Uber Go Style Guide

Table of Contents

Introduction

Styles are the conventions that govern our code. The term style is a bit of a
misnomer, since these conventions cover far more than just source file
formatting—gofmt handles that for us.

The goal of this guide is to manage this complexity by describing in detail the
Dos and Don’ts of writing Go code at Uber. These rules exist to keep the code
base manageable while still allowing engineers to use Go language features
productively.

This guide was originally created by Prashant Varanasi and Simon Newton as
a way to bring some colleagues up to speed with using Go. Over the years it has
been amended based on feedback from others.

This documents idiomatic conventions in Go code that we follow at Uber. A lot
of these are general guidelines for Go, while others extend upon external
resources:

  1. Effective Go
  2. The Go common mistakes guide

All code should be error-free when run through golint and go vet. We
recommend setting up your editor to:

  • Run goimports on save
  • Run golint and go vet to check for errors

You can find information in editor support for Go tools here:
https://github.com/golang/go/wiki/IDEsAndTextEditorPlugins

Guidelines

Pointers to Interfaces

You almost never need a pointer to an interface. You should be passing
interfaces as values—the underlying data can still be a pointer.

An interface is two fields:

  1. A pointer to some type-specific information. You can think of this as
    “type.”
  2. Data pointer. If the data stored is a pointer, it’s stored directly. If
    the data stored is a value, then a pointer to the value is stored.

If you want interface methods to modify the underlying data, you must use a
pointer.

Receivers and Interfaces

Methods with value receivers can be called on pointers as well as values.

For example,

type S struct {
  data string
}

func (s S) Read() string {
  return s.data
}

func (s *S) Write(str string) {
  s.data = str
}

sVals := map[int]S{1: {"A"}}

// You can only call Read using a value
sVals[1].Read()

// This will not compile:
//  sVals[1].Write("test")

sPtrs := map[int]*S{1: {"A"}}

// You can call both Read and Write using a pointer
sPtrs[1].Read()
sPtrs[1].Write("test")

Similarly, an interface can be satisfied by a pointer, even if the method has a
value receiver.

type F interface {
  f()
}

type S1 struct{}

func (s S1) f() {}

type S2 struct{}

func (s *S2) f() {}

s1Val := S1{}
s1Ptr := &S1{}
s2Val := S2{}
s2Ptr := &S2{}

var i F
i = s1Val
i = s1Ptr
i = s2Ptr

// The following doesn't compile, since s2Val is a value, and there is no value receiver for f.
//   i = s2Val

Effective Go has a good write up on Pointers vs. Values.

Zero-value Mutexes are Valid

The zero-value of sync.Mutex and sync.RWMutex is valid, so you almost
never need a pointer to a mutex.

Bad Good
“`go
mu := new(sync.Mutex)
mu.Lock()
“`
“`go
var mu sync.Mutex
mu.Lock()
“`

If you use a struct by pointer, then the mutex can be a non-pointer field.

Unexported structs that use a mutex to protect fields of the struct may embed
the mutex.

“`go
type smap struct {
sync.Mutex // only for unexported types

data map[string]string
}

func newSMap() *smap {
return &smap{
data: make(map[string]string),
}
}

func (m *smap) Get(k string) string {
m.Lock()
defer m.Unlock()

return m.data[k]
}
“`

“`go
type SMap struct {
mu sync.Mutex

data map[string]string
}

func NewSMap() *SMap {
return &SMap{
data: make(map[string]string),
}
}

func (m *SMap) Get(k string) string {
m.mu.Lock()
defer m.mu.Unlock()

return m.data[k]
}
“`

Embed for private types or types that need to implement the Mutex interface. For exported types, use a private field.

Copy Slices and Maps at Boundaries

Slices and maps contain pointers to the underlying data so be wary of scenarios
when they need to be copied.

Receiving Slices and Maps

Keep in mind that users can modify a map or slice you received as an argument
if you store a reference to it.

Bad Good
“`go
func (d *Driver) SetTrips(trips []Trip) {
d.trips = trips
}

trips := …
d1.SetTrips(trips)

// Did you mean to modify d1.trips?
trips[0] = …
“`

“`go
func (d *Driver) SetTrips(trips []Trip) {
d.trips = make([]Trip, len(trips))
copy(d.trips, trips)
}

trips := …
d1.SetTrips(trips)

// We can now modify trips[0] without affecting d1.trips.
trips[0] = …
“`

Returning Slices and Maps

Similarly, be wary of user modifications to maps or slices exposing internal
state.

Bad Good
“`go
type Stats struct {
sync.Mutex

counters map[string]int
}

// Snapshot returns the current stats.
func (s *Stats) Snapshot() map[string]int {
s.Lock()
defer s.Unlock()

return s.counters
}

// snapshot is no longer protected by the lock!
snapshot := stats.Snapshot()
“`

“`go
type Stats struct {
sync.Mutex

counters map[string]int
}

func (s *Stats) Snapshot() map[string]int {
s.Lock()
defer s.Unlock()

result := make(map[string]int, len(s.counters))
for k, v := range s.counters {
result[k] = v
}
return result
}

// Snapshot is now a copy.
snapshot := stats.Snapshot()
“`

Defer to Clean Up

Use defer to clean up resources such as files and locks.

Bad Good
“`go
p.Lock()
if p.count < 10 {
p.Unlock()
return p.count
}

p.count++
newCount := p.count
p.Unlock()

return newCount

// easy to miss unlocks due to multiple returns
“`

“`go
p.Lock()
defer p.Unlock()

if p.count < 10 {
return p.count
}

p.count++
return p.count

// more readable
“`

Defer has an extremely small overhead and should be avoided only if you can
prove that your function execution time is in the order of nanoseconds. The
readability win of using defers is worth the miniscule cost of using them. This
is especially true for larger methods that have more than simple memory
accesses, where the other computations are more significant than the defer.

Channel Size is One or None

Channels should usually have a size of one or be unbuffered. By default,
channels are unbuffered and have a size of zero. Any other size
must be subject to a high level of scrutiny. Consider how the size is
determined, what prevents the channel from filling up under load and blocking
writers, and what happens when this occurs.

Bad Good
“`go
// Ought to be enough for anybody!
c := make(chan int, 64)
“`
“`go
// Size of one
c := make(chan int, 1) // or
// Unbuffered channel, size of zero
c := make(chan int)
“`

Start Enums at One

The standard way of introducing enumerations in Go is to declare a custom type
and a const group with iota. Since variables have a 0 default value, you
should usually start your enums on a non-zero value.

Bad Good
“`go
type Operation int

const (
Add Operation = iota
Subtract
Multiply
)

// Add=0, Subtract=1, Multiply=2
“`

“`go
type Operation int

const (
Add Operation = iota + 1
Subtract
Multiply
)

// Add=1, Subtract=2, Multiply=3
“`

There are cases where using the zero value makes sense, for example when the
zero value case is the desirable default behavior.

type LogOutput int

const (
  LogToStdout LogOutput = iota
  LogToFile
  LogToRemote
)

// LogToStdout=0, LogToFile=1, LogToRemote=2

Error Types

There are various options for declaring errors:

  • [errors.New] for errors with simple static strings
  • [fmt.Errorf] for formatted error strings
  • Custom types that implement an Error() method
  • Wrapped errors using ["pkg/errors".Wrap]

When returning errors, consider the following to determine the best choice:

  • Is this a simple error that needs no extra information? If so, [errors.New]
    should suffice.
  • Do the clients need to detect and handle this error? If so, you should use a
    custom type, and implement the Error() method.
  • Are you propagating an error returned by a downstream function? If so, check
    the section on error wrapping.
  • Otherwise, [fmt.Errorf] is okay.

If the client needs to detect the error, and you have created a simple error
using [errors.New], use a var for the error.

Bad Good
“`go
// package foo

func Open() error {
return errors.New(“could not open”)
}

// package bar

func use() {
if err := foo.Open(); err != nil {
if err.Error() == “could not open” {
// handle
} else {
panic(“unknown error”)
}
}
}
“`

“`go
// package foo

var ErrCouldNotOpen = errors.New(“could not open”)

func Open() error {
return ErrCouldNotOpen
}

// package bar

if err := foo.Open(); err != nil {
if err == foo.ErrCouldNotOpen {
// handle
} else {
panic(“unknown error”)
}
}
“`

If you have an error that clients may need to detect, and you would like to add
more information to it (e.g., it is not a static string), then you should use a
custom type.

Bad Good
“`go
func open(file string) error {
return fmt.Errorf(“file %q not found”, file)
}

func use() {
if err := open(); err != nil {
if strings.Contains(err.Error(), “not found”) {
// handle
} else {
panic(“unknown error”)
}
}
}
“`

“`go
type errNotFound struct {
file string
}

func (e errNotFound) Error() string {
return fmt.Sprintf(“file %q not found”, e.file)
}

func open(file string) error {
return errNotFound{file: file}
}

func use() {
if err := open(); err != nil {
if _, ok := err.(errNotFound); ok {
// handle
} else {
panic(“unknown error”)
}
}
}
“`

Be careful with exporting custom error types directly since they become part of
the public API of the package. It is preferable to expose matcher functions to
check the error instead.

// package foo

type errNotFound struct {
  file string
}

func (e errNotFound) Error() string {
  return fmt.Sprintf("file %q not found", e.file)
}

func IsNotFoundError(err error) bool {
  _, ok := err.(errNotFound)
  return ok
}

func Open(file string) error {
  return errNotFound{file: file}
}

// package bar

if err := foo.Open("foo"); err != nil {
  if foo.IsNotFoundError(err) {
    // handle
  } else {
    panic("unknown error")
  }
}

Error Wrapping

There are three main options for propagating errors if a call fails:

  • Return the original error if there is no additional context to add and you
    want to maintain the original error type.
  • Add context using ["pkg/errors".Wrap] so that the error message provides
    more context and ["pkg/errors".Cause] can be used to extract the original
    error.
  • Use [fmt.Errorf] if the callers do not need to detect or handle that
    specific error case.

It is recommended to add context where possible so that instead of a vague
error such as “connection refused”, you get more useful errors such as
“call service foo: connection refused”.

When adding context to returned errors, keep the context succinct by avoiding
phrases like “failed to”, which state the obvious and pile up as the error
percolates up through the stack:

Bad Good
“`go
s, err := store.New()
if err != nil {
return fmt.Errorf(
“failed to create new store: %s”, err)
}
“`
“`go
s, err := store.New()
if err != nil {
return fmt.Errorf(
“new store: %s”, err)
}
“`
“`
failed to x: failed to y: failed to create new store: the error
“`
“`
x: y: new store: the error
“`

However once the error is sent to another system, it should be clear the
message is an error (e.g. an err tag or “Failed” prefix in logs).

See also Don’t just check errors, handle them gracefully.

Handle Type Assertion Failures

The single return value form of a type assertion will panic on an incorrect
type. Therefore, always use the “comma ok” idiom.

Bad Good
“`go
t := i.(string)
“`
“`go
t, ok := i.(string)
if !ok {
// handle the error gracefully
}
“`

Don’t Panic

Code running in production must avoid panics. Panics are a major source of
cascading failures. If an error occurs, the function must return an error and
allow the caller to decide how to handle it.

Bad Good
“`go
func foo(bar string) {
if len(bar) == 0 {
panic(“bar must not be empty”)
}
// …
}

func main() {
if len(os.Args) != 2 {
fmt.Println(“USAGE: foo <bar>”)
os.Exit(1)
}
foo(os.Args[1])
}
“`

“`go
func foo(bar string) error {
if len(bar) == 0
return errors.New(“bar must not be empty”)
}
// …
return nil
}

func main() {
if len(os.Args) != 2 {
fmt.Println(“USAGE: foo <bar>”)
os.Exit(1)
}
if err := foo(os.Args[1]); err != nil {
panic(err)
}
}
“`

Panic/recover is not an error handling strategy. A program must panic only when
something irrecoverable happens such as a nil dereference. An exception to this is
program initialization: bad things at program startup that should abort the
program may cause panic.

var _statusTemplate = template.Must(template.New("name").Parse("_statusHTML"))

Even in tests, prefer t.Fatal or t.FailNow over panics to ensure that the
test is marked as failed.

Bad Good
“`go
// func TestFoo(t *testing.T)

f, err := ioutil.TempFile(“”, “test”)
if err != nil {
panic(“failed to set up test”)
}
“`

“`go
// func TestFoo(t *testing.T)

f, err := ioutil.TempFile(“”, “test”)
if err != nil {
t.Fatal(“failed to set up test”)
}
“`

Use go.uber.org/atomic

Atomic operations with the sync/atomic package operate on the raw types
(int32, int64, etc.) so it is easy to forget to use the atomic operation to
read or modify the variables.

go.uber.org/atomic adds type safety to these operations by hiding the
underlying type. Additionally, it includes a convenient atomic.Bool type.

Bad Good
“`go
type foo struct {
running int32 // atomic
}

func (f* foo) start() {
if atomic.SwapInt32(&f.running, 1) == 1 {
// already running…
return
}
// start the Foo
}

func (f *foo) isRunning() bool {
return f.running == 1 // race!
}
“`

“`go
type foo struct {
running atomic.Bool
}

func (f *foo) start() {
if f.running.Swap(true) {
// already running…
return
}
// start the Foo
}

func (f *foo) isRunning() bool {
return f.running.Load()
}
“`

Performance

Performance-specific guidelines apply only to the hot path.

Prefer strconv over fmt

When converting primitives to/from strings, strconv is faster than
fmt.

Bad Good
“`go
for i := 0; i < b.N; i++ {
s := fmt.Sprint(rand.Int())
}
“`
“`go
for i := 0; i < b.N; i++ {
s := strconv.Itoa(rand.Int())
}
“`
“`
BenchmarkFmtSprint-4 143 ns/op 2 allocs/op
“`
“`
BenchmarkStrconv-4 64.2 ns/op 1 allocs/op
“`

Avoid string-to-byte conversion

Do not create byte slices from a fixed string repeatedly. Instead, perform the
conversion once and capture the result.

Bad Good
“`go
for i := 0; i < b.N; i++ {
w.Write([]byte(“Hello world”))
}
“`
“`go
data := []byte(“Hello world”)
for i := 0; i < b.N; i++ {
w.Write(data)
}
“`
“`
BenchmarkBad-4 50000000 22.2 ns/op
“`
“`
BenchmarkGood-4 500000000 3.25 ns/op
“`

Style

Group Similar Declarations

Go supports grouping similar declarations.

Bad Good
“`go
import “a”
import “b”
“`
“`go
import (
“a”
“b”
)
“`

This also applies to constants, variables, and type declarations.

Bad Good
“`go

const a = 1
const b = 2

var a = 1
var b = 2

type Area float64
type Volume float64
“`

“`go
const (
a = 1
b = 2
)

var (
a = 1
b = 2
)

type (
Area float64
Volume float64
)
“`

Only group related declarations. Do not group declarations that are unrelated.

Bad Good
“`go
type Operation int

const (
Add Operation = iota + 1
Subtract
Multiply
ENV_VAR = “MY_ENV”
)
“`

“`go
type Operation int

const (
Add Operation = iota + 1
Subtract
Multiply
)

const ENV_VAR = “MY_ENV”
“`

Groups are not limited in where they can be used. For example, you can use them
inside of functions.

Bad Good
“`go
func f() string {
var red = color.New(0xff0000)
var green = color.New(0x00ff00)
var blue = color.New(0x0000ff)


}
“`

“`go
func f() string {
var (
red = color.New(0xff0000)
green = color.New(0x00ff00)
blue = color.New(0x0000ff)
)


}
“`

Import Group Ordering

There should be two import groups:

  • Standard library
  • Everything else

This is the grouping applied by goimports by default.

Bad Good
“`go
import (
“fmt”
“os”
“go.uber.org/atomic”
“golang.org/x/sync/errgroup”
)
“`
“`go
import (
“fmt”
“os”

“go.uber.org/atomic”
“golang.org/x/sync/errgroup”
)
“`

Package Names

When naming packages, choose a name that is:

  • All lower-case. No capitals or underscores.
  • Does not need to be renamed using named imports at most call sites.
  • Short and succinct. Remember that the name is identified in full at every call
    site.
  • Not plural. For example, net/url, not net/urls.
  • Not “common”, “util”, “shared”, or “lib”. These are bad, uninformative names.

See also Package Names and Style guideline for Go packages.

Function Names

We follow the Go community’s convention of using MixedCaps for function
names
. An exception is made for test functions, which may contain underscores
for the purpose of grouping related test cases, e.g.,
TestMyFunction_WhatIsBeingTested.

Import Aliasing

Import aliasing must be used if the package name does not match the last
element of the import path.

import (
  "net/http"

  client "example.com/client-go"
  trace "example.com/trace/v2"
)

In all other scenarios, import aliases should be avoided unless there is a
direct conflict between imports.

Bad Good
“`go
import (
“fmt”
“os”

nettrace “golang.net/x/trace”
)
“`

“`go
import (
“fmt”
“os”
“runtime/trace”

nettrace “golang.net/x/trace”
)
“`

Function Grouping and Ordering

  • Functions should be sorted in rough call order.
  • Functions in a file should be grouped by receiver.

Therefore, exported functions should appear first in a file, after
struct, const, var definitions.

A newXYZ()/NewXYZ() may appear after the type is defined, but before the
rest of the methods on the receiver.

Since functions are grouped by receiver, plain utility functions should appear
towards the end of the file.

Bad Good
“`go
func (s *something) Cost() {
return calcCost(s.weights)
}

type something struct{ … }

func calcCost(n []int) int {…}

func (s *something) Stop() {…}

func newSomething() *something {
return &something{}
}
“`

“`go
type something struct{ … }

func newSomething() *something {
return &something{}
}

func (s *something) Cost() {
return calcCost(s.weights)
}

func (s *something) Stop() {…}

func calcCost(n []int) int {…}
“`

Reduce Nesting

Code should reduce nesting where possible by handling error cases/special
conditions first and returning early or continuing the loop. Reduce the amount
of code that is nested multiple levels.

Bad Good
“`go
for _, v := range data {
if v.F1 == 1 {
v = process(v)
if err := v.Call(); err == nil {
v.Send()
} else {
return err
}
} else {
log.Printf(“Invalid v: %v”, v)
}
}
“`
“`go
for _, v := range data {
if v.F1 != 1 {
log.Printf(“Invalid v: %v”, v)
continue
}

v = process(v)
if err := v.Call(); err != nil {
return err
}
v.Send()
}
“`

Unnecessary Else

If a variable is set in both branches of an if, it can be replaced with a
single if.

Bad Good
“`go
var a int
if b {
a = 100
} else {
a = 10
}
“`
“`go
a := 10
if b {
a = 100
}
“`

Top-level Variable Declarations

At the top level, use the standard var keyword. Do not specify the type,
unless it is not the same type as the expression.

Bad Good
“`go
var _s string = F()

func F() string { return “A” }
“`

“`go
var _s = F()
// Since F already states that it returns a string, we don’t need to specify
// the type again.

func F() string { return “A” }
“`

Specify the type if the type of the expression does not match the desired type
exactly.

type myError struct{}

func (myError) Error() string { return "error" }

func F() myError { return myError{} }

var _e error = F()
// F returns an object of type myError but we want error.

Prefix Unexported Globals with _

Prefix unexported top-level vars and consts with _ to make it clear when
they are used that they are global symbols.

Exception: Unexported error values, which should be prefixed with err.

Rationale: Top-level variables and constants have a package scope. Using a
generic name makes it easy to accidentally use the wrong value in a different
file.

Bad Good
“`go
// foo.go

const (
defaultPort = 8080
defaultUser = “user”
)

// bar.go

func Bar() {
defaultPort := 9090

fmt.Println(“Default port”, defaultPort)

// We will not see a compile error if the first line of
// Bar() is deleted.
}
“`

“`go
// foo.go

const (
_defaultPort = 8080
_defaultUser = “user”
)
“`

Embedding in Structs

Embedded types (such as mutexes) should be at the top of the field list of a
struct, and there must be an empty line separating embedded fields from regular
fields.

Bad Good
“`go
type Client struct {
version int
http.Client
}
“`
“`go
type Client struct {
http.Client

version int
}
“`

Use Field Names to initialize Structs

You should almost always specify field names when initializing structs. This is
now enforced by [go vet].

Bad Good
“`go
k := User{“John”, “Doe”, true}
“`
“`go
k := User{
FirstName: “John”,
LastName: “Doe”,
Admin: true,
}
“`

Exception: Field names may be omitted in test tables when there are 3 or
fewer fields.

tests := []struct{
  op Operation
  want string
}{
  {Add, "add"},
  {Subtract, "subtract"},
}

Local Variable Declarations

Short variable declarations (:=) should be used if a variable is being set to
some value explicitly.

Bad Good
“`go
var s = “foo”
“`
“`go
s := “foo”
“`

However, there are cases where the default value is clearer when the var
keyword is use. Declaring Empty Slices, for example.

Bad Good
“`go
func f(list []int) {
filtered := []int{}
for _, v := range list {
if v > 10 {
filtered = append(filtered, v)
}
}
}
“`
“`go
func f(list []int) {
var filtered []int
for _, v := range list {
if v > 10 {
filtered = append(filtered, v)
}
}
}
“`

nil is a valid slice

nil is a valid slice of length 0. This means that,

  • You should not return a slice of length zero explicitly. Return nil
    instead.
Bad Good
“`go
if x == “” {
return []int{}
}
“`
“`go
if x == “” {
return nil
}
“`
  • To check if a slice is empty, always use len(s) == 0. Do not check for
    nil.
Bad Good
“`go
func isEmpty(s []string) bool {
return s == nil
}
“`
“`go
func isEmpty(s []string) bool {
return len(s) == 0
}
“`
  • The zero value (a slice declared with var) is usable immediately without
    make().
Bad Good
“`go
nums := []int{}
// or, nums := make([]int)

if add1 {
nums = append(nums, 1)
}

if add2 {
nums = append(nums, 2)
}
“`

“`go
var nums []int

if add1 {
nums = append(nums, 1)
}

if add2 {
nums = append(nums, 2)
}
“`

Reduce Scope of Variables

Where possible, reduce scope of variables. Do not reduce the scope if it
conflicts with Reduce Nesting.

Bad Good
“`go
err := ioutil.WriteFile(name, data, 0644)
if err != nil {
return err
}
“`
“`go
if err := ioutil.WriteFile(name, data, 0644); err != nil {
return err
}
“`

If you need a result of a function call outside of the if, then you should not
try to reduce the scope.

Bad Good
“`go
if data, err := ioutil.ReadFile(name); err == nil {
err = cfg.Decode(data)
if err != nil {
return err
}

fmt.Println(cfg)
return nil
} else {
return err
}
“`

“`go
data, err := ioutil.ReadFile(name)
if err != nil {
return err
}

if err := cfg.Decode(data); err != nil {
return err
}

fmt.Println(cfg)
return nil
“`

Avoid Naked Parameters

Naked parameters in function calls can hurt readability. Add C-style comments
(/* ... */) for parameter names when their meaning is not obvious.

Bad Good
“`go
// func printInfo(name string, isLocal, done bool)

printInfo(“foo”, true, true)
“`

“`go
// func printInfo(name string, isLocal, done bool)

printInfo(“foo”, true /* isLocal */, true /* done */)
“`

Better yet, replace naked bool types with custom types for more readable and
type-safe code. This allows more than just two states (true/false) for that
parameter in the future.

type Region int

const (
  UnknownRegion Region = iota
  Local
)

type Status int

const (
  StatusReady = iota + 1
  StatusDone
  // Maybe we will have a StatusInProgress in the future.
)

func printInfo(name string, region Region, status Status)

Use Raw String Literals to Avoid Escaping

Go supports raw string literals,
which can span multiple lines and include quotes. Use these to avoid
hand-escaped strings which are much harder to read.

Bad Good
“`go
wantError := “unknown name:\”test\””
“`
“`go
wantError := `unknown error:”test”`
“`

</td></tr>
</tbody></table>

<h3>Initializing Struct References</h3>

Use <code>&T{}</code> instead of <code>new(T)</code> when initializing struct references so that it
is consistent with the struct initialization.

<table>
<thead><tr><th>Bad</th><th>Good</th></tr></thead>
<tbody>
<tr><td>

“`go
sval := T{Name: “foo”}

// inconsistent
sptr := new(T)
sptr.Name = “bar”
“`

</td><td>

“`go
sval := T{Name: “foo”}

sptr := &T{Name: “bar”}
“`

</td></tr>
</tbody></table>

<h3>Format Strings outside Printf</h3>

If you declare format strings for <code>Printf</code>-style functions outside a string
literal, make them <code>const</code> values.

This helps <code>go vet</code> perform static analysis of the format string.

<table>
<thead><tr><th>Bad</th><th>Good</th></tr></thead>
<tbody>
<tr><td>

“`go
msg := “unexpected values %v, %v\n”
fmt.Printf(msg, 1, 2)
“`

</td><td>

“`go
const msg = “unexpected values %v, %v\n”
fmt.Printf(msg, 1, 2)
“`

</td></tr>
</tbody></table>

<h3>Naming Printf-style Functions</h3>

When you declare a <code>Printf</code>-style function, make sure that <code>go vet</code> can detect
it and check the format string.

This means that you should use pre-defined <code>Printf</code>-style function
names if possible. <code>go vet</code> will check these by default. See <a href=”https://golang.org/cmd/vet/#hdr-Printf_family”>Printf family</a>
for more information.

If using the pre-defined names is not an option, end the name you choose with
f: <code>Wrapf</code>, not <code>Wrap</code>. <code>go vet</code> can be asked to check specific <code>Printf</code>-style
names but they must end with f.

<pre><code class=”language-shell “>$ go vet -printfuncs=wrapf,statusf
</code></pre>

See also <a href=”https://kuzminva.wordpress.com/2017/11/07/go-vet-printf-family-check/”>go vet: Printf family check</a>.

<h2>Patterns</h2>

<h3>Test Tables</h3>

Use table-driven tests with <a href=”https://blog.golang.org/subtests”>subtests</a> to avoid duplicating code when the core
test logic is repetitive.

<table>
<thead><tr><th>Bad</th><th>Good</th></tr></thead>
<tbody>
<tr><td>

“`go
// func TestSplitHostPort(t *testing.T)

host, port, err := net.SplitHostPort(“192.0.2.0:8000”)
require.NoError(t, err)
assert.Equal(t, “192.0.2.0”, host)
assert.Equal(t, “8000”, port)

host, port, err = net.SplitHostPort(“192.0.2.0:http”)
require.NoError(t, err)
assert.Equal(t, “192.0.2.0”, host)
assert.Equal(t, “http”, port)

host, port, err = net.SplitHostPort(“:8000”)
require.NoError(t, err)
assert.Equal(t, “”, host)
assert.Equal(t, “8000”, port)

host, port, err = net.SplitHostPort(“1:8”)
require.NoError(t, err)
assert.Equal(t, “1”, host)
assert.Equal(t, “8”, port)
“`

</td><td>

“`go
// func TestSplitHostPort(t *testing.T)

tests := []struct{
give string
wantHost string
wantPort string
}{
{
give: “192.0.2.0:8000”,
wantHost: “192.0.2.0”,
wantPort: “8000”,
},
{
give: “192.0.2.0:http”,
wantHost: “192.0.2.0”,
wantPort: “http”,
},
{
give: “:8000”,
wantHost: “”,
wantPort: “8000”,
},
{
give: “1:8”,
wantHost: “1”,
wantPort: “8”,
},
}

for _, tt := range tests {
t.Run(tt.give, func(t *testing.T) {
host, port, err := net.SplitHostPort(tt.give)
require.NoError(t, err)
assert.Equal(t, tt.wantHost, host)
assert.Equal(t, tt.wantPort, port)
})
}
“`

</td></tr>
</tbody></table>

Test tables make it easier to add context to error messages, reduce duplicate
logic, and add new test cases.

We follow the convention that the slice of structs is referred to as <code>tests</code>
and each test case <code>tt</code>. Further, we encourage explicating the input and output
values for each test case with <code>give</code> and <code>want</code> prefixes.

<pre><code class=”language-go “>tests := []struct{
give string
wantHost string
wantPort string
}{
// …
}

for _, tt := range tests {
// …
}
</code></pre>

<h3>Functional Options</h3>

Functional options is a pattern in which you declare an opaque <code>Option</code> type
that records information in some internal struct. You accept a variadic number
of these options and act upon the full information recorded by the options on
the internal struct.

Use this pattern for optional arguments in constructors and other public APIs
that you foresee needing to expand, especially if you already have three or
more arguments on those functions.

<table>
<thead><tr><th>Bad</th><th>Good</th></tr></thead>
<tbody>
<tr><td>

“`go
// package db

func Connect(
addr string,
timeout time.Duration,
caching bool,
) (*Connection, error) {
// …
}

// Timeout and caching must always be provided,
// even if the user wants to use the default.

db.Connect(addr, db.DefaultTimeout, db.DefaultCaching)
db.Connect(addr, newTimeout, db.DefaultCaching)
db.Connect(addr, db.DefaultTimeout, false /* caching */)
db.Connect(addr, newTimeout, false /* caching */)
“`

</td><td>

“`go
type options struct {
timeout time.Duration
caching bool
}

// Option overrides behavior of Connect.
type Option interface {
apply(*options)
}

type optionFunc func(*options)

func (f optionFunc) apply(o *options) {
f(o)
}

func WithTimeout(t time.Duration) Option {
return optionFunc(func(o *options) {
o.timeout = t
})
}

func WithCaching(cache bool) Option {
return optionFunc(func(o *options) {
o.caching = cache
})
}

// Connect creates a connection.
func Connect(
addr string,
opts …Option,
) (*Connection, error) {
options := options{
timeout: defaultTimeout,
caching: defaultCaching,
}

for _, o := range opts {
o.apply(&options)
}

// …
}

// Options must be provided only if needed.

db.Connect(addr)
db.Connect(addr, db.WithTimeout(newTimeout))
db.Connect(addr, db.WithCaching(false))
db.Connect(
addr,
db.WithCaching(false),
db.WithTimeout(newTimeout),
)
“`

See also,

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