How to compare Go errors

by

Declaring an error, and comparing it with ‘==‘ (as in err == myPkg.ErrTokenExpired) is no longer the best practice with Go 1.13 (Q3 2019)

The release notes mentions:

Go 1.13 contains support for error wrapping, as first proposed in the Error Values proposal and discussed on the associated issue.

An error e can wrap another error w by providing an Unwrap method that returns w.
Both e and w are available to programs, allowing e to provide additional context to w or to reinterpret it while still allowing programs to make decisions based on w.

To support wrapping, fmt.Errorf now has a %w verb for creating wrapped errors, and three new functions in the errors package ( errors.Unwrap, errors.Is and errors.As) simplify unwrapping and inspecting wrapped errors.

So the Error Value FAQ explains:

You need to be prepared that errors you get may be wrapped.

If you currently compare errors using ==, use errors.Is instead.
Example:

if err == io.ErrUnexpectedEOF

becomes

if errors.Is(err, io.ErrUnexpectedEOF)
  • Checks of the form if err != nil need not be changed.
  • Comparisons to io.EOF need not be changed, because io.EOF should never be wrapped.

If you check for an error type using a type assertion or type switch, use errors.As instead. Example:

if e, ok := err.(*os.PathError); ok

becomes

var e *os.PathError
if errors.As(err, &e)

Also use this pattern to check whether an error implements an interface. (This is one of those rare cases when a pointer to an interface is appropriate.)

Rewrite a type switch as a sequence of if-elses.

More Related Contents:

  1. Multiple values in single-value context
  2. Close multiple goroutine if an error occurs in one in go
  3. Golang ascending order
  4. Convert map values to plain strings without brackets? [closed]
  5. Golang libphonenumber [closed]
  6. What are the use(s) for tags in Go?
  7. is there a way to create an instance of a struct from a string?
  8. How to avoid annoying error “declared and not used”
  9. How to broadcast message using channel
  10. Splitting client/server code
  11. Pass method argument to function
  12. How safe are Golang maps for concurrent Read/Write operations?
  13. In Go is naming the receiver variable ‘self’ misleading or good practice?
  14. Golang. What to use? http.ServeFile(..) or http.FileServer(..)?
  15. Custom type passed to function as a parameter
  16. Goroutine does not execute if time.Sleep included
  17. Struct in for loop initializer
  18. How can I “go run” a project with multiple files in the main package?
  19. Is it possible to have nested templates in Go using the standard library?
  20. Go error: cannot use generic type without instantiation
  21. How can I instantiate a non-nil pointer of type argument with generic Go?
  22. Function signature with no function body
  23. How to clone a structure with unexported field?
  24. Function declaration syntax: things in parenthesis before function name
  25. How to import local packages without gopath
  26. How can I open files relative to my GOPATH?
  27. Difference between any/interface{} as constraint vs. type of argument?
  28. Streaming commands output progress
  29. Slice chunking in Go
  30. Handle maps with same key type but different value type

Leave a Comment