How to parse json to get all values of a specific key within an array?

Question

I'm having trouble trying to get a list of values from a specific key inside an json array using python. Using the JSON example below, I am trying to create a list which consists only the values of the name key. Original JSON:

[
    {
        "id": 1,
        "name": "Bulbasaur",
        "type": [
            "grass",
            "poison"
        ]
    },
    {
        "id": 2,
        "name": "Ivysaur",
        "type": [
            "grass",
            "poison"
        ]
    }
]

Expected:

["Bulbasaur", "Ivysaur"]

Below is the code of my approach:

import json
try:
    with open("./simple.json", 'r') as f:
        contents = json.load(f)
except Exception as e:
    print(e)

print(contents[:]["name"])

I'm trying to go to an approach where i don't need to loop every single index and append them, something like the code above. Is this approach possible using python' json library?

Answer 1

You cannot do contents[:]["name"] since contents is a list is a dictionary with integer indexes, and you cannot access an element from it using a string name.

To fix that, you would want to iterate over the list and get the value for key name for each item

import json
contents = []

try:
    with open("./simple.json", 'r') as f:
        contents = json.load(f)
except Exception as e:
    print(e)


li = [item.get('name') for item in contents]
print(li)

The output will be

['Bulbasaur', 'Ivysaur']

Answer 2

This is not a real answer to the question. The real answer is to use a list comprehension. However, you can make a class that allows you to use specifically the syntax you tried in the question. The general idea is to subclass list so that a slice like [:] returns a special view (another class) into the list. This special view will then allow retrieval and assignment from all the dictionaries simultaneously.

class DictView:
    """
    A special class for getting and setting multiple dictionaries
    simultaneously. This class is not meant to be instantiated
    in its own, but rather in response to a slice operation on UniformDictList.
    """
    def __init__(parent, slice):
        self.parent = parent
        self.range = range(*slice.indices(len(parent)))

    def keys(self):
        """
        Retreives a set of all the keys that are shared across all
        indexed dictionaries. This method makes `DictView` appear as
        a genuine mapping type to `dict`.
        """
        key_set = set()
        for k in self.range:
            key_set &= self.parent.keys()
        return key_set

    def __getitem__(self, key):
        """
        Retreives a list of values corresponding to all the indexed
        values for `key` in the parent. Any missing key will raise
        a `KeyError`.
        """
        return [self.parent[k][key] for k in self.range]

    def get(self, key, default=None):
        """
        Retreives a list of values corresponding to all the indexed
        values for `key` in the parent. Any missing key will return
        `default`.
        """
        return [self.parent[k].get(key, default) for k in self.range]

    def __setitem__(self, key, value):
        """
        Set all the values in the indexed dictionaries for `key` to `value`.
        """
        for k in self.range:
            self.parent[k][key] = value

    def update(self, *args, **kwargs):
        """
        Update all the indexed dictionaries in the parent with the specified
        values. Arguments are the same as to `dict.update`.
        """
        for k in self.range:
             self.parent[k].update(*args, **kwargs)


class UniformDictList(list):
    def __getitem__(self, key):
        if isinstance(key, slice):
            return DictView(self, key)
        return super().__getitem__(key)

Your original code would now work out of the box with just one additional wrap in UniformDictList:

import json
try:
    with open("./simple.json", 'r') as f:
        contents = UniformDictList(json.load(f))
except Exception as e:
    print(e)

print(contents[:]["name"])

Answer 3

Try this with list comprehensions:

print([d["name"] for d in contents])