blocks|key|234929|text|你想要的东西叫做defaultdict|type|unstyled|depth|inlineStyleRanges|entityRanges|data|234930|请参阅http://docs.python.org/library/collections.html#collections.defaultdict|offset|length|234931|entityMap|0|LINK|mutability|MUTABLE|url|http://docs.python.org/library/collections.html#collections.defaultdict^0|0|3|1Z|0|0^^$0|@$1|2|3|4|5|6|7|N|8|@]|9|@]|A|$]]|$1|B|3|C|5|6|7|O|8|@]|9|@$D|P|E|Q|1|R]]|A|$]]|$1|F|3|-4|5|6|7|S|8|@]|9|@]|A|$]]]|G|$H|$5|I|J|K|A|$L|M]]]]

What you want is called a defaultdict

See <a href="http://docs.python.org/library/collections.html#collections.defaultdict">http://docs.python.org/library/collections.html#collections.defaultdict</a>

blocks|key|452928|text|你有很多选择。我喜欢使用Counter|type|unstyled|depth|inlineStyleRanges|offset|length|style|CODE|entityRanges|data|452929|>>>+from+collections+import+Counter
>>>+d+=+Counter()
>>>+d[12]+%2B=+3
>>>+d
Counter({12:+3})|code-block|syntax|javascript|452930|或defaultdict|452931|>>>+from+collections+import+defaultdict
>>>+d+=+defaultdict(int)++#+int()+==+0,+so+the+default+value+for+each+key+is+0
>>>+d[12]+%2B=+3
>>>+d
defaultdict(<function+<lambda>+at+0x7ff2fe7d37d0>,+{12:+3})|452932|entityMap|0|LINK|mutability|MUTABLE|url|https://docs.python.org/3/library/collections.html#collections.Counter|1|https://docs.python.org/3/library/collections.html#collections.defaultdict^0|C|7|C|7|0|0|0|1|B|1|B|1|0|0^^$0|@$1|2|3|4|5|6|7|Y|8|@$9|Z|A|10|B|C]]|D|@$9|11|A|12|1|13]]|E|$]]|$1|F|3|G|5|H|7|14|8|@]|D|@]|E|$I|J]]|$1|K|3|L|5|6|7|15|8|@$9|16|A|17|B|C]]|D|@$9|18|A|19|1|1A]]|E|$]]|$1|M|3|N|5|H|7|1B|8|@]|D|@]|E|$I|J]]|$1|O|3|-4|5|6|7|1C|8|@]|D|@]|E|$]]]|P|$Q|$5|R|S|T|E|$U|V]]|W|$5|R|S|T|E|$U|X]]]]

You have quite a few options. I like using <a href="https://docs.python.org/3/library/collections.html#collections.Counter" rel="noreferrer"><code>Counter</code></a>:

<pre><code>&gt;&gt;&gt; from collections import Counter
&gt;&gt;&gt; d = Counter()
&gt;&gt;&gt; d[12] += 3
&gt;&gt;&gt; d
Counter({12: 3})
</code></pre>

Or <a href="https://docs.python.org/3/library/collections.html#collections.defaultdict" rel="noreferrer"><code>defaultdict</code></a>:

<pre><code>&gt;&gt;&gt; from collections import defaultdict
&gt;&gt;&gt; d = defaultdict(int) # int() == 0, so the default value for each key is 0
&gt;&gt;&gt; d[12] += 3
&gt;&gt;&gt; d
defaultdict(&lt;function &lt;lambda&gt; at 0x7ff2fe7d37d0&gt;, {12: 3})
</code></pre>

blocks|key|452948|text|转换：|type|unstyled|depth|inlineStyleRanges|entityRanges|data|452949|if+key+in+my_dict:
++my_dict[key]+%2B=+num
else:
++my_dict[key]+=+num|code-block|syntax|javascript|452950|使用setdefault转换成以下代码|offset|length|style|CODE|452951|my_dict[key]+=+my_dict.setdefault(key,+0) %2B+num|452952|entityMap|0|LINK|mutability|MUTABLE|url|http://docs.python.org/library/stdtypes.html#dict.setdefault^0|0|0|2|A|2|A|0|0|0^^$0|@$1|2|3|4|5|6|7|W|8|@]|9|@]|A|$]]|$1|B|3|C|5|D|7|X|8|@]|9|@]|A|$E|F]]|$1|G|3|H|5|6|7|Y|8|@$I|Z|J|10|K|L]]|9|@$I|11|J|12|1|13]]|A|$]]|$1|M|3|N|5|D|7|14|8|@]|9|@]|A|$E|F]]|$1|O|3|-4|5|6|7|15|8|@]|9|@]|A|$]]]|P|$Q|$5|R|S|T|A|$U|V]]]]

transform:

<pre><code>if key in my_dict:
 my_dict[key] += num
else:
 my_dict[key] = num
</code></pre>

into the following using <a href="http://docs.python.org/library/stdtypes.html#dict.setdefault" rel="noreferrer"><code>setdefault</code></a>:

<pre><code>my_dict[key] = my_dict.setdefault(key, 0) + num
</code></pre>

blocks|key|234986|text|另一种选择是：|type|unstyled|depth|inlineStyleRanges|entityRanges|data|234987|my_dict[key]+=+my_dict.get(key,+0)+%2B+num|code-block|syntax|javascript|234988|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|$]]

An alternative is:

<pre><code>my_dict[key] = my_dict.get(key, 0) + num
</code></pre>

blocks|key|452990|text|还有一点不同的setdefault方式：|type|unstyled|depth|inlineStyleRanges|offset|length|style|CODE|entityRanges|data|452991|my_dict.setdefault(key,+0)
my_dict[key]+%2B=+num|code-block|syntax|javascript|452992|如果与其他逻辑相结合，这可能具有一些优点。|452993|entityMap^0|7|A|0|0|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]]|$1|K|3|L|5|6|7|S|8|@]|D|@]|E|$]]|$1|M|3|-4|5|6|7|T|8|@]|D|@]|E|$]]]|N|$]]

There is also a little bit different <code>setdefault</code> way:

<pre><code>my_dict.setdefault(key, 0)
my_dict[key] += num
</code></pre>

Which may have some advantages if combined with other logic.

blocks|key|453022|text|可以使用.get或.setdefault中的任何一个：|type|unstyled|depth|inlineStyleRanges|offset|length|style|CODE|entityRanges|data|453023|.get()如果没有有效的密钥，则在函数中传递默认值|453024|my_dict[key]+=+my_dict.get(key,+0)+%2B+num|code-block|syntax|javascript|453025|.setdefault+()创建一个传入了默认值的密钥|453026|my_dict[key]+=+my_dict.setdefault(key,+0)+%2B+num|453027|entityMap^0|4|4|9|B|0|0|6|0|0|0|E|0|0^^$0|@$1|2|3|4|5|6|7|S|8|@$9|T|A|U|B|C]|$9|V|A|W|B|C]]|D|@]|E|$]]|$1|F|3|G|5|6|7|X|8|@$9|Y|A|Z|B|C]]|D|@]|E|$]]|$1|H|3|I|5|J|7|10|8|@]|D|@]|E|$K|L]]|$1|M|3|N|5|6|7|11|8|@$9|12|A|13|B|C]]|D|@]|E|$]]|$1|O|3|P|5|J|7|14|8|@]|D|@]|E|$K|L]]|$1|Q|3|-4|5|6|7|15|8|@]|D|@]|E|$]]]|R|$]]

Any one of <code>.get</code> or <code>.setdefault</code> can be used:

<code>.get()</code> give default value passed in the function if there is no valid key

<pre><code>my_dict[key] = my_dict.get(key, 0) + num
</code></pre>

<code>.setdefault ()</code> create a key with default value passed

<pre><code>my_dict[key] = my_dict.setdefault(key, 0) + num
</code></pre>

In Python it's annoying to have to check whether a key is in the dictionary first before incrementing it:

<pre><code>if key in my_dict:
 my_dict[key] += num
else:
 my_dict[key] = num
</code></pre>

Is there a shorter substitute for the four lines above?

Python dictionary increment

Python

在Python中，在递增关键字之前，必须先检查它是否在字典中，这很烦人：if key in my_dict:  my_dict[key] += numelse:  my_dict[key] = num上面的四行有没有更短的替代行？

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