing__
KeyError:+'password'|431548|entityMap|0|LINK|mutability|MUTABLE|url|https://docs.python.org/3/reference/datamodel.html?highlight=__missing__#object.__missing__^0|0|8|4|J|D|J|D|0|0|0|0|0^^$0|@$1|2|3|4|5|6|7|Y|8|@]|9|@]|A|$]]|$1|B|3|C|5|6|7|Z|8|@$D|10|E|11|F|G]|$D|12|E|13|F|G]]|9|@$D|14|E|15|1|16]]|A|$]]|$1|H|3|I|5|J|7|17|8|@]|9|@]|A|$K|L]]|$1|M|3|N|5|6|7|18|8|@]|9|@]|A|$]]|$1|O|3|P|5|J|7|19|8|@]|9|@]|A|$K|L]]|$1|Q|3|-4|5|6|7|1A|8|@]|9|@]|A|$]]]|R|$S|$5|T|U|V|A|$W|X]]]]

(this is a late answer)

An alternative is to subclass the <code>dict</code> class and implement the <a href="https://docs.python.org/3/reference/datamodel.html?highlight=__missing__#object.__missing__" rel="noreferrer"><code>__missing__()</code></a> method, like this:

<pre><code>class ConnectionDetails(dict):
 def __missing__(self, key):
 if key == 'host':
 return "localhost"
 raise KeyError(key)
</code></pre>

Examples:

<pre><code>&gt;&gt;&gt; connection_details = ConnectionDetails(port=80)

&gt;&gt;&gt; connection_details['host']
'localhost'

&gt;&gt;&gt; connection_details['port']
80

&gt;&gt;&gt; connection_details['password']
Traceback (most recent call last):
 File "python", line 1, in &lt;module&gt;
 File "python", line 6, in __missing__
KeyError: 'password'
</code></pre>

blocks|key|645853|text|测试@Tim+Pietzcker对Python3.3.5的PyPy+(5.2.0-alpha0)中的情况的怀疑，我发现.get()和if/else方式确实执行类似。实际上，在if/else的情况下，如果条件和赋值涉及相同的键，则甚至只有一次查找(与上一种有两次查找的情况相比)。|type|unstyled|depth|inlineStyleRanges|offset|length|style|CODE|entityRanges|data|645854|>>>>+timeit.timeit(setup="d={1:2,+3:4,+5:6,+7:8,+9:0}",
....+stmt="try:\n+a=d[1]\nexcept+KeyError:\n+a=10")
0.011889292989508249
>>>>+timeit.timeit(setup="d={1:2,+3:4,+5:6,+7:8,+9:0}",
....+stmt="try:\n+a=d[2]\nexcept+KeyError:\n+a=10")
0.07310474599944428
>>>>+timeit.timeit(setup="d={1:2,+3:4,+5:6,+7:8,+9:0}",
....+stmt="a=d.get(1,+10)")
0.010391917996457778
>>>>+timeit.timeit(setup="d={1:2,+3:4,+5:6,+7:8,+9:0}",
....+stmt="a=d.get(2,+10)")
0.009348208011942916
>>>>+timeit.timeit(setup="d={1:2,+3:4,+5:6,+7:8,+9:0}",
....+stmt="if+1+in+d:\n+a=d[1]\nelse:\n+a=10")
0.011475925013655797
>>>>+timeit.timeit(setup="d={1:2,+3:4,+5:6,+7:8,+9:0}",
....+stmt="if+2+in+d:\n+a=d[2]\nelse:\n+a=10")
0.009605801998986863
>>>>+timeit.timeit(setup="d={1:2,+3:4,+5:6,+7:8,+9:0}",
....+stmt="if+2+in+d:\n+a=d[2]\nelse:\n+a=d[1]")
0.017342638995614834|code-block|syntax|javascript|645855|entityMap^0|1N|6|1U|2|1X|4|0|0^^$0|@$1|2|3|4|5|6|7|M|8|@$9|N|A|O|B|C]|$9|P|A|Q|B|C]|$9|R|A|S|B|C]]|D|@]|E|$]]|$1|F|3|G|5|H|7|T|8|@]|D|@]|E|$I|J]]|$1|K|3|-4|5|6|7|U|8|@]|D|@]|E|$]]]|L|$]]

Testing @Tim Pietzcker's suspicion about the situation in PyPy (5.2.0-alpha0) for Python 3.3.5, I find that indeed both <code>.get()</code> and the <code>if</code>/<code>else</code> way perform similar. Actually it seems that in the if/else case there is even only a single lookup if the condition and the assignment involve the same key (compare with the last case where there is two lookups).

<pre><code>&gt;&gt;&gt;&gt; timeit.timeit(setup="d={1:2, 3:4, 5:6, 7:8, 9:0}",
.... stmt="try:\n a=d[1]\nexcept KeyError:\n a=10")
0.011889292989508249
&gt;&gt;&gt;&gt; timeit.timeit(setup="d={1:2, 3:4, 5:6, 7:8, 9:0}",
.... stmt="try:\n a=d[2]\nexcept KeyError:\n a=10")
0.07310474599944428
&gt;&gt;&gt;&gt; timeit.timeit(setup="d={1:2, 3:4, 5:6, 7:8, 9:0}",
.... stmt="a=d.get(1, 10)")
0.010391917996457778
&gt;&gt;&gt;&gt; timeit.timeit(setup="d={1:2, 3:4, 5:6, 7:8, 9:0}",
.... stmt="a=d.get(2, 10)")
0.009348208011942916
&gt;&gt;&gt;&gt; timeit.timeit(setup="d={1:2, 3:4, 5:6, 7:8, 9:0}",
.... stmt="if 1 in d:\n a=d[1]\nelse:\n a=10")
0.011475925013655797
&gt;&gt;&gt;&gt; timeit.timeit(setup="d={1:2, 3:4, 5:6, 7:8, 9:0}",
.... stmt="if 2 in d:\n a=d[2]\nelse:\n a=10")
0.009605801998986863
&gt;&gt;&gt;&gt; timeit.timeit(setup="d={1:2, 3:4, 5:6, 7:8, 9:0}",
.... stmt="if 2 in d:\n a=d[2]\nelse:\n a=d[1]")
0.017342638995614834
</code></pre>

blocks|key|645936|text|为此，您可以使用lamba函数作为一行程序。创建一个新的对象connectionDetails2，它可以像函数一样访问...|type|unstyled|depth|inlineStyleRanges|offset|length|style|CODE|entityRanges|data|645937|connectionDetails2+=+lambda+k:+connectionDetails[k]+if+k+in+connectionDetails.keys()+else+"DEFAULT"|code-block|syntax|javascript|645938|现在使用|645939|connectionDetails2(k)|645940|而不是|645941|connectionDetails[k]|645942|如果键中有k，则返回字典值，否则返回"DEFAULT"|645943|entityMap^0|U|I|0|0|0|0|0|0|5|1|I|9|0^^$0|@$1|2|3|4|5|6|7|W|8|@$9|X|A|Y|B|C]]|D|@]|E|$]]|$1|F|3|G|5|H|7|Z|8|@]|D|@]|E|$I|J]]|$1|K|3|L|5|6|7|10|8|@]|D|@]|E|$]]|$1|M|3|N|5|H|7|11|8|@]|D|@]|E|$I|J]]|$1|O|3|P|5|6|7|12|8|@]|D|@]|E|$]]|$1|Q|3|R|5|H|7|13|8|@]|D|@]|E|$I|J]]|$1|S|3|T|5|6|7|14|8|@$9|15|A|16|B|C]|$9|17|A|18|B|C]]|D|@]|E|$]]|$1|U|3|-4|5|6|7|19|8|@]|D|@]|E|$]]]|V|$]]

You can use a lamba function for this as a one-liner. Make a new object <code>connectionDetails2</code> which is accessed like a function...

<pre><code>connectionDetails2 = lambda k: connectionDetails[k] if k in connectionDetails.keys() else "DEFAULT"
</code></pre>

Now use 

<pre><code>connectionDetails2(k)
</code></pre>

instead of 

<pre><code>connectionDetails[k]
</code></pre>

which returns the dictionary value if <code>k</code> is in the keys, otherwise it returns <code>"DEFAULT"</code>

Assuming <code>connectionDetails</code> is a Python dictionary, what's the best, most elegant, most "pythonic" way of refactoring code like this?

<pre><code>if "host" in connectionDetails:
 host = connectionDetails["host"]
else:
 host = someDefaultValue
</code></pre>

Dictionaries and default values

Python

假设connectionDetails是一个Python字典，那么像这样重构代码的最好、最优雅、最“Python”的方式是什么？if "host" in connectionDetails:    host = connectionDetails["host"]else:    host = someDefaultValue

问字典和默认值
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回答 8

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