Example snippet showing additional logic added to Flask API POST method.

users_post2.py

        ...

        # read our CSV
        data = pd.read_csv('users.csv')

        if args['userId'] in list(data['userId']):
            return {
                'message': f"'{args['userId']}' already exists."
            }, 401
        else:
            # create new dataframe containing new values
            new_data = pd.DataFrame({
                'userId': args['userId'],
                'name': args['name'],
                'city': args['city'],
                'locations': [[]]
            })
            # add the newly provided values
            data = data.append(new_data, ignore_index=True)
            data.to_csv('users.csv', index=False)  # save back to CSV
            return {'data': data.to_dict()}, 200  # return data with 200 OK