Python:list()作为字典的默认值 [英] Python: list() as default value for dictionary

问题描述

我有如下的Python代码:

I have Python code that looks like:

if key in dict:
  dict[key].append(some_value)
else:
  dict[key] = [some_value]

但是我认为应该有一些方法可以解决这个'if'语句.我尝试过

but I figure there should be some method to get around this 'if' statement. I tried

dict.setdefault(key, [])
dict[key].append(some_value)

和

dict[key] = dict.get(key, []).append(some_value)

，但都抱怨"TypeError:无法散列的类型:'list'".有什么建议吗?谢谢！

but both complain about "TypeError: unhashable type: 'list'". Any recommendations? Thanks!

推荐答案

最好的方法是使用 collections.defaultdict ，默认值为list

from collections import defaultdict
dct = defaultdict(list)

然后只使用:

dct[key].append(some_value)

，如果键尚未在映射中，则词典将为您创建一个新列表. collections.defaultdict是dict的子类，否则表现得像普通的dict对象.

and the dictionary will create a new list for you if the key is not yet in the mapping. collections.defaultdict is a subclass of dict and otherwise behaves just like a normal dict object.

使用标准dict时，dict.setdefault()会为您正确设置dct[key]为默认值，因此该版本应该可以正常工作.您可以使用.append()链接该呼叫:

When using a standard dict, dict.setdefault() correctly sets dct[key] for you to the default, so that version should have worked just fine. You can chain that call with .append():

>>> dct = {}
>>> dct.setdefault('foo', []).append('bar')  # returns None!
>>> dct
{'foo': ['bar']}

但是，通过使用dct[key] = dct.get(...).append()，您可以替换 dct[key]的值，并输出.append()，即None.

However, by using dct[key] = dct.get(...).append() you replace the value for dct[key] with the output of .append(), which is None.

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