附加到 Python 字典中的列表
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Appending to list in Python dictionary
提问by Michael Murphy
Is there a more elegant way to write this code?
有没有更优雅的方式来编写这段代码？
What I am doing: I have keys and dates. There can be a number of dates assigned to a key and so I am creating a dictionary of lists of dates to represent this. The following code works fine, but I was hoping for a more elegant and Pythonic method.
我在做什么：我有钥匙和日期。可以有多个日期分配给一个键，因此我正在创建一个日期列表字典来表示这一点。下面的代码工作正常，但我希望有一个更优雅和 Pythonic 的方法。
dates_dict = dict()
for key, date in cur:
if key in dates_dict:
dates_dict[key].append(date)
else:
dates_dict[key] = [date]
I was expecting the below to work, but I keep getting a NoneType has no attribute append error.
我期待下面的工作，但我不断收到 NoneType has no attribute append 错误。
dates_dict = dict()
for key, date in cur:
dates_dict[key] = dates_dict.get(key, []).append(date)
This probably has something to do with the fact that
这可能与以下事实有关
print([].append(1))
None
but why?
但为什么？
采纳答案by thefourtheye
list.appendreturns None, since it is an in-place operation and you are assigning it back to dates_dict[key]. So, the next time when you do dates_dict.get(key, []).appendyou are actually doing None.append. That is why it is failing. Instead, you can simply do
list.append返回None，因为它是一个就地操作并且您将其分配回dates_dict[key]. 所以，下次你做的时候，你dates_dict.get(key, []).append实际上是在做None.append. 这就是它失败的原因。相反，你可以简单地做
dates_dict.setdefault(key, []).append(date)
But, we have collections.defaultdictfor this purpose only. You can do something like this
但是，我们只是collections.defaultdict为了这个目的。你可以做这样的事情
from collections import defaultdict
dates_dict = defaultdict(list)
for key, date in cur:
dates_dict[key].append(date)
This will create a new list object, if the keyis not found in the dictionary.
如果key在字典中找不到，这将创建一个新的列表对象。
Note:Since the defaultdictwill create a new list if the key is not found in the dictionary, this will have unintented side-effects. For example, if you simply want to retrieve a value for the key, which is not there, it will create a new list and return it.
注意：由于defaultdict如果在字典中找不到键，将创建一个新列表，这将产生意想不到的副作用。例如，如果您只想检索键的值，而该值不存在，它将创建一个新列表并返回它。
回答by Padraic Cunningham
dates_dict[key] = dates_dict.get(key, []).append(date)sets dates_dict[key]to Noneas list.appendreturns None.
dates_dict[key] = dates_dict.get(key, []).append(date)设置 dates_dict[key]于None作为list.append回报None。
In [5]: l = [1,2,3]
In [6]: var = l.append(3)
In [7]: print var
None
You should use collections.defaultdict
import collections
dates_dict = collections.defaultdict(list)
回答by jfs
Is there a more elegant way to write this code?
有没有更优雅的方式来编写这段代码？
from collections import defaultdict
dates_dict = defaultdict(list)
for key, date in cur:
dates_dict[key].append(date)
