JSON Python parsing error KeyError: 'vrm'

I have stitched together the below python code and I’m struggling with a KeyError: 'vrm'.

My aim is to parse the JSON string and only print the specific JSON objects.

#!/usr/bin/python3
import json

class Vehicle:
    def __init__(self,responseInfo, code, desc, data, vrm, vin, engineNumber, capId, capCode, manufacturer, model, vehicleType, registrationDate, firstRegistrationDate, body, doors, engineSize, fuel, fuelDelivery, transmission, isScraped, isExported, isImported, images, exactMatch, url, mainImage):
        self.responseInfo = responseInfo
        self.code = code
        self.desc = desc
        self.data = data
        self.vrm = vrm
        self.vin = vin
        self.engineNumber = engineNumber
        self.capId = capId
        self.capCode = capCode
        self.manufacturer = manufacturer
        self.model = model
        self.vehicleType = vehicleType
        self.registrationDate = registrationDate
        self.firstRegistrationDate = firstRegistrationDate
        self.body = body
        self.doors = doors
        self.engineSize = engineSize
        self.fuel = fuel
        self.fuelDelivery = fuelDelivery
        self.transmission = transmission
        self.isScraped = isScraped
        self.isExported = isExported
        self.isImported = isImported
        self.images = images
        self.exactMatch = exactMatch
        self.url = url
        self.mainImage = mainImage

def vehicleDecoder(obj):
        return Vehicle(obj['vrm'], obj['vin'], obj['engineNumber'], obj['capId'], obj['capCode'], obj['manufacturer'], obj['model'], obj['vehicleType'], obj['registrationDate'], obj['firstRegistrationDate'], obj['body'], obj['doors'], obj['engineSize'], obj['fuel'], obj['fuelDelivery'], obj['transmission'], obj['isScraped'], obj['isExported'], obj['isImported'], obj['images'], obj['exactMatch'], obj['url'], obj['mainImage'])

vehicleObj = json.loads('{"responseInfo":{"code":0,"desc":"Success"},"data":{"vrm":"MA71VWG","vin":"WBATS120009H40802","engineNumber":"A1566412","capId":91400,"capCode":"BMX320MS 5EXTA4 4","manufacturer":"BMW","model":"X3","vehicleType":"Car","registrationDate":"27/09/21","firstRegistrationDate":"27/09/21","body":"Estate","doors":"5","engineSize":"1998","fuel":"Petrol/PlugIn Elec Hybrid","fuelDelivery":"Turbo","transmission":"Automatic","isScraped":"No","isExported":"No","isImported":"No","images":[{"viewpoint":"Front Three Quarter","exactMatch":true,"url":"https://soap.cap.co.uk/images/VehicleImage.aspx?SUBID=171774&HASHCODE=92A987528AB2F2215A7372DADD8CE57C&DB=CAR&CAPID=91400&WIDTH=&HEIGHT=&IMAGETEXT=&VIEWPOINT=3","mainImage":true}]}}')
print(vehicleObj) #works
#vehicleObj = (vehicleObj['data'])
print(vehicleObj) #works
vehicleObj = json.dumps(vehicleObj)
print(vehicleObj)
vehicleObj = json.loads(str(vehicleObj), object_hook=vehicleDecoder)
#print(vehicleObj)
print(vehicleObj.vrm, vehicleObj.vin, vehicleObj.engineNumber, vehicleObj.capId, vehicleObj.capCode, vehicleObj.manufacturer, vehicleObj.model, vehicleObj.vehicleType, vehicleObj.registrationDate, vehicleObj.firstRegistrationDate, vehicleObj.body, vehicleObj.doors, vehicleObj.engineSize, vehicleObj.fuel, vehicleObj.fuelDelivery, vehicleObj.transmission, vehicleObj.isScraped, vehicleObj.isExported, vehicleObj.isImported)

Error output below

Traceback (most recent call last):
  File "C:\Users\jack\Desktop\Python-Project\test.py", line 42, in <module>
    vehicleObj = json.loads(str(vehicleObj), object_hook=vehicleDecoder)
  File "C:\Python\Python39\lib\json\__init__.py", line 359, in loads
    return cls(**kw).decode(s)
  File "C:\Python\Python39\lib\json\decoder.py", line 337, in decode
    obj, end = self.raw_decode(s, idx=_w(s, 0).end())
  File "C:\Python\Python39\lib\json\decoder.py", line 353, in raw_decode
    obj, end = self.scan_once(s, idx)
  File "C:\Users\jack\Desktop\Python-Project\test.py", line 34, in vehicleDecoder
    return Vehicle(obj['vrm'], obj['vin'], obj['engineNumber'], obj['capId'], obj['capCode'], obj['manufacturer'], obj['model'], obj['vehicleType'], obj['registrationDate'], obj['firstRegistrationDate'], obj['body'], obj['doors'], obj['engineSize'], obj['fuel'], obj['fuelDelivery'], obj['transmission'], obj['isScraped'], obj['isExported'], obj['isImported'], obj['images'], obj['exactMatch'], obj['url'], obj['mainImage'])
KeyError: 'vrm'

Can anyone spot where I’ve gone wrong or have a better solution to parse JSON output?

>Solution :

If you reduce this example down to the minimum required to trigger the problem, the issue is much more clear:

import json


class Vehicle:
    def __init__(self, vrm):
        self.vrm = vrm


def vehicleDecoder(obj):
        return Vehicle(obj['vrm'])

vehicle_str = '{"data":{"vrm":"MA71VWG"}}'

vehicleObj = json.loads(vehicle_str, object_hook=vehicleDecoder)

This still creates a KeyError. Why? When loading a json, the object hook is called on every object. You probably intended for it to be called on the object containing the "vrm" key, but technically the {"data": ... part is an object too.

How can you fix this? Don’t use object_hook. Parse the json like this:

vehicleJson = json.loads(...)
vehicleObj = vehicleDecoder(vehicleJson['data'])