flask: how to make validation on Request JSON and JSON schema?

Question

In flask-restplus API, I need to make validation on request JSON data where I already defined request body schema with api.model . Basically I want to pass input JSON data to API function where I have to validate input JSON data before using API function. To do so, I used RequestParser for doing this task, but the API function was expecting proper JSON data as parameters after request JSON is validated and parsed. To do request JSON validation, first I have to parse received input JSON data, parse its JSON body, validate each then reconstructs it as JSON object, and pass to the API function. Is there any easier way to do this?

input JSON data

{
  "body": {
    "gender": "M",
    "PCT": {
      "value": 12,
      "device": "roche_cobas"
    },
    "IL6": {
      "value": 12,
      "device": "roche_cobas"
    },
    "CRP": {
      "value": 12,
      "device": "roche_cobas"
    }
  }
}

my current attempt in flask

from flask_restplus import Api, Namespace, Resource, fields, reqparse, inputs
from flask import Flask, request, make_response, Response, jsonify

app = Flask(__name__)
api = Api(app)
ns = Namespace('')

feature = api.model('feature', {
    'value': fields.Integer(required=True),
    'time': fields.Date(required=True)
})

features = api.model('featureList', {
    'age': fields.String,
    'gender': fields.String(required=True),
    'time': fields.Date,
    'features': fields.List(fields.Nested(feature, required=True))
    })

@ns.route('/hello')
class helloResource(Resource):
    @ns.expect(features)
    def post(self):
        json_dict = request.json  ## get input JSON data
        ## need to parse json_dict to validate expected argument in JSON body
        root_parser = reqparse.RequestParser()
        root_parser.add_argument('body', type=dict)
        root_args = root_parser.parse_args()

        jsbody_parser = reqparse.RequestParser()
        jsbody_parser.add_argument('age', type=dict, location = ('body',))
        jsbody_parser.add_argument('gender', type=dict, location=('body',))
        ## IL6, CRP could be something else, how to use **kwargs here
        jsbody_parser.add_argument('IL6', type=dict, location=('body',))
        jsbody_parser.add_argument('PCT', type=dict, location=('body',))
        jsbody_parser.add_argument('CRP', type=dict, location=('body',))
        jsbody_parser = jsbody_parser.parse_args(req=root_args)

        ## after validate each argument on input JSON request body, all needs to be constructed as JSON data
        json_data = json.dumps(jsonify(jsbody_parser))   ## how can I get JSON again from jsbody_parser
        func_output = my_funcs(json_data)
        rest = make_response(jsonify(str(func_output)), 200)
        return rest

if __name__ == '__main__':
    api.add_namespace(ns) 
    app.run(debug=True)

update: dummy api function

Here is dummy function that expecting json data after validation:

import json
def my_funcs(json_data):
    a =json.loads(json_data)
    for k,v in a.iteritems():
           print k,v
    return jsonify(a)

current output of above attempt :

I have this on response body:

{
  "errors": {
    "gender": "'gender' is a required property"
  },
  "message": "Input payload validation failed"
}

obviously, request JSON input is not handled and not validated in my attempt. I think I have to pass json_dict to RequestParser object, but still can't validate request JSON here. how to make this happen?

I have to validate expected arguments from JSON body, after validation, I want to construct JSON body that gonna be used as a parameter for API function. How can I make this happen? any workaround to achieve this?

parsed JSON must pass to my_funcs

in my post, request JSON data should be parsed, such as age , gender should be validated as expected arguments in the request JSON, then jsonify added arguments as JSON and pass the my_funcs . how to make this happen easily in fl

I want to make sure flask should parse JSON body and add arguments as it expected, otherwise throw up error. for example:

{
      "body": {
        "car": "M",
        "PCT": {
          "value": 12,
          "device": "roche_cobas"
        },
        "IL6": {
          "device": "roche_cobas"
        },
        "CRP": {
          "value": 12
        }
      }
    }

if I give JSON data like above for making POST request to a server endpoint, it should give the error. How to make this happen? how to validate POST request JSON for flask API call?

Answer 1

As I tried to convey in our conversation it appears you are after a serilzation and deserilization tool. I have found Marshmallow to be an exceptional tool for this (it is not the only one). Here's a working example of using Marshmallow to validate a request body, converting the validated data back to a JSON string and passing it to a function for manipulation, and returning a response with JSON data:

from json import dumps, loads
from flask import Flask, jsonify, request
from marshmallow import Schema, fields, ValidationError

class BaseSchema(Schema):
    age = fields.Integer(required=True)
    gender = fields.String(required=True)

class ExtendedSchema(BaseSchema):
    # have a look at the examples in the Marshmallow docs for more complex data structures, such as nested fields.
    IL6 = fields.String()
    PCT = fields.String()
    CRP = fields.String()

def my_func(json_str:str):
    """ Your Function that Requires JSON string"""

    a_dict = loads(json_str)

    return a_dict

app = Flask(__name__)

@app.route('/base', methods=["POST"])
def base():
    # Get Request body from JSON
    request_data = request.json
    schema = BaseSchema()
    try:
        # Validate request body against schema data types
        result = schema.load(request_data)
    except ValidationError as err:
        # Return a nice message if validation fails
        return jsonify(err.messages), 400

    # Convert request body back to JSON str
    data_now_json_str = dumps(result)

    response_data = my_func(data_now_json_str)

    # Send data back as JSON
    return jsonify(response_data), 200


@app.route('/extended', methods=["POST"])
def extended():
    """ Same as base function but with more fields in Schema """
    request_data = request.json
    schema = ExtendedSchema()
    try:
        result = schema.load(request_data)
    except ValidationError as err:
        return jsonify(err.messages), 400

    data_now_json_str = dumps(result)

    response_data = my_func(data_now_json_str)

    return jsonify(response_data), 200

Here's some quick tests to show validation, as well as extending the fields in your request body:

import requests
# Request fails validation
base_data = {
    'age': 42,
    }
r1 = requests.post('http://127.0.0.1:5000/base', json=base_data)
print(r1.content)

# Request passes validation
base_data = {
    'age': 42,
    'gender': 'hobbit'
    }
r2 = requests.post('http://127.0.0.1:5000/base', json=base_data)
print(r2.content)

# Request with extended properties
extended_data = {
    'age': 42,
    'gender': 'hobbit',
    'IL6': 'Five',
    'PCT': 'Four',
    'CRP': 'Three'}

r3 = requests.post('http://127.0.0.1:5000/extended', json=extended_data)
print(r3.content)

Hope this helps gets you where you're going.