aasumitro/uber-go.md

## uber-go.md

      
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              uber-go.md
            
          
Uber Go Style Guide

Table of Contents


Introduction
Guidelines

Pointers to Interfaces
Verify Interface Compliance
Receivers and Interfaces
Zero-value Mutexes are Valid
Copy Slices and Maps at Boundaries
Defer to Clean Up
Channel Size is One or None
Start Enums at One
Use "time" to handle time
Error Types
Error Wrapping
Handle Type Assertion Failures
Don't Panic
Use go.uber.org/atomic
Avoid Mutable Globals
Avoid Embedding Types in Public Structs
Avoid Using Built-In Names
Avoid init()


Performance

Prefer strconv over fmt
Avoid string-to-byte conversion
Prefer Specifying Container Capacity

Specifying Map Capacity Hints
Specifying Slice Capacity


Style

Be Consistent
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
Initializing Maps
Format Strings outside Printf
Naming Printf-style Functions


Patterns

Test Tables
Functional Options


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:

Effective Go
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:

A pointer to some type-specific information. You can think of this as
"type."
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.
Verify Interface Compliance

Verify interface compliance at compile time where appropriate. This includes:

Exported types that are required to implement specific interfaces as part of
their API contract
Exported or unexported types that are part of a collection of types
implementing the same interface
Other cases where violating an interface would break users


Bad Good


type Handler struct {
  // ...
}


func (h *Handler) ServeHTTP(
  w http.ResponseWriter,
  r *http.Request,
) {
  ...
}

type Handler struct {
  // ...
}

var _ http.Handler = (*Handler)(nil)

func (h *Handler) ServeHTTP(
  w http.ResponseWriter,
  r *http.Request,
) {
  // ...
}


The statement var _ http.Handler = (*Handler)(nil) will fail to compile if
*Handler ever stops matching the http.Handler interface.
The right hand side of the assignment should be the zero value of the asserted
type. This is nil for pointer types (like *Handler), slices, and maps, and
an empty struct for struct types.
type LogHandler struct {
  h   http.Handler
  log *zap.Logger
}

var _ http.Handler = LogHandler{}

func (h LogHandler) ServeHTTP(
  w http.ResponseWriter,
  r *http.Request,
) {
  // ...
}
Receivers and Interfaces

Methods with value receivers can be called on pointers as well as values.
Methods with pointer receivers can only be called on pointers or addressable 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


mu := new(sync.Mutex)
mu.Lock()

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.


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]
}

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


func (d *Driver) SetTrips(trips []Trip) {
  d.trips = trips
}

trips := ...
d1.SetTrips(trips)

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


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


type Stats struct {
  mu sync.Mutex
  counters map[string]int
}

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

  return s.counters
}

// snapshot is no longer protected by the mutex, so any
// access to the snapshot is subject to data races.
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
}

// 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


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
Use "time" to handle time

Time is complicated. Incorrect assumptions often made about time include the
following.

A day has 24 hours
An hour has 60 minutes
A week has 7 days
A year has 365 days
And a lot more

For example, 1 means that adding 24 hours to a time instant will not always
yield a new calendar day.
Therefore, always use the "time" package when dealing with time because it
helps deal with these incorrect assumptions in a safer, more accurate manner.
Use time.Time for instants of time

Use time.Time when dealing with instants of time, and the methods on
time.Time when comparing, adding, or subtracting time.

Bad Good


func isActive(now, start, stop int) bool {
  return start <= now && now < stop
}

func isActive(now, start, stop time.Time) bool {
  return (start.Before(now) || start.Equal(now)) && now.Before(stop)
}


Use time.Duration for periods of time

Use time.Duration when dealing with periods of time.

Bad Good


func poll(delay int) {
  for {
    // ...
    time.Sleep(time.Duration(delay) * time.Millisecond)
  }
}

poll(10) // was it seconds or milliseconds?

func poll(delay time.Duration) {
  for {
    // ...
    time.Sleep(delay)
  }
}

poll(10*time.Second)


Going back to the example of adding 24 hours to a time instant, the method we
use to add time depends on intent. If we want the same time of the day, but on
the next calendar day, we should use Time.AddDate. However, if we want an
instant of time guaranteed to be 24 hours after the previous time, we should
use Time.Add.
newDay := t.AddDate(0 /* years */, 0, /* months */, 1 /* days */)
maybeNewDay := t.Add(24 * time.Hour)
Use time.Time and time.Duration with external systems

Use time.Duration and time.Time in interactions with external systems when
possible. For example:

Command-line flags: flag supports time.Duration via
time.ParseDuration
JSON: encoding/json supports encoding time.Time as an RFC 3339
string via its UnmarshalJSON method
SQL: database/sql supports converting DATETIME or TIMESTAMP columns
into time.Time and back if the underlying driver supports it
YAML: gopkg.in/yaml.v2 supports time.Time as an RFC 3339 string, and
time.Duration via time.ParseDuration.

When it is not possible to use time.Duration in these interactions, use
int or float64 and include the unit in the name of the field.
For example, since encoding/json does not support time.Duration, the unit
is included in the name of the field.

Bad Good


// {"interval": 2}
type Config struct {
  Interval int `json:"interval"`
}

// {"intervalMillis": 2000}
type Config struct {
  IntervalMillis int `json:"intervalMillis"`
}


When it is not possible to use time.Time in these interactions, unless an
alternative is agreed upon, use string and format timestamps as defined in
RFC 3339. This format is used by default by Time.UnmarshalText and is
available for use in Time.Format and time.Parse via time.RFC3339.
Although this tends to not be a problem in practice, keep in mind that the
"time" package does not support parsing timestamps with leap seconds
(8728), nor does it account for leap seconds in calculations (15190). If
you compare two instants of time, the difference will not include the leap
seconds that may have occurred between those two instants.

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("testfile.txt"); 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("testfile.txt"); 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()
}


Avoid Mutable Globals

Avoid mutating global variables, instead opting for dependency injection.
This applies to function pointers as well as other kinds of values.

Bad Good


// sign.go

var _timeNow = time.Now

func sign(msg string) string {
  now := _timeNow()
  return signWithTime(msg, now)
}

// sign.go

type signer struct {
  now func() time.Time
}

func newSigner() *signer {
  return &signer{
    now: time.Now,
  }
}

func (s *signer) Sign(msg string) string {
  now := s.now()
  return signWithTime(msg, now)
}


// sign_test.go

func TestSign(t *testing.T) {
  oldTimeNow := _timeNow
  _timeNow = func() time.Time {
    return someFixedTime
  }
  defer func() { _timeNow = oldTimeNow }()

  assert.Equal(t, want, sign(give))
}

// sign_test.go

func TestSigner(t *testing.T) {
  s := newSigner()
  s.now = func() time.Time {
    return someFixedTime
  }

  assert.Equal(t, want, s.Sign(give))
}


Avoid Embedding Types in Public Structs

These embedded types leak implementation details, inhibit type evolution, and
obscure documentation.
Assuming you have implemented a variety of list types using a shared
AbstractList, avoid embedding the AbstractList in your concrete list
implementations.
Instead, hand-write only the methods to your concrete list that will delegate
to the abstract list.
type AbstractList struct {}

// Add adds an entity to the list.
func (l *AbstractList) Add(e Entity) {
  // ...
}

// Remove removes an entity from the list.
func (l *AbstractList) Remove(e Entity) {
  // ...
}

Bad Good


// ConcreteList is a list of entities.
type ConcreteList struct {
  *AbstractList
}

// ConcreteList is a list of entities.
type ConcreteList struct {
  list *AbstractList
}

// Add adds an entity to the list.
func (l *ConcreteList) Add(e Entity) {
  return l.list.Add(e)
}

// Remove removes an entity from the list.
func (l *ConcreteList) Remove(e Entity) {
  return l.list.Remove(e)
}


Go allows type embedding as a compromise between inheritance and composition.
The outer type gets implicit copies of the embedded type's methods.
These methods, by default, delegate to the same method of the embedded
instance.
The struct also gains a field by the same name as the type.
So, if the embedded type is public, the field is public.
To maintain backward compatibility, every future version of the outer type must
keep the embedded type.
An embedded type is rarely necessary.
It is a convenience that helps you avoid writing tedious delegate methods.
Even embedding a compatible AbstractList interface, instead of the struct,
would offer the developer more flexibility to change in the future, but still
leak the detail that the concrete lists use an abstract implementation.

Bad Good


// AbstractList is a generalized implementation
// for various kinds of lists of entities.
type AbstractList interface {
  Add(Entity)
  Remove(Entity)
}

// ConcreteList is a list of entities.
type ConcreteList struct {
  AbstractList
}

// ConcreteList is a list of entities.
type ConcreteList struct {
  list *AbstractList
}

// Add adds an entity to the list.
func (l *ConcreteList) Add(e Entity) {
  return l.list.Add(e)
}

// Remove removes an entity from the list.
func (l *ConcreteList) Remove(e Entity) {
  return l.list.Remove(e)
}


Either with an embedded struct or an embedded interface, the embedded type
places limits on the evolution of the type.

Adding methods to an embedded interface is a breaking change.
Removing methods from an embedded struct is a breaking change.
Removing the embedded type is a breaking change.
Replacing the embedded type, even with an alternative that satisfies the same
interface, is a breaking change.

Although writing these delegate methods is tedious, the additional effort hides
an implementation detail, leaves more opportunities for change, and also
eliminates indirection for discovering the full List interface in
documentation.
Avoid Using Built-In Names

The Go language specification outlines several built-in,
predeclared identifiers that should not be used as names within Go programs.
Depending on context, reusing these identifiers as names will either shadow
the original within the current lexical scope (and any nested scopes) or make
affected code confusing. In the best case, the compiler will complain; in the
worst case, such code may introduce latent, hard-to-grep bugs.

Bad Good


var error string
// `error` shadows the builtin

// or

func handleErrorMessage(error string) {
    // `error` shadows the builtin
}

var errorMessage string
// `error` refers to the builtin

// or

func handleErrorMessage(msg string) {
    // `error` refers to the builtin
}


type Foo struct {
    // While these fields technically don't
    // constitute shadowing, grepping for
    // `error` or `string` strings is now
    // ambiguous.
    error  error
    string string
}

func (f Foo) Error() error {
    // `error` and `f.error` are
    // visually similar
    return f.error
}

func (f Foo) String() string {
    // `string` and `f.string` are
    // visually similar
    return f.string
}

type Foo struct {
    // `error` and `string` strings are
    // now unambiguous.
    err error
    str string
}

func (f Foo) Error() error {
    return f.err
}

func (f Foo) String() string {
    return f.str
}


Note that the compiler will not generate errors when using predeclared
identifiers, but tools such as go vet should correctly point out these and
other cases of shadowing.
Avoid init()

Avoid init() where possible. When init() is unavoidable or desirable, code
should attempt to:

Be completely deterministic, regardless of program environment or invocation.
Avoid depending on the ordering or side-effects of other init() functions.
While init() ordering is well-known, code can change, and thus
relationships between init() functions can make code brittle and
error-prone.
Avoid accessing or manipulating global or environment state, such as machine
information, environment variables, working directory, program
arguments/inputs, etc.
Avoid I/O, including both filesystem, network, and system calls.

Code that cannot satisfy these requirements likely belongs as a helper to be
called as part of main() (or elsewhere in a program's lifecycle), or be
written as part of main() itself. In particular, libraries that are intended
to be used by other programs should take special care to be completely
deterministic and not perform "init magic".

Bad Good


type Foo struct {
    // ...
}

var _defaultFoo Foo

func init() {
    _defaultFoo = Foo{
        // ...
    }
}

var _defaultFoo = Foo{
    // ...
}

// or, better, for testability:

var _defaultFoo = defaultFoo()

func defaultFoo() Foo {
    return Foo{
        // ...
    }
}


type Config struct {
    // ...
}

var _config Config

func init() {
    // Bad: based on current directory
    cwd, _ := os.Getwd()

    // Bad: I/O
    raw, _ := ioutil.ReadFile(
        path.Join(cwd, "config", "config.yaml"),
    )

    yaml.Unmarshal(raw, &_config)
}

type Config struct {
    // ...
}

func loadConfig() Config {
    cwd, err := os.Getwd()
    // handle err

    raw, err := ioutil.ReadFile(
        path.Join(cwd, "config", "config.yaml"),
    )
    // handle err

    var config Config
    yaml.Unmarshal(raw, &config)

    return config
}


Considering the above, some situations in which init() may be preferable or
necessary might include:

Complex expressions that cannot be represented as single assignments.
Pluggable hooks, such as database/sql dialects, encoding type registries, etc.
Optimizations to Google Cloud Functions and other forms of deterministic
precomputation.

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


Prefer Specifying Container Capacity

Specify container capacity where possible in order to allocate memory for the
container up front. This minimizes subsequent allocations (by copying and
resizing of the container) as elements are added.
Specifying Map Capacity Hints

Where possible, provide capacity hints when initializing
maps with make().
make(map[T1]T2, hint)
Providing a capacity hint to make() tries to right-size the
map at initialization time, which reduces the need for growing
the map and allocations as elements are added to the map.
Note that, unlike slices, map capacity hints do not guarantee complete,
preemptive allocation, but are used to approximate the number of hashmap buckets
required. Consequently, allocations may still occur when adding elements to the
map, even up to the specified capacity.

Bad Good


m := make(map[string]os.FileInfo)

files, _ := ioutil.ReadDir("./files")
for _, f := range files {
    m[f.Name()] = f
}

files, _ := ioutil.ReadDir("./files")

m := make(map[string]os.FileInfo, len(files))
for _, f := range files {
    m[f.Name()] = f
}


m is created without a size hint; there may be more
allocations at assignment time.

m is created with a size hint; there may be fewer
allocations at assignment time.


Specifying Slice Capacity

Where possible, provide capacity hints when initializing slices with make(),
particularly when appending.
make([]T, length, capacity)
Unlike maps, slice capacity is not a hint: the compiler will allocate enough
memory for the capacity of the slice as provided to make(), which means that
subsequent append() operations will incur zero allocations (until the length
of the slice matches the capacity, after which any appends will require a resize
to hold additional elements).

Bad Good


for n := 0; n < b.N; n++ {
  data := make([]int, 0)
  for k := 0; k < size; k++{
    data = append(data, k)
  }
}

for n := 0; n < b.N; n++ {
  data := make([]int, 0, size)
  for k := 0; k < size; k++{
    data = append(data, k)
  }
}


BenchmarkBad-4    100000000    2.48s


BenchmarkGood-4   100000000    0.21s


Style

Be Consistent

Some of the guidelines outlined in this document can be evaluated objectively;
others are situational, contextual, or subjective.
Above all else, be consistent.
Consistent code is easier to maintain, is easier to rationalize, requires less
cognitive overhead, and is easier to migrate or update as new conventions emerge
or classes of bugs are fixed.
Conversely, having multiple disparate or conflicting styles within a single
codebase causes maintenance overhead, uncertainty, and cognitive dissonance,
all of which can directly contribute to lower velocity, painful code reviews,
and bugs.
When applying these guidelines to a codebase, it is recommended that changes
are made at a package (or larger) level: application at a sub-package level
violates the above concern by introducing multiple styles into the same code.
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
}


Embedding should provide tangible benefit, like adding or augmenting
functionality in a semantically-appropriate way. It should do this with zero
adverse user-facing effects (see also: Avoid Embedding Types in Public Structs).
Embedding should not:

Be purely cosmetic or convenience-oriented.
Make outer types more difficult to construct or use.
Affect outer types' zero values. If the outer type has a useful zero value, it
should still have a useful zero value after embedding the inner type.
Expose unrelated functions or fields from the outer type as a side-effect of
embedding the inner type.
Expose unexported types.
Affect outer types' copy semantics.
Change the outer type's API or type semantics.
Embed a non-canonical form of the inner type.
Expose implementation details of the outer type.
Allow users to observe or control type internals.
Change the general behavior of inner functions through wrapping in a way that
would reasonably surprise users.

Simply put, embed consciously and intentionally. A good litmus test is, "would
all of these exported inner methods/fields be added directly to the outer type";
if the answer is "some" or "no", don't embed the inner type - use a field
instead.

Bad Good


type A struct {
    // Bad: A.Lock() and A.Unlock() are
    //      now available, provide no
    //      functional benefit, and allow
    //      users to control details about
    //      the internals of A.
    sync.Mutex
}

type countingWriteCloser struct {
    // Good: Write() is provided at this
    //       outer layer for a specific
    //       purpose, and delegates work
    //       to the inner type's Write().
    io.WriteCloser

    count int
}

func (w *countingWriteCloser) Write(bs []byte) (int, error) {
    w.count += len(bs)
    return w.WriteCloser.Write(bs)
}


type Book struct {
    // Bad: pointer changes zero value usefulness
    io.ReadWriter

    // other fields
}

// later

var b Book
b.Read(...)  // panic: nil pointer
b.String()   // panic: nil pointer
b.Write(...) // panic: nil pointer

type Book struct {
    // Good: has useful zero value
    bytes.Buffer

    // other fields
}

// later

var b Book
b.Read(...)  // ok
b.String()   // ok
b.Write(...) // ok


type Client struct {
    sync.Mutex
    sync.WaitGroup
    bytes.Buffer
    url.URL
}

type Client struct {
    mtx sync.Mutex
    wg  sync.WaitGroup
    buf bytes.Buffer
    url url.URL
}


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 used. 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)
}


Remember that, while it is a valid slice, a nil slice is not equivalent to an
allocated slice of length 0 - one is nil and the other is not - and the two may
be treated differently in different situations (such as serialization).
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 Status = 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"}


Initializing Maps

Prefer make(..) for empty maps, and maps populated
programmatically. This makes map initialization visually
distinct from declaration, and it makes it easy to add size
hints later if available.

Bad Good


var (
  // m1 is safe to read and write;
  // m2 will panic on writes.
  m1 = map[T1]T2{}
  m2 map[T1]T2
)

var (
  // m1 is safe to read and write;
  // m2 will panic on writes.
  m1 = make(map[T1]T2)
  m2 map[T1]T2
)


Declaration and initialization are visually similar.

Declaration and initialization are visually distinct.


Where possible, provide capacity hints when initializing
maps with make(). See
Prefer Specifying Map Capacity Hints
for more information.
On the other hand, if the map holds a fixed list of elements,
use map literals to initialize the map.

Bad Good


m := make(map[T1]T2, 3)
m[k1] = v1
m[k2] = v2
m[k3] = v3

m := map[T1]T2{
  k1: v1,
  k2: v2,
  k3: v3,
}


The basic rule of thumb is to use map literals when adding a fixed set of
elements at initialization time, otherwise use make (and specify a size hint
if available).
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 predefined Printf-style function
names if possible. go vet will check these by default. See Printf family
for more information.
If using the predefined 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 Open(
  addr string,
  cache bool,
  logger *zap.Logger
) (*Connection, error) {
  // ...
}

// package db

type Option interface {
  // ...
}

func WithCache(c bool) Option {
  // ...
}

func WithLogger(log *zap.Logger) Option {
  // ...
}

// Open creates a connection.
func Open(
  addr string,
  opts ...Option,
) (*Connection, error) {
  // ...
}


The cache and logger parameters must always be provided, even if the user
wants to use the default.
db.Open(addr, db.DefaultCache, zap.NewNop())
db.Open(addr, db.DefaultCache, log)
db.Open(addr, false /* cache */, zap.NewNop())
db.Open(addr, false /* cache */, log)

Options are provided only if needed.
db.Open(addr)
db.Open(addr, db.WithLogger(log))
db.Open(addr, db.WithCache(false))
db.Open(
  addr,
  db.WithCache(false),
  db.WithLogger(log),
)


Our suggested way of implementing this pattern is with an Option interface
that holds an unexported method, recording options on an unexported options
struct.
type options struct {
  cache  bool
  logger *zap.Logger
}

type Option interface {
  apply(*options)
}

type cacheOption bool

func (c cacheOption) apply(opts *options) {
  opts.cache = bool(c)
}

func WithCache(c bool) Option {
  return cacheOption(c)
}

type loggerOption struct {
  Log *zap.Logger
}

func (l loggerOption) apply(opts *options) {
  opts.logger = l.Log
}

func WithLogger(log *zap.Logger) Option {
  return loggerOption{Log: log}
}

// Open creates a connection.
func Open(
  addr string,
  opts ...Option,
) (*Connection, error) {
  options := options{
    cache:  defaultCache,
    logger: zap.NewNop(),
  }

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

  // ...
}
Note that there's a method of implementing this pattern with closures but we
believe that the pattern above provides more flexibility for authors and is
easier to debug and test for users. In particular, it allows options to be
compared against each other in tests and mocks, versus closures where this is
impossible. Further, it lets options implement other interfaces, including
fmt.Stringer which allows for user-readable string representations of the
options.
See also,

Self-referential functions and the design of options
Functional options for friendly APIs
Bad	Good
var ( // m1 is safe to read and write; // m2 will panic on writes. m1 = map[T1]T2{} m2 map[T1]T2 )	var ( // m1 is safe to read and write; // m2 will panic on writes. m1 = make(map[T1]T2) m2 map[T1]T2 )
Declaration and initialization are visually similar.	Declaration and initialization are visually distinct.