how to access keys in a rquest sent via POST method

Question

in the below posted webservice, i obtain the data posted from the front-end using json.loads the result of the print statment is posted below in the sample-data section. the problem i have is when i try to obtain the values included in the request.data, for example, treatmentGeometryAsJSONInEPSG3857. the error i am getting when i run the code is:

treatmentAsGeoJSONInEPSG3857 = json.loads(request.data)['body'][config['FRONT_END_KEYS']['key_treatmentGeometryAsJSONInEPSG3857']]
TypeError: string indices must be integers

please let me know how to access the contents of the posted request

i referred to the following posts as well but none contains an answer to my question

https://stackoverflow.com/questions/10434599/get-the-data-received-in-a-flask-request
https://www.digitalocean.com/community/tutorials/processing-incoming-request-data-in-flask-de

code:

@app.route("/experimentExistenceCheck/", methods=['POST','OPTIONS'])
    def experimentExistenceCheck():
    preFlight = FlaskAccessControlUtils.preFlightCheck(request)
    if preFlight: return preFlight
    
    data = json.loads(request.data)['body']
    print("--------->",data)
    
    treatmentAsGeoJSONInEPSG3857 = json.loads(request.data)['body'][config['FRONT_END_KEYS']['key_treatmentGeometryAsJSONInEPSG3857']]
    selectedSiteId = json.loads(request.data)['body'][config['FRONT_END_KEYS']['selectedSiteID']]
    threshold = json.loads(request.data)['body'][config['FRONT_END_KEYS']['threshold']]
    visualizationOperationID = json.loads(request.data)['body'][config['FRONT_END_KEYS']['visualizationOperationID']]
    print("--------->",treatmentAsGeoJSONInEPSG3857)
    print("--------->",selectedSiteId)
    print("--------->",threshold)
    print("--------->",visualizationOperationID)
    

sample data:

{"threshold":"1","visualizationOperationID":2,"treatmentGeometryAsJSONInEPSG3857":"{\"coordinates\":[[[745037.9841857546,6644742.79192291],[744938.5789015774,6644804.979114908],[744973.912533394,6644856.330659814],[745025.2640783006,6644838.428286361],[745022.4373877553,6644802.623539453],[744997.9394030292,6644768.703252909],[745037.9841857546,6644742.79192291]]],\"type\":\"Polygon\"}","selectedSiteID":"202108041239"}