CODEWITHSUNDEEP
jsonpython-3.xdictionary
maurera
maurera

Reputation: 1679

In Python, how to concisely get nested values in json data?

I have data loaded from JSON and am trying to extract arbitrary nested values using a list as input, where the list corresponds to the names of successive children. I want a function get_value(data,lookup) that returns the value from data by treating each entry in lookup as a nested child.

In the example below, when lookup=['alldata','TimeSeries','rates'], the return value should be [1.3241,1.3233].

json_data = {'alldata':{'name':'CAD/USD','TimeSeries':{'dates':['2018-01-01','2018-01-02'],'rates':[1.3241,1.3233]}}}

def get_value(data,lookup):
    res = data
    for item in lookup:
        res = res[item]
    return res

lookup = ['alldata','TimeSeries','rates']
get_value(json_data,lookup)

My example works, but there are two problems:

  1. It's inefficient - In my for loop, I copy the whole TimeSeries object to res, only to then replace it with the rates list. As @Andrej Kesely explained, res is a reference at each iteration, so data isn't being copied.
  2. It's not concise - I was hoping to be able to find a concise (eg one or two line) way of extracting the data using something like list comprehension syntax

Upvotes: 0

Views: 151

Answers (2)

Andrej Kesely
Andrej Kesely

Reputation: 195573

If you want one-liner and you are using Python 3.8, you can use assignment expression ("walrus operator"):

json_data = {'alldata':{'name':'CAD/USD','TimeSeries':{'dates':['2018-01-01','2018-01-02'],'rates':[1.3241,1.3233]}}}

def get_value(data,lookup):
    return [data:=data[item] for item in lookup][-1]

lookup = ['alldata','TimeSeries','rates']
print( get_value(json_data,lookup) )

Prints:

[1.3241, 1.3233]

Upvotes: 1

Dane Johnson
Dane Johnson

Reputation: 334

I don't think you can do it without a loop, but you could use a reducer here to increase readability.

functools.reduce(dict.get, lookup, json_data)

Upvotes: 1

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