import "github.com/pkg/errors"
Package errors provides simple error handling primitives.
The traditional error handling idiom in Go is roughly akin to
if err != nil {
return err
}
which when applied recursively up the call stack results in error reports without context or debugging information. The errors package allows programmers to add context to the failure path in their code in a way that does not destroy the original value of the error.
The errors.Wrap function returns a new error that adds context to the original error by recording a stack trace at the point Wrap is called, together with the supplied message. For example
_, err := ioutil.ReadAll(r)
if err != nil {
return errors.Wrap(err, "read failed")
}
If additional control is required, the errors.WithStack and errors.WithMessage functions destructure errors.Wrap into its component operations: annotating an error with a stack trace and with a message, respectively.
Using errors.Wrap constructs a stack of errors, adding context to the preceding error. Depending on the nature of the error it may be necessary to reverse the operation of errors.Wrap to retrieve the original error for inspection. Any error value which implements this interface
type causer interface {
Cause() error
}
can be inspected by errors.Cause. errors.Cause will recursively retrieve the topmost error that does not implement causer, which is assumed to be the original cause. For example:
switch err := errors.Cause(err).(type) {
case *MyError:
// handle specifically
default:
// unknown error
}
Although the causer interface is not exported by this package, it is considered a part of its stable public interface.
All error values returned from this package implement fmt.Formatter and can be formatted by the fmt package. The following verbs are supported:
%s print the error. If the error has a Cause it will be
printed recursively.
%v see %s
%+v extended format. Each Frame of the error's StackTrace will
be printed in detail.
New, Errorf, Wrap, and Wrapf record a stack trace at the point they are invoked. This information can be retrieved with the following interface:
type stackTracer interface {
StackTrace() errors.StackTrace
}
The returned errors.StackTrace type is defined as
type StackTrace []Frame
The Frame type represents a call site in the stack trace. Frame supports the fmt.Formatter interface that can be used for printing information about the stack trace of this error. For example:
if err, ok := err.(stackTracer); ok {
for _, f := range err.StackTrace() {
fmt.Printf("%+s:%d\n", f, f)
}
}
Although the stackTracer interface is not exported by this package, it is considered a part of its stable public interface.
See the documentation for Frame.Format for more details.
Code:
type stackTracer interface {
StackTrace() errors.StackTrace
}
err, ok := errors.Cause(fn()).(stackTracer)
if !ok {
panic("oops, err does not implement stackTracer")
}
st := err.StackTrace()
fmt.Printf("%+v", st[0:2]) // top two frames
// Example output:
// github.com/pkg/errors_test.fn
// /home/dfc/src/github.com/pkg/errors/example_test.go:47
// github.com/pkg/errors_test.Example_stackTrace
// /home/dfc/src/github.com/pkg/errors/example_test.go:127
As finds the first error in err's chain that matches target, and if so, sets target to that error value and returns true.
The chain consists of err itself followed by the sequence of errors obtained by repeatedly calling Unwrap.
An error matches target if the error's concrete value is assignable to the value pointed to by target, or if the error has a method As(interface{}) bool such that As(target) returns true. In the latter case, the As method is responsible for setting target.
As will panic if target is not a non-nil pointer to either a type that implements error, or to any interface type. As returns false if err is nil.
Cause returns the underlying cause of the error, if possible. An error value has a cause if it implements the following interface:
type causer interface {
Cause() error
}
If the error does not implement Cause, the original error will be returned. If the error is nil, nil will be returned without further investigation.
Code:
err := fn() fmt.Println(err) fmt.Println(errors.Cause(err))
Output:
outer: middle: inner: error error
Code:
err := errors.Wrap(func() error {
return func() error {
return errors.New("hello world")
}()
}(), "failed")
fmt.Printf("%v", err)
Output:
failed: hello world
Errorf formats according to a format specifier and returns the string as a value that satisfies error. Errorf also records the stack trace at the point it was called.
Code:
err := errors.Errorf("whoops: %s", "foo")
fmt.Printf("%+v", err)
// Example output:
// whoops: foo
// github.com/pkg/errors_test.ExampleErrorf
// /home/dfc/src/github.com/pkg/errors/example_test.go:101
// testing.runExample
// /home/dfc/go/src/testing/example.go:114
// testing.RunExamples
// /home/dfc/go/src/testing/example.go:38
// testing.(*M).Run
// /home/dfc/go/src/testing/testing.go:744
// main.main
// /github.com/pkg/errors/_test/_testmain.go:102
// runtime.main
// /home/dfc/go/src/runtime/proc.go:183
// runtime.goexit
// /home/dfc/go/src/runtime/asm_amd64.s:2059
Is reports whether any error in err's chain matches target.
The chain consists of err itself followed by the sequence of errors obtained by repeatedly calling Unwrap.
An error is considered to match a target if it is equal to that target or if it implements a method Is(error) bool such that Is(target) returns true.
New returns an error with the supplied message. New also records the stack trace at the point it was called.
Code:
err := errors.New("whoops")
fmt.Println(err)
Output:
whoops
Code:
err := errors.New("whoops")
fmt.Printf("%+v", err)
// Example output:
// whoops
// github.com/pkg/errors_test.ExampleNew_printf
// /home/dfc/src/github.com/pkg/errors/example_test.go:17
// testing.runExample
// /home/dfc/go/src/testing/example.go:114
// testing.RunExamples
// /home/dfc/go/src/testing/example.go:38
// testing.(*M).Run
// /home/dfc/go/src/testing/testing.go:744
// main.main
// /github.com/pkg/errors/_test/_testmain.go:106
// runtime.main
// /home/dfc/go/src/runtime/proc.go:183
// runtime.goexit
// /home/dfc/go/src/runtime/asm_amd64.s:2059
Unwrap returns the result of calling the Unwrap method on err, if err's type contains an Unwrap method returning error. Otherwise, Unwrap returns nil.
WithMessage annotates err with a new message. If err is nil, WithMessage returns nil.
Code:
cause := errors.New("whoops")
err := errors.WithMessage(cause, "oh noes")
fmt.Println(err)
Output:
oh noes: whoops
WithMessagef annotates err with the format specifier. If err is nil, WithMessagef returns nil.
WithStack annotates err with a stack trace at the point WithStack was called. If err is nil, WithStack returns nil.
Code:
cause := errors.New("whoops")
err := errors.WithStack(cause)
fmt.Println(err)
Output:
whoops
Code:
cause := errors.New("whoops")
err := errors.WithStack(cause)
fmt.Printf("%+v", err)
// Example Output:
// whoops
// github.com/pkg/errors_test.ExampleWithStack_printf
// /home/fabstu/go/src/github.com/pkg/errors/example_test.go:55
// testing.runExample
// /usr/lib/go/src/testing/example.go:114
// testing.RunExamples
// /usr/lib/go/src/testing/example.go:38
// testing.(*M).Run
// /usr/lib/go/src/testing/testing.go:744
// main.main
// github.com/pkg/errors/_test/_testmain.go:106
// runtime.main
// /usr/lib/go/src/runtime/proc.go:183
// runtime.goexit
// /usr/lib/go/src/runtime/asm_amd64.s:2086
// github.com/pkg/errors_test.ExampleWithStack_printf
// /home/fabstu/go/src/github.com/pkg/errors/example_test.go:56
// testing.runExample
// /usr/lib/go/src/testing/example.go:114
// testing.RunExamples
// /usr/lib/go/src/testing/example.go:38
// testing.(*M).Run
// /usr/lib/go/src/testing/testing.go:744
// main.main
// github.com/pkg/errors/_test/_testmain.go:106
// runtime.main
// /usr/lib/go/src/runtime/proc.go:183
// runtime.goexit
// /usr/lib/go/src/runtime/asm_amd64.s:2086
Wrap returns an error annotating err with a stack trace at the point Wrap is called, and the supplied message. If err is nil, Wrap returns nil.
Code:
cause := errors.New("whoops")
err := errors.Wrap(cause, "oh noes")
fmt.Println(err)
Output:
oh noes: whoops
Code:
err := fn()
fmt.Printf("%+v\n", err)
// Example output:
// error
// github.com/pkg/errors_test.fn
// /home/dfc/src/github.com/pkg/errors/example_test.go:47
// github.com/pkg/errors_test.ExampleCause_printf
// /home/dfc/src/github.com/pkg/errors/example_test.go:63
// testing.runExample
// /home/dfc/go/src/testing/example.go:114
// testing.RunExamples
// /home/dfc/go/src/testing/example.go:38
// testing.(*M).Run
// /home/dfc/go/src/testing/testing.go:744
// main.main
// /github.com/pkg/errors/_test/_testmain.go:104
// runtime.main
// /home/dfc/go/src/runtime/proc.go:183
// runtime.goexit
// /home/dfc/go/src/runtime/asm_amd64.s:2059
// github.com/pkg/errors_test.fn
// /home/dfc/src/github.com/pkg/errors/example_test.go:48: inner
// github.com/pkg/errors_test.fn
// /home/dfc/src/github.com/pkg/errors/example_test.go:49: middle
// github.com/pkg/errors_test.fn
// /home/dfc/src/github.com/pkg/errors/example_test.go:50: outer
Wrapf returns an error annotating err with a stack trace at the point Wrapf is called, and the format specifier. If err is nil, Wrapf returns nil.
Code:
cause := errors.New("whoops")
err := errors.Wrapf(cause, "oh noes #%d", 2)
fmt.Println(err)
Output:
oh noes #2: whoops
Frame represents a program counter inside a stack frame. For historical reasons if Frame is interpreted as a uintptr its value represents the program counter + 1.
Format formats the frame according to the fmt.Formatter interface.
%s source file %d source line %n function name %v equivalent to %s:%d
Format accepts flags that alter the printing of some verbs, as follows:
%+s function name and path of source file relative to the compile time
GOPATH separated by \n\t (<funcname>\n\t<path>)
%+v equivalent to %+s:%d
MarshalText formats a stacktrace Frame as a text string. The output is the same as that of fmt.Sprintf("%+v", f), but without newlines or tabs.
StackTrace is stack of Frames from innermost (newest) to outermost (oldest).
func (st StackTrace) Format(s fmt.State, verb rune)
Format formats the stack of Frames according to the fmt.Formatter interface.
%s lists source files for each Frame in the stack %v lists the source file and line number for each Frame in the stack
Format accepts flags that alter the printing of some verbs, as follows:
%+v Prints filename, function, and line number for each Frame in the stack.
Package errors imports 7 packages (graph) and is imported by 44007 packages. Updated 2020-01-15. Refresh now. Tools for package owners.