How can I get the specified key value in a nested dictionary in a most effective way？

Question

There is a nested dictionery like ：

data_dict = {
 "picture":"xxx.jpg",
 "link_data":{
     "picture":"xxxxx.jpg", 
     ... 
     "child_attachments":{
            "picture":"xxxxx.jpg", 
              ... 
       }
  }
  ...
}

The problem is at every level of the dictionary, the key picture may exist, how can I get the picture's value in a most effective way?

Here's my trial, but failed:

def get_picture_url(data):
    for key, value in data.items():
        if key == "picture":
            return data[key]
        else:
            if isinstance(value, dict):
                return get_picture_url(value)

get_picture_url(data_dict)

Related: https://stackoverflow.com/questions/10399614/accessing-value-inside-nested-dictionaries — bhansa, Apr 12 '19 at 12:36
Which particular value are you interested in? There are lots of keys that are `"picture"`. — quamrana, Apr 12 '19 at 12:42
Thanks for replying, there will only be one `picture` , i just want to show that it may exist at every level — jia Jimmy, Apr 12 '19 at 13:27

score 0 · Answer 1 · answered Apr 12 '19 at 12:41

You are not checking the returned value of the recursive call to get_picture_url.

This should give you the top most picture in your dict:

def get_picture_url(data, picture_key="picture"):
    if not isinstance(data, dict):
        return None
    picture_url = data.get(picture_key)
    if picture_url is not None:
        return picture_url
    for value in data.values():
        picture_url = get_picture_url(value)
        if picture_url is not None:
            return picture_url
    return None

Óscar López · Accepted Answer · 2019-04-12T13:04:19.070

This should work for the general case of an arbitrarily nested dictionary with JSON-like structure:

def get_picture(data):
    # you can remove this case if the
    # input doesn't contain lists
    if isinstance(data, list):
        ans = []
        for e in data:
            ans += get_picture(e)
        return ans
    elif not isinstance(data, dict):
        return []
    else:
        ans = []
        for k, v in data.items():
            if k == 'picture':
                ans.append(v)
            else:
                ans += get_picture(v)
        return ans

It'll traverse all levels of the data structure, looking for keys named 'picture' and accumulating all of their values in a single output list. If you're sure that there are no lists in the input, we can simplify the solution a bit:

def get_picture(data):
    ans = []
    if isinstance(data, dict):
        for k, v in data.items():
            if k == 'picture':
                ans.append(v)
            else:
                ans += get_picture(v)
    return ans

Either way, it works as expected for your sample input:

data_dict = {
 "picture":"xxx.jpg",
 "link_data":{
    "picture":"xxxx.jpg",
    "child_attachments":{
        "picture":"xxxxx.jpg"
    }
  }
}

get_picture(data_dict)
=> ['xxx.jpg', 'xxxx.jpg', 'xxxxx.jpg']

How can I get the specified key value in a nested dictionary in a most effective way？

2 Answers2