Catching panics in Golang

Question

With the following code, if no file argument is given, a panic is thrown for line 9 panic: runtime error: index out of range as expected.

How can I 'catch' this panic and handle it when directly when passing something to it (os.Args[1]) that causes the panic? Much like try/catch in PHP or try/except in Python.

I've had a search here on StackOverflow but I've not found anything that answers this as such.

package main

import (
    "fmt"
    "os"
)

func main() {
    file, err := os.Open(os.Args[1])
    if err != nil {
        fmt.Println("Could not open file")
    }
    fmt.Printf("%s", file)
}

Answer 1

I had to catch panics in a test case. I got redirected here.

func.go

var errUnexpectedClose = errors.New("Unexpected Close")
func closeTransaction(a bool) {
    if a == true {
        panic(errUnexpectedClose)
    }
}

func_test.go

func TestExpectedPanic() {
    got := panicValue(func() { closeTransaction(true) })
    a, ok := got.(error)
    if a != errUnexpectedClose || !ok {
        t.Error("Expected ", errUnexpectedClose.Error())
    }
}

func panicValue(fn func()) (recovered interface{}) {
    defer func() {
        recovered = recover()
    }()
    fn()
    return
}

Used from https://github.com/golang/go/commit/e4f1d9cf2e948eb0f0bb91d7c253ab61dfff3a59 (ref from VonC)

Answer 2

We can manage panic without halting process using recover. By calling recover in any function using defer it will return the execution to calling function. Recover returns two values one is boolean and other one is interface to recover. Using type assertion we can get underlying error value You can also print underlying error using using recover.

defer func() {
    if r := recover(); r != nil {
        var ok bool
        err, ok = r.(error)
        if !ok {
            err = fmt.Errorf("pkg: %v", r)
        }
    }
}()

Answer 3

First: You wouldn't want to do this. Try-catch-style error handling is no error handling. In Go you would check len(os.Args) first and access element 1 only if present.

For the rare cases you need to catch panics (and your case is not one of them!) use defer in combination with recover. See http://golang.org/doc/effective_go.html#recover

Answer 4

Note that the recover treatment of a panic Execution error (such as attempting to index an array out of bounds trigger) might change with go 1.7 after issue 14965

See CL 21214 and its test:

runtime: make execution error panic values implement the Error interface

Make execution panics implement Error as mandated by Run-time panics (specs), instead of panics with strings.

When you recover a panic error, you would be able to do:

if _, ok := recovered.(runtime.Error); !ok {

---

This is still being evaluated, and as Dave Cheney. mentions:

I don't know what people are currently doing but from my POV this has been broken for a long time and nobody has complained so they are either explicitly relying on the broken behaviour, or nobody cares. Either way I think it's a good idea to avoid making this change.

Answer 5

Some Golang official packages use panic/defer+recover as throw/catch, but only when they need to unwind a large call stack. In Golang's json package using panic/defer+recover as throw/catch is the most elegant solution.

from http://blog.golang.org/defer-panic-and-recover

For a real-world example of panic and recover, see the json package from the Go standard library. It decodes JSON-encoded data with a set of recursive functions. When malformed JSON is encountered, the parser calls panic to unwind the stack to the top-level function call, which recovers from the panic and returns an appropriate error value (see the 'error' and 'unmarshal' methods of the decodeState type in decode.go).

Search for d.error( at http://golang.org/src/encoding/json/decode.go

In your example the "idiomatic" solution is to check the parameters before using them, as other solutions have pointed.

But, if you want/need to catch anything you can do:

package main

import (
    "fmt"
    "os"
)

func main() {

    defer func() { //catch or finally
        if err := recover(); err != nil { //catch
            fmt.Fprintf(os.Stderr, "Exception: %v\n", err)
            os.Exit(1)
        }
    }()

    file, err := os.Open(os.Args[1])
    if err != nil {
        fmt.Println("Could not open file")
    }

    fmt.Printf("%s", file)
}

Answer 6

Go is not python, you should properly check for args before you use it:

func main() {
    if len(os.Args) != 2 {
         fmt.Printf("usage: %s [filename]\n", os.Args[0])
         os.Exit(1)
    }
    file, err := os.Open(os.Args[1])
    if err != nil {
        log.Fatal(err)
    }
    fmt.Printf("%s", file)
}