Error handling using JSONDecoder in Swift

Question

I am using JSONDecoder() in Swift and need to get better error messages.

Within the debug description (for example) I can see messages like "The given data was not valid JSON", but I need to know it is that rather than a network error (for example).

let decoder = JSONDecoder()
if let data = data {
   do {
      // process data

   } catch let error {
      // can access error.localizedDescription but seemingly nothing else
   }
}

I tried to cast to a DecodingError, but this does not seem to reveal more information. I certainly don't need the string - even an error code is much more helpful than this...

You have to cast the error to specific type to access the properties. Check out documentation for more details. docs.swift.org/swift-book/LanguageGuide/ErrorHandling.html — Aks
– Aks, Commented Mar 28, 2019 at 4:13
I know, I tried DecodingError. But it has no more detail than error alone. — WishIHadThreeGuns
– WishIHadThreeGuns, Commented Mar 28, 2019 at 4:16
Can you post the code you have tried? It will help us to get more context of your issue. — Aks
– Aks, Commented Mar 28, 2019 at 4:22
print ("Error in reading data", error as! DecodingError) print ("error", error.localizedDescription) let decerr = error as! DecodingError print (decerr.errorDescription) print ("resaon", decerr.errorDescription) — WishIHadThreeGuns
– WishIHadThreeGuns, Commented Mar 28, 2019 at 4:23

Haroldo Gondim · Accepted Answer · 2023-05-18 17:07:11Z

112

Never print error.localizedDescription in a decoding catch block. This returns a quite meaningless generic error message. Print always the error instance. Then you get the desired information.

let decoder = JSONDecoder()
if let data = data {
   do {
      // process data

   } catch  {
      print(error)
   }
}

Or for the full set of errors use

let decoder = JSONDecoder()
if let data = data {
    do {
       // process data
    } catch let DecodingError.dataCorrupted(context) {
        print(context)
    } catch let DecodingError.keyNotFound(key, context) {
        print("Key '\(key)' not found:", context.debugDescription)
        print("codingPath:", context.codingPath)
    } catch let DecodingError.valueNotFound(value, context) {
        print("Value '\(value)' not found:", context.debugDescription)
        print("codingPath:", context.codingPath)
    } catch let DecodingError.typeMismatch(type, context)  {
        print("Type '\(type)' mismatch:", context.debugDescription)
        print("codingPath:", context.codingPath)
    } catch {
        print("error: ", error)
    }
}

edited May 18, 2023 at 17:07

Haroldo Gondim

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answered Mar 28, 2019 at 6:09

vadian

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6 Comments

Peter Schorn Over a year ago

localizedDescription should only be used if you need to display something to the end user.

rustyMagnet Over a year ago

I really like this answer. The only difference - for my own debugging -was omitting the context variable as it was too verbose. catch let DecodingError.keyNotFound(key, _) { print("[!]Key '\(key)' not found")

stack_underflow Over a year ago

Can you get access to the property that was type mismatched? Say a field came back as an Int when I'm expecting a String, I want to set a default value for that property in that case

vadian Over a year ago

@infiltratingtree This makes no sense, because when an error occurs the decoding process will be interrupted and terminated.

stack_underflow Over a year ago

Maybe I'm not explaining properly. I want to know which field caused the decoding error. Is there any way to get that info?

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