README
¶
errors 
Package errors provides simple error handling primitives.
go get github.com/pkg/errors
The traditional error handling idiom in Go is roughly akin to
if err != nil {
return err
}
which 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.
Adding context to an error
The errors.Wrap function returns a new error that adds context to the original error. For example
_, err := ioutil.ReadAll(r)
if err != nil {
return errors.Wrap(err, "read failed")
}
Retrieving the cause of an error
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 can be inspected by errors.Cause.
type causer interface {
Cause() error
}
errors.Cause will recursively retrieve the topmost error which 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
}
Read the package documentation for more information.
Contributing
We welcome pull requests, bug fixes and issue reports. With that said, the bar for adding new symbols to this package is intentionally set high.
Before proposing a change, please discuss your change by raising an issue.
License
BSD-2-Clause
Documentation
¶
Overview ¶
Package errors provides simple error handling primitives.
The traditional error handling idiom in Go is roughly akin to
if err != nil {
return err
}
which 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.
Adding context to an error ¶
The errors.Annotate function returns a new error that adds context to the original error by recording a stack trace at the point Annotate is called, and the supplied message. For example
_, err := ioutil.ReadAll(r)
if err != nil {
return errors.Annotate(err, "read failed")
}
If additional control is required the errors.AddStack and errors.WithMessage functions destructure errors.Annotate into its component operations of annotating an error with a stack trace and an a message, respectively.
Retrieving the cause of an error ¶
Using errors.Annotate 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.Annotate 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 which 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
}
causer interface is not exported by this package, but is considered a part of stable public API. errors.Unwrap is also available: this will retrieve the next error in the chain.
Formatted printing of errors ¶
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.
Retrieving the stack trace of an error or wrapper ¶
New, Errorf, Annotate, and Annotatef record a stack trace at the point they are invoked. This information can be retrieved with the StackTracer interface that returns a StackTrace. Where errors.StackTrace 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 stacked := errors.GetStackTracer(err); stacked != nil {
for _, f := range stacked.StackTrace() {
fmt.Printf("%+s:%d", f)
}
}
See the documentation for Frame.Format for more details.
errors.Find can be used to search for an error in the error chain.
Example (StackTrace) ¶
package main
import (
"fmt"
"github.com/pkg/errors"
)
func fn() error {
e1 := errors.New("error")
e2 := errors.Wrap(e1, "inner")
e3 := errors.Wrap(e2, "middle")
return errors.Wrap(e3, "outer")
}
func main() {
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
}
Output:
Index ¶
- func AddStack(err error) error
- func AlreadyExistsf(format string, args ...interface{}) error
- func Annotate(err error, message string) error
- func Annotatef(err error, format string, args ...interface{}) error
- func BadRequestf(format string, args ...interface{}) error
- func Cause(err error) error
- func ErrorStack(err error) string
- func Errorf(format string, args ...interface{}) error
- func Errors(err error) []error
- func Find(origErr error, test func(error) bool) error
- func HasStack(err error) bool
- func IsAlreadyExists(err error) bool
- func IsNotFound(err error) bool
- func New(message string) error
- func NewNoStackError(msg string) error
- func NotFoundf(format string, args ...interface{}) error
- func NotSupportedf(format string, args ...interface{}) error
- func NotValidf(format string, args ...interface{}) error
- func SuspendStack(err error) error
- func Trace(err error) error
- func Unwrap(err error) error
- func WalkDeep(err error, visitor func(err error) bool) bool
- func WithMessage(err error, message string) error
- func WithStack(err error) error
- func Wrap(err error, message string) error
- func Wrapf(err error, format string, args ...interface{}) error
- type ErrorGroup
- type Frame
- type StackTrace
- type StackTraceAware
- type StackTracer
Examples ¶
Constants ¶
This section is empty.
Variables ¶
This section is empty.
Functions ¶
func AddStack ¶ added in v0.9.0
AddStack is similar to WithStack. However, it will first check with HasStack to see if a stack trace already exists in the causer chain before creating another one.
func AlreadyExistsf ¶ added in v0.11.0
AlreadyExistsf represents an error with already exists message.
func BadRequestf ¶ added in v0.9.0
BadRequestf represents an error with bad request message.
func Cause ¶
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.
Example ¶
package main
import (
"fmt"
"github.com/pkg/errors"
)
func fn() error {
e1 := errors.New("error")
e2 := errors.Wrap(e1, "inner")
e3 := errors.Wrap(e2, "middle")
return errors.Wrap(e3, "outer")
}
func main() {
err := fn()
fmt.Println(err)
fmt.Println(errors.Cause(err))
}
Output: outer: middle: inner: error error
Example (Printf) ¶
package main
import (
"fmt"
"github.com/pkg/errors"
)
func main() {
err := errors.Wrap(func() error {
return func() error {
return errors.Errorf("hello %s", fmt.Sprintf("world"))
}()
}(), "failed")
fmt.Printf("%v", err)
}
Output: failed: hello world
func ErrorStack ¶ added in v0.9.0
ErrorStack will format a stack trace if it is available, otherwise it will be Error() If the error is nil, the empty string is returned Note that this just calls fmt.Sprintf("%+v", err)
func Errorf ¶ added in v0.3.0
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.
Example (Extended) ¶
package main
import (
"fmt"
"github.com/pkg/errors"
)
func main() {
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
}
Output:
func Errors ¶ added in v0.10.0
Errors uses the ErrorGroup interface to return a slice of errors. If the ErrorGroup interface is not implemented it returns an array containing just the given error.
func Find ¶ added in v0.9.0
Find an error in the chain that matches a test function. returns nil if no error is found.
func IsAlreadyExists ¶ added in v0.11.0
IsAlreadyExists reports whether err was already exists error.
func IsNotFound ¶ added in v0.11.0
IsNotFound reports whether err was not found error.
func New ¶
New returns an error with the supplied message. New also records the stack trace at the point it was called.
Example ¶
package main
import (
"fmt"
"github.com/pkg/errors"
)
func main() {
err := errors.New("whoops")
fmt.Println(err)
}
Output: whoops
Example (Printf) ¶
package main
import (
"fmt"
"github.com/pkg/errors"
)
func main() {
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
}
Output:
func NewNoStackError ¶ added in v0.11.2
NewNoStackError creates error without error stack later duplicate trace will no longer generate Stack too.
func NotSupportedf ¶ added in v0.9.0
NotSupportedf represents an error with not supported message.
func SuspendStack ¶ added in v0.11.3
SuspendStack suspends stack generate for error.
func Unwrap ¶ added in v0.9.0
Unwrap uses causer to return the next error in the chain or nil. This goes one-level deeper, whereas Cause goes as far as possible
func WalkDeep ¶ added in v0.9.0
WalkDeep does a depth-first traversal of all errors. Any ErrorGroup is traversed (after going deep). The visitor function can return true to end the traversal early In that case, WalkDeep will return true, otherwise false.
func WithMessage ¶ added in v0.8.0
WithMessage annotates err with a new message. If err is nil, WithMessage returns nil.
Example ¶
package main
import (
"fmt"
"github.com/pkg/errors"
)
func main() {
cause := errors.New("whoops")
err := errors.WithMessage(cause, "oh noes")
fmt.Println(err)
}
Output: oh noes: whoops
func WithStack ¶ added in v0.8.0
WithStack annotates err with a stack trace at the point WithStack was called. If err is nil, WithStack returns nil.
For most use cases this is deprecated and AddStack should be used (which will ensure just one stack trace). However, one may want to use this in some situations, for example to create a 2nd trace across a goroutine.
Example ¶
package main
import (
"fmt"
"github.com/pkg/errors"
)
func main() {
cause := errors.New("whoops")
err := errors.WithStack(cause)
fmt.Println(err)
}
Output: whoops
Example (Printf) ¶
package main
import (
"fmt"
"github.com/pkg/errors"
)
func main() {
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
}
Output:
func Wrap ¶
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.
For most use cases this is deprecated in favor of Annotate. Annotate avoids creating duplicate stack traces.
Example ¶
package main
import (
"fmt"
"github.com/pkg/errors"
)
func main() {
cause := errors.New("whoops")
err := errors.Wrap(cause, "oh noes")
fmt.Println(err)
}
Output: oh noes: whoops
Example (Extended) ¶
package main
import (
"fmt"
"github.com/pkg/errors"
)
func fn() error {
e1 := errors.New("error")
e2 := errors.Wrap(e1, "inner")
e3 := errors.Wrap(e2, "middle")
return errors.Wrap(e3, "outer")
}
func main() {
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
}
Output:
func Wrapf ¶ added in v0.2.0
Wrapf returns an error annotating err with a stack trace at the point Wrapf is call, and the format specifier. If err is nil, Wrapf returns nil.
For most use cases this is deprecated in favor of Annotatef. Annotatef avoids creating duplicate stack traces.
Example ¶
package main
import (
"fmt"
"github.com/pkg/errors"
)
func main() {
cause := errors.New("whoops")
err := errors.Wrapf(cause, "oh noes #%d", 2)
fmt.Println(err)
}
Output: oh noes #2: whoops
Types ¶
type ErrorGroup ¶ added in v0.9.0
type ErrorGroup interface {
Errors() []error
}
ErrorGroup is an interface for multiple errors that are not a chain. This happens for example when executing multiple operations in parallel.
type Frame ¶ added in v0.6.0
type Frame uintptr
Frame represents a program counter inside a stack frame.
func (Frame) Format ¶ added in v0.6.0
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
type StackTrace ¶ added in v0.7.0
type StackTrace []Frame
StackTrace is stack of Frames from innermost (newest) to outermost (oldest).
func (StackTrace) Format ¶ added in v0.7.0
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.
type StackTraceAware ¶ added in v0.9.0
type StackTraceAware interface {
HasStack() bool
}
StackTraceAware is an optimization to avoid repetitive traversals of an error chain. HasStack checks for this marker first. Annotate/Wrap and Annotatef/Wrapf will produce this marker.
type StackTracer ¶ added in v0.9.0
type StackTracer interface {
StackTrace() StackTrace
}
StackTracer retrieves the StackTrace Generally you would want to use the GetStackTracer function to do that.
func GetStackTracer ¶ added in v0.9.0
func GetStackTracer(origErr error) StackTracer
GetStackTracer will return the first StackTracer in the causer chain. This function is used by AddStack to avoid creating redundant stack traces.
You can also use the StackTracer interface on the returned error to get the stack trace.
func NewStack ¶ added in v0.9.0
func NewStack(skip int) StackTracer
NewStack is for library implementers that want to generate a stack trace. Normally you should insted use AddStack to get an error with a stack trace.
The result of this function can be turned into a stack trace by calling .StackTrace()
This function takes an argument for the number of stack frames to skip. This avoids putting stack generation function calls like this one in the stack trace. A value of 0 will give you the line that called NewStack(0) A library author wrapping this in their own function will want to use a value of at least 1.
Source Files