Python字典中的Keyerror

Question

I am stack in something really simple, but I cannot find the right way. I have a dictionary like this one:

mydic= {'a': {'mylist':[..,..,..]}, 'b': {'mylist':[..,..,..]}}

I am trying to iterate through mylist and create a new subdictionary with the results of a function.

for i in mydic:
    for j in mydic[i]['mylist']:
         mydic[i]['thenewkey'][j] = myfunction(j)

The find dictionary would be like this:

mydic= {'a': {'mylist':[..,..,..], 'thenewkey':{'..':'..', '..':'..'}}, 'b': {'mylist':[..,..,..],'thenewkey':{'..':'..', '..':'..'}}}

But when I run the code, I have a key error on thenewkey . Any idea?

Answer 1

在分配给mydic[i]['thenewkey'][j]之前，您必须先创建mydic[i]['thenewkey'] mydic[i]['thenewkey'][j] 。

Answer 2

As already said, you have to create the thenewkey entry before you can add items to it, ie you'd have to add mydic[i]['thenewkey'] = {} to your outer loop. You could use a defaultdict(dict) to make Python automatically insert missing entries, but since you have both list and dict entries, this does not seem like a good idea.

That said, using a dictionary comprehension makes it a bit shorter and IMHO much more readable:

for i in mydic:
    mydic[i]['thenewkey'] = {j: myfunction(j) for j in mydic[i]['mylist']}