动态创建字典键，我可以附加其他值？

Question

I have a dictionary that I'll be adding data to that looks like this:

mydict = {
    'key1': ['First string message', 'Second string message'],
    'key2': ['String message']
}

I am iterating through a json object that has two keys - key and message . I want to add every key into mydict and append all message s that have the same key .

Example:

[
    {
        'key': 'key1',
        'message': 'First string message'
    },
    {
        'key': 'key2',
        'message': 'String message'
    },
    {
        'key': 'key1',
        'message': 'Second string message'
    },
    {
        'key': 'key3',
        'message': 'Brand new string message'
    }
]

I am having a problem on building my dictionary dynamically though. How can I dynamically create my keys so that I can append additional strings?

One of the ways I've tried is this:

import json
response = [
    {
        'key': 'key1',
        'message': 'First string message'
    },
    {
        'key': 'key2',
        'message': 'String message'
    },
    {
        'key': 'key1',
        'message': 'Second string message'
    },
    {
        'key': 'key3',
        'message': 'Brand new string message'
    }
]
mydict = {}
for r in response:
    mydict[r['key']] = mydict.get(r['key'], [r['message']])

print(mydict)
{'key1': ['First string message'], 'key2': ['String message'], 'key3': ['Brand new string message']}

This, obviously, fails my expected result because key1 doesn't contain both strings.

I tried this:

for r in response:
    mydict[r['key']] = mydict.get(r['key'], [r['message']]) + r['message']

But this fails with TypeError: can only concatenate list (not "str") to list

How can I dynamically create my keys so that I can append additional strings?

Answer 1

You should use either defaultdict or setdefault . I prefer defaultdict :

from collections import defaultdict

response = [
    {
        'key': 'key1',
        'message': 'First string message'
    },
    {
        'key': 'key2',
        'message': 'String message'
    },
    {
        'key': 'key1',
        'message': 'Second string message'
    },
    {
        'key': 'key3',
        'message': 'Brand new string message'
    }
]

mydict = defaultdict(list)

for r in response:
    mydict[r['key']].append(r['message'])

With setdefault you can make mydict a regular dictionary:

mydict = {}
for r in response:
    mydict.setdefault(r['key'], []).append(r['message'])