uber-go-style-guide-kr

Translated in Korean

Currently WIP, but translation will done by 20th of Oct, 2019

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Uber의 Go언어 스타일 가이드 (Uber's Go Style Guide)

• uber-go-style-guide-kr
• Uber의 Go언어 스타일 가이드 (Uber's Go Style Guide)
  • 소개 (Introduction)
  • 가이드라인 (Guidelines)
    • 인터페이스에 대한 포인터 (Pointers to Interfaces)
    • 수신자(Receivers)와 인터페이스(Interfaces)
    • 제로 값 뮤텍스(Zero-value Mutexes)는 유효하다
    • 슬라이스 복사(Copy Slices)와 바운더리 에서의 맵(Maps at Boundaries)
      • Slices와 Maps의 수신(receiving)
      • 슬라이스(Slices)와 맵(Maps)의 리턴
    • Defer to Clean Up
    • Channel Size is One or None
    • Start Enums at One
    • Error Types
    • Error Wrapping
    • Handle Type Assertion Failures
    • Don't Panic
    • Use go.uber.org/atomic
  • Performance
    • Prefer strconv over fmt
    • Avoid string-to-byte conversion
  • Style
    • Group Similar Declarations
    • Import Group Ordering
    • Package Names
    • Function Names
    • Import Aliasing
    • Function Grouping and Ordering
    • Reduce Nesting
    • Unnecessary Else
    • Top-level Variable Declarations
    • Prefix Unexported Globals with _
    • Embedding in Structs
    • Use Field Names to initialize Structs
    • Local Variable Declarations
    • nil is a valid slice
    • Reduce Scope of Variables
    • Avoid Naked Parameters
    • Use Raw String Literals to Avoid Escaping
    • Initializing Struct References
    • Format Strings outside Printf
    • Naming Printf-style Functions
  • Patterns
    • Test Tables
    • Functional Options

소개 (Introduction)

스타일은 코드를 통제하는(govern) 관습이다. 이러한 관습(convention)은 소스파일 포맷팅 (e.g. gofmt)보다 더 많은 영역을 다루기(cover) 때문에, "스타일" 이라는 단어 자체가 약간 부적절 할 수 있다.

본 가이드의 목표는 Uber에서 Go 코드를 작성할 때 해야 할 것과 하지 말아야 할 것 (Dos and Don'ts)에 대하여 자세하게 설명하여 이러한 복잡성을 관리하는 것이다. 이런 규칙들은 엔지니어들이 Go 언어의 특성을(feature) 생산적으로개계속 사용할 수 있도록 코드 베이스를 관리가능하게 유지하기위해 존재한다.

이 가이드는 원래 Prashant Varanasi와 Simon Newton이 동료들에게 Go를 사용하면서 개발속도 향상을 도모하기 위해 소개되었다. 또한, 수 년에 거쳐서 다른 사람들로부터의 피드백을 통해서 개정되 오고 있다.

이 문서는 Uber에서의 엔지니어들이 지향하는 Go언어 코드의 관용적 규칙을 설명한다. 상당 수의 규칙들은 Go언어에 대한 일반적인 가이드라인이며, 다른 부분에 대해서는 외부 레퍼런스에 의해 확장된다 (아래 참고)

1. Effective Go
2. The Go common mistakes guide

모든 코드는 golint 와 go vet를 실행할 때 에러가 없어야 한다. 또한 우리는 여러분들의 에디터를 아래와 같이 설정하기를 권고한다:

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

아래의 링크를 통해서 Go 툴을 지원하는 에디터에 대한 정보를 얻을 수 있다: https://github.com/golang/go/wiki/IDEsAndTextEditorPlugins

가이드라인 (Guidelines)

인터페이스에 대한 포인터 (Pointers to Interfaces)

일반적으로 인터페이스에 대한 포인터는 거의 필요하지 않을 것이다. 여러분들은 인터페이스를 값(value)으로서 전달(passing)해야 할 것이며, 인터페이스에 대한 기본 데이터(underlying data)는 여전히 포인터가 될 수 있다.

한 인터페이스는 두 가지 필드이다:

1. 타입-특정 정보(type-specific information)에 대한 포인터. 여러분들을 이것을 "타입"으로 간주할 수 있다.
2. 데이터 포인터. 저장된 데이터가 포인터일 경우, 이것은 직접적으로 저장될 수 있다. 만약, 저장된 데이터가 값(value)인 경우, 값에 대한 포인터가 저장된다.

만약 여러분들이 기본 데이터(underlying data) 수정하기 위한 인터페이스 메서드 (interface methods)를 원한다면, 여러분들은 반드시 포인터를 사용해야 한다.

수신자(Receivers)와 인터페이스(Interfaces)

값 수신자 (value receivers)와 메서드(Methods)는 포인터 혹은 값에 의해서 호출 될 수 있다.

예를 들면,

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"}}

// 오직 값만 사용하여 Read를 호출 할 수 있다.
sVals[1].Read()

// 아래 코드는 컴파일 되지 않을 것:
//  sVals[1].Write("test")

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

// 포인터를 사용하여 Read와 Write 모두 호출 할 수 있다.
sPtrs[1].Read()
sPtrs[1].Write("test")

마찬가지로, 메서드가 값 수신자(value receiver)를 가지고 있다고 하더라도 포인터가 인터페이스를 충족시킬 수 있다.

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

// s2Val이 값이고 f에 대한 수신자가 없기 때문에, 아래의 코드는 컴파일 되지 않는다.
//   i = s2Val

Effective Go에 Pointers vs. Values에 대한 좋은 글이 있으니 참고하기 바란다.

제로 값 뮤텍스(Zero-value Mutexes)는 유효하다

sync.Mutex와 sync.RWMutex 의 제로 값은 유효하므로, 거의 대부분의 경우 뮤텍스에 대한 포인터는 필요로 하지 않는다.

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

포인터로 구조체를 사용할 경우, 뮤텍스는 포인터가 아닌 필드(non-pointer field)가 될 수 있다.

구조체의 필드를 보호하기 위해 뮤텍스를 사용한 수출되지 않는 구조체(unexported structs)는 뮤텍스를 포함(embed) 할 수 있다.

type smap struct { sync.Mutex // 오직 수출되지 않은 타입을 위해서 사용 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] }	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] }
뮤텍스 인터페이스를 구현해야 하는 전용 타입(private type) 혹은 타입에 포함됨.	수출되는 타입(exported type)에 대해서는 전용 필드 (private field)를 사용함.

슬라이스 복사(Copy Slices)와 바운더리 에서의 맵(Maps at Boundaries)

슬라이스(Slices)와 맵(maps)은 기본 데이터(underlying data)에 대한 포인터를 포함하고 있으므로 이들을 복사 해야 할 때의 시나리오에 대해서 주의할 필요가 있다.

Slices와 Maps의 수신(receiving)

참조/레퍼런스(reference)를 저장할 경우, 사용자는 인수(argument)로 받는 맵 혹은 슬라이스를 수정할 수 있음을 명심하라.

</tbody턴

Bad	Good
func (d *Driver) SetTrips(trips []Trip) { d.trips = trips } trips := ... d1.SetTrips(trips) // d1.trips을 수정할 것을 의미하는가? trips[0] = ...	func (d *Driver) SetTrips(trips []Trip) { d.trips = make([]Trip, len(trips)) copy(d.trips, trips) } trips := ... d1.SetTrips(trips) // 이제 d1.trips에 영향을 주지 않고서 trips[0]을 수정 할 수 있다. trips[0] = ...

슬라이스(Slices)와 맵(Maps)의 리턴

마찬가지로, 내부 상태(internal status)를 노출시키는 슬라이스나 맵에 대한 사용자의 수정에 주의하라.

Bad	Good
type Stats struct { mu sync.Mutex counters map[string]int } // Snapshot은 현재의 stats을 반환(return)한다 func (s *Stats) Snapshot() map[string]int { s.mu.Lock() defer s.mu.Unlock() return s.counters } // snapshot은 더이상 뮤텍스에 의해서 보호되지 않는다. // 따라서, snapshot에 대한 access가 안정되지 않는다. (any access to the snapshot is racy.) snapshot := stats.Snapshot()	type Stats struct { mu sync.Mutex counters map[string]int } func (s *Stats) Snapshot() map[string]int { s.mu.Lock() defer s.mu.Unlock() result := make(map[string]int, len(s.counters)) for k, v := range s.counters { result[k] = v } return result } // Snapshot는 카피(copy)다. snapshot := stats.Snapshot()

Defer to Clean Up

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

Bad	Good
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	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
// Ought to be enough for anybody! c := make(chan int, 64)	// 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
type Operation int const ( Add Operation = iota Subtract Multiply ) // Add=0, Subtract=1, Multiply=2	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
// 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") } } }	// 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
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") } } }	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
s, err := store.New() if err != nil { return fmt.Errorf( "failed to create new store: %s", err) }	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
t := i.(string)	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
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]) }	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
// func TestFoo(t *testing.T) f, err := ioutil.TempFile("", "test") if err != nil { panic("failed to set up test") }	// 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
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! }	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
for i := 0; i < b.N; i++ { s := fmt.Sprint(rand.Int()) }	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
for i := 0; i < b.N; i++ { w.Write([]byte("Hello world")) }	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
import "a" import "b"	import ( "a" "b" )

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

Bad	Good
const a = 1 const b = 2 var a = 1 var b = 2 type Area float64 type Volume float64	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
type Operation int const ( Add Operation = iota + 1 Subtract Multiply ENV_VAR = "MY_ENV" )	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
func f() string { var red = color.New(0xff0000) var green = color.New(0x00ff00) var blue = color.New(0x0000ff) ... }	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
import ( "fmt" "os" "go.uber.org/atomic" "golang.org/x/sync/errgroup" )	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
import ( "fmt" "os" nettrace "golang.net/x/trace" )	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
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{} }	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
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) } }	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
var a int if b { a = 100 } else { a = 10 }	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
var _s string = F() func F() string { return "A" }	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
// 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. }	// 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
type Client struct { version int http.Client }	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
k := User{"John", "Doe", true}	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
var s = "foo"	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
func f(list []int) { filtered := []int{} for _, v := range list { if v > 10 { filtered = append(filtered, v) } } }	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
  if x == "" { return []int{} }	if x == "" { return nil }

• To check if a slice is empty, always use len(s) == 0. Do not check for nil.

  Bad	Good
  func isEmpty(s []string) bool { return s == nil }	func isEmpty(s []string) bool { return len(s) == 0 }

• The zero value (a slice declared with var) is usable immediately without make().

  Bad	Good
  nums := []int{} // or, nums := make([]int) if add1 { nums = append(nums, 1) } if add2 { nums = append(nums, 2) }	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
err := ioutil.WriteFile(name, data, 0644) if err != nil { return err }	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
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 }	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
// func printInfo(name string, isLocal, done bool) printInfo("foo", true, true)	// 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
wantError := "unknown name:\"test\""	wantError := `unknown error:"test"`

Initializing Struct References

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

Bad	Good
sval := T{Name: "foo"} // inconsistent sptr := new(T) sptr.Name = "bar"	sval := T{Name: "foo"} sptr := &T{Name: "bar"}

Format Strings outside Printf

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

This helps go vet perform static analysis of the format string.

Bad	Good
msg := "unexpected values %v, %v\n" fmt.Printf(msg, 1, 2)	const msg = "unexpected values %v, %v\n" fmt.Printf(msg, 1, 2)

Naming Printf-style Functions

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

This means that you should use pre-defined Printf-style function names if possible. go vet will check these by default. See Printf family for more information.

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

$ go vet -printfuncs=wrapf,statusf

See also go vet: Printf family check.

Patterns

Test Tables

Use table-driven tests with subtests to avoid duplicating code when the core test logic is repetitive.

Bad

Good

// 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)

// 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) }) }

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 tests and each test case tt. Further, we encourage explicating the input and output values for each test case with give and want prefixes.

tests := []struct{
  give     string
  wantHost string
  wantPort string
}{
  // ...
}

for _, tt := range tests {
  // ...
}

Functional Options

Functional options is a pattern in which you declare an opaque Option 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.

Bad

Good

// 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 */)

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,

• Self-referential functions and the design of options

• Functional options for friendly APIs