Swift Codable is a little weird...

file.swift

import Foundation

// When using swift codable to parse JSON into a struct
let json = [
    "non_accessible": true,
    "index": 7
    ] as [String : Any]

struct Info: Codable {
    let index: Int
    var isPreferred: Bool? = false
    var nonAccessible: Bool? = false

    enum CodingKeys: String, CodingKey {
        case index
        case isPreferred = "is_preferred"
        case nonAccessible = "non_accessible"
    }
}

let data = try! JSONSerialization.data(withJSONObject: json, options: .prettyPrinted)

let jsonDecoder = JSONDecoder.init()
let info = try! jsonDecoder.decode(Info.self, from: data)

// What's the value of `info.isPreferred`? `false`, or `nil`?