github.com/graybobo/golang.org-package-offline-cache@v0.0.0-20200626051047-6608995c132f/x/blog/content/error-handling-and-go.article (about) 1 Error handling and Go 2 12 Jul 2011 3 Tags: error, interface, type, technical 4 5 Andrew Gerrand 6 7 * Introduction 8 9 If you have written any Go code you have probably encountered the built-in `error` type. Go code uses `error` values to indicate an abnormal state. For example, the `os.Open` function returns a non-nil `error` value when it fails to open a file. 10 11 func Open(name string) (file *File, err error) 12 13 The following code uses `os.Open` to open a file. If an error occurs it calls `log.Fatal` to print the error message and stop. 14 15 f, err := os.Open("filename.ext") 16 if err != nil { 17 log.Fatal(err) 18 } 19 // do something with the open *File f 20 21 You can get a lot done in Go knowing just this about the `error` type, but in this article we'll take a closer look at `error` and discuss some good practices for error handling in Go. 22 23 * The error type 24 25 The `error` type is an interface type. An `error` variable represents any value that can describe itself as a string. Here is the interface's declaration: 26 27 type error interface { 28 Error() string 29 } 30 31 The `error` type, as with all built in types, is [[http://golang.org/doc/go_spec.html#Predeclared_identifiers][predeclared]] in the [[http://golang.org/doc/go_spec.html#Blocks][universe block]]. 32 33 The most commonly-used `error` implementation is the [[http://golang.org/pkg/errors/][errors]] package's unexported `errorString` type. 34 35 // errorString is a trivial implementation of error. 36 type errorString struct { 37 s string 38 } 39 40 func (e *errorString) Error() string { 41 return e.s 42 } 43 44 You can construct one of these values with the `errors.New` function. It takes a string that it converts to an `errors.errorString` and returns as an `error` value. 45 46 // New returns an error that formats as the given text. 47 func New(text string) error { 48 return &errorString{text} 49 } 50 51 Here's how you might use `errors.New`: 52 53 func Sqrt(f float64) (float64, error) { 54 if f < 0 { 55 return 0, errors.New("math: square root of negative number") 56 } 57 // implementation 58 } 59 60 A caller passing a negative argument to `Sqrt` receives a non-nil `error` value (whose concrete representation is an `errors.errorString` value). The caller can access the error string ("math: square root of...") by calling the `error`'s `Error` method, or by just printing it: 61 62 f, err := Sqrt(-1) 63 if err != nil { 64 fmt.Println(err) 65 } 66 67 The [[http://golang.org/pkg/fmt/][fmt]] package formats an `error` value by calling its `Error()`string` method. 68 69 It is the error implementation's responsibility to summarize the context. The error returned by `os.Open` formats as "open /etc/passwd: permission denied," not just "permission denied." The error returned by our `Sqrt` is missing information about the invalid argument. 70 71 To add that information, a useful function is the `fmt` package's `Errorf`. It formats a string according to `Printf`'s rules and returns it as an `error` created by `errors.New`. 72 73 if f < 0 { 74 return 0, fmt.Errorf("math: square root of negative number %g", f) 75 } 76 77 In many cases `fmt.Errorf` is good enough, but since `error` is an interface, you can use arbitrary data structures as error values, to allow callers to inspect the details of the error. 78 79 For instance, our hypothetical callers might want to recover the invalid argument passed to `Sqrt`. We can enable that by defining a new error implementation instead of using `errors.errorString`: 80 81 type NegativeSqrtError float64 82 83 func (f NegativeSqrtError) Error() string { 84 return fmt.Sprintf("math: square root of negative number %g", float64(f)) 85 } 86 87 A sophisticated caller can then use a [[http://golang.org/doc/go_spec.html#Type_assertions][type assertion]] to check for a `NegativeSqrtError` and handle it specially, while callers that just pass the error to `fmt.Println` or `log.Fatal` will see no change in behavior. 88 89 As another example, the [[http://golang.org/pkg/encoding/json/][json]] package specifies a `SyntaxError` type that the `json.Decode` function returns when it encounters a syntax error parsing a JSON blob. 90 91 type SyntaxError struct { 92 msg string // description of error 93 Offset int64 // error occurred after reading Offset bytes 94 } 95 96 func (e *SyntaxError) Error() string { return e.msg } 97 98 The `Offset` field isn't even shown in the default formatting of the error, but callers can use it to add file and line information to their error messages: 99 100 if err := dec.Decode(&val); err != nil { 101 if serr, ok := err.(*json.SyntaxError); ok { 102 line, col := findLine(f, serr.Offset) 103 return fmt.Errorf("%s:%d:%d: %v", f.Name(), line, col, err) 104 } 105 return err 106 } 107 108 (This is a slightly simplified version of some [[http://camlistore.org/code/?p=camlistore.git;a=blob;f=lib/go/camli/jsonconfig/eval.go#l68][actual code]] from the [[http://camlistore.org][Camlistore]] project.) 109 110 The `error` interface requires only a `Error` method; specific error implementations might have additional methods. For instance, the [[http://golang.org/pkg/net/][net]] package returns errors of type `error`, following the usual convention, but some of the error implementations have additional methods defined by the `net.Error` interface: 111 112 package net 113 114 type Error interface { 115 error 116 Timeout() bool // Is the error a timeout? 117 Temporary() bool // Is the error temporary? 118 } 119 120 Client code can test for a `net.Error` with a type assertion and then distinguish transient network errors from permanent ones. For instance, a web crawler might sleep and retry when it encounters a temporary error and give up otherwise. 121 122 if nerr, ok := err.(net.Error); ok && nerr.Temporary() { 123 time.Sleep(1e9) 124 continue 125 } 126 if err != nil { 127 log.Fatal(err) 128 } 129 130 * Simplifying repetitive error handling 131 132 In Go, error handling is important. The language's design and conventions encourage you to explicitly check for errors where they occur (as distinct from the convention in other languages of throwing exceptions and sometimes catching them). In some cases this makes Go code verbose, but fortunately there are some techniques you can use to minimize repetitive error handling. 133 134 Consider an [[http://code.google.com/appengine/docs/go/][App Engine]] application with an HTTP handler that retrieves a record from the datastore and formats it with a template. 135 136 func init() { 137 http.HandleFunc("/view", viewRecord) 138 } 139 140 func viewRecord(w http.ResponseWriter, r *http.Request) { 141 c := appengine.NewContext(r) 142 key := datastore.NewKey(c, "Record", r.FormValue("id"), 0, nil) 143 record := new(Record) 144 if err := datastore.Get(c, key, record); err != nil { 145 http.Error(w, err.Error(), 500) 146 return 147 } 148 if err := viewTemplate.Execute(w, record); err != nil { 149 http.Error(w, err.Error(), 500) 150 } 151 } 152 153 This function handles errors returned by the `datastore.Get` function and `viewTemplate`'s `Execute` method. In both cases, it presents a simple error message to the user with the HTTP status code 500 ("Internal Server Error"). This looks like a manageable amount of code, but add some more HTTP handlers and you quickly end up with many copies of identical error handling code. 154 155 To reduce the repetition we can define our own HTTP `appHandler` type that includes an `error` return value: 156 157 type appHandler func(http.ResponseWriter, *http.Request) error 158 159 Then we can change our `viewRecord` function to return errors: 160 161 func viewRecord(w http.ResponseWriter, r *http.Request) error { 162 c := appengine.NewContext(r) 163 key := datastore.NewKey(c, "Record", r.FormValue("id"), 0, nil) 164 record := new(Record) 165 if err := datastore.Get(c, key, record); err != nil { 166 return err 167 } 168 return viewTemplate.Execute(w, record) 169 } 170 171 This is simpler than the original version, but the [[http://golang.org/pkg/net/http/][http]] package doesn't understand functions that return `error`. To fix this we can implement the `http.Handler` interface's `ServeHTTP` method on `appHandler`: 172 173 func (fn appHandler) ServeHTTP(w http.ResponseWriter, r *http.Request) { 174 if err := fn(w, r); err != nil { 175 http.Error(w, err.Error(), 500) 176 } 177 } 178 179 The `ServeHTTP` method calls the `appHandler` function and displays the returned error (if any) to the user. Notice that the method's receiver, `fn`, is a function. (Go can do that!) The method invokes the function by calling the receiver in the expression `fn(w,`r)`. 180 181 Now when registering `viewRecord` with the http package we use the `Handle` function (instead of `HandleFunc`) as `appHandler` is an `http.Handler` (not an `http.HandlerFunc`). 182 183 func init() { 184 http.Handle("/view", appHandler(viewRecord)) 185 } 186 187 With this basic error handling infrastructure in place, we can make it more user friendly. Rather than just displaying the error string, it would be better to give the user a simple error message with an appropriate HTTP status code, while logging the full error to the App Engine developer console for debugging purposes. 188 189 To do this we create an `appError` struct containing an `error` and some other fields: 190 191 type appError struct { 192 Error error 193 Message string 194 Code int 195 } 196 197 Next we modify the appHandler type to return `*appError` values: 198 199 type appHandler func(http.ResponseWriter, *http.Request) *appError 200 201 (It's usually a mistake to pass back the concrete type of an error rather than `error`, for reasons discussed in [[http://golang.org/doc/go_faq.html#nil_error][the Go FAQ]], but it's the right thing to do here because `ServeHTTP` is the only place that sees the value and uses its contents.) 202 203 And make `appHandler`'s `ServeHTTP` method display the `appError`'s `Message` to the user with the correct HTTP status `Code` and log the full `Error` to the developer console: 204 205 func (fn appHandler) ServeHTTP(w http.ResponseWriter, r *http.Request) { 206 if e := fn(w, r); e != nil { // e is *appError, not os.Error. 207 c := appengine.NewContext(r) 208 c.Errorf("%v", e.Error) 209 http.Error(w, e.Message, e.Code) 210 } 211 } 212 213 Finally, we update `viewRecord` to the new function signature and have it return more context when it encounters an error: 214 215 func viewRecord(w http.ResponseWriter, r *http.Request) *appError { 216 c := appengine.NewContext(r) 217 key := datastore.NewKey(c, "Record", r.FormValue("id"), 0, nil) 218 record := new(Record) 219 if err := datastore.Get(c, key, record); err != nil { 220 return &appError{err, "Record not found", 404} 221 } 222 if err := viewTemplate.Execute(w, record); err != nil { 223 return &appError{err, "Can't display record", 500} 224 } 225 return nil 226 } 227 228 This version of `viewRecord` is the same length as the original, but now each of those lines has specific meaning and we are providing a friendlier user experience. 229 230 It doesn't end there; we can further improve the error handling in our application. Some ideas: 231 232 - give the error handler a pretty HTML template, 233 234 - make debugging easier by writing the stack trace to the HTTP response when the user is an administrator, 235 236 - write a constructor function for `appError` that stores the stack trace for easier debugging, 237 238 - recover from panics inside the `appHandler`, logging the error to the console as "Critical," while telling the user "a serious error has occurred." This is a nice touch to avoid exposing the user to inscrutable error messages caused by programming errors. See the [[http://golang.org/doc/articles/defer_panic_recover.html][Defer, Panic, and Recover]] article for more details. 239 240 * Conclusion 241 242 Proper error handling is an essential requirement of good software. By employing the techniques described in this post you should be able to write more reliable and succinct Go code.