Appending to list in Python dictionary [duplicate]
list.append
returns 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, []).append
you are actually doing None.append
. That is why it is failing. Instead, you can simply do
dates_dict.setdefault(key, []).append(date)
But, we have collections.defaultdict
for this purpose only. You can do something like this
from collections import defaultdictdates_dict = defaultdict(list)for key, date in cur: dates_dict[key].append(date)
This will create a new list object, if the key
is not found in the dictionary.
Note: Since the defaultdict
will 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.
Is there a more elegant way to write this code?
from collections import defaultdictdates_dict = defaultdict(list)for key, date in cur: dates_dict[key].append(date)
dates_dict[key] = dates_dict.get(key, []).append(date)
sets dates_dict[key]
to None
as list.append
returns None
.
In [5]: l = [1,2,3]In [6]: var = l.append(3)In [7]: print varNone
You should use collections.defaultdict
import collectionsdates_dict = collections.defaultdict(list)