A simple assertion library using Go generics

This library is inspired by testify/require, but with a significantly reduced API surface based on empirical use of that package.

It also provides much nicer diff output, eg.

=== RUN   TestFail
    assert_test.go:14: Expected values to be equal:
         assert.Data{
        -  Str: "foo",
        +  Str: "far",
           Num: 10,
         }
--- FAIL: TestFail (0.00s)

API

Import then use as assert:

import "github.com/alecthomas/assert/v2"

This library has the following API. For all functions, msgAndArgs is used to format error messages using the fmt package.

// Equal asserts that "expected" and "actual" are equal using google/go-cmp.
//
// If they are not, a diff of the Go representation of the values will be displayed.
func Equal[T comparable](t testing.TB, expected, actual T, msgAndArgs ...interface{})

// NotEqual asserts that "expected" is not equal to "actual" using google/go-cmp.
//
// If they are equal the expected value will be displayed.
func NotEqual[T comparable](t testing.TB, expected, actual T, msgAndArgs ...interface{})

// Zero asserts that a value is its zero value.
func Zero[T comparable](t testing.TB, value T, msgAndArgs ...interface{})

// NotZero asserts that a value is not its zero value.
func NotZero[T comparable](t testing.TB, value T, msgAndArgs ...interface{})

// Contains asserts that "haystack" contains "needle".
func Contains(t testing.TB, haystack string, needle string, msgAndArgs ...interface{})

// NotContains asserts that "haystack" does not contain "needle".
func NotContains(t testing.TB, haystack string, needle string, msgAndArgs ...interface{})

// EqualError asserts that either an error is non-nil and that its message is what is expected,
// or that error is nil if the expected message is empty.
func EqualError(t testing.TB, err error, errString string, msgAndArgs...interface{})

// Error asserts that an error is not nil.
func Error(t testing.TB, err error, msgAndArgs ...interface{})

// NoError asserts that an error is nil.
func NoError(t testing.TB, err error, msgAndArgs ...interface{})

// IsError asserts than any error in "err"'s tree matches "target".
func IsError(t testing.TB, err, target error, msgAndArgs ...interface{})

// NotIsError asserts than no error in "err"'s tree matches "target".
func NotIsError(t testing.TB, err, target error, msgAndArgs ...interface{})

// Panics asserts that the given function panics.
func Panics(t testing.TB, fn func(), msgAndArgs ...interface{})

// NotPanics asserts that the given function does not panic.
func NotPanics(t testing.TB, fn func(), msgAndArgs ...interface{})

// Compare two values for equality and return true or false.
func Compare[T any](t testing.TB, x, y T) bool

// True asserts that an expression is true.
func True(t testing.TB, ok bool, msgAndArgs ...interface{})

// False asserts that an expression is false.
func False(t testing.TB, ok bool, msgAndArgs ...interface{})

Evaluation process

Our empirical data of testify usage comes from a monorepo with around 50K lines of tests.

These are the usage counts for all testify functions, normalised to the base (not Printf()) non-negative(not No(t)?) case for each core function.

2240 Error
1314 Equal
 219 True
 210 Nil
 167 Empty
 107 Contains
  79 Len
  61 False
  24 EqualValues
  20 EqualError
  17 Zero
  15 Fail
  15 ElementsMatch
   9 Panics
   7 IsType
   6 FileExists
   4 JSONEq
   3 PanicsWithValue
   3 Eventually

The decision for each function was:

Keep

• Error(t, err) -> frequently used, keep
• Equal(t, expected, actual) -> frequently used, keep but make type safe
• True(t, expr) -> frequently used, keep
• False(t, expr) -> frequently used, keep
• Empty(t, thing) -> require.Equal(t, len(thing), 0)
• Contains(t, haystack string, needle string) - the only variant used in our codebase, keep as concrete type
• Zero(t, value) -> make type safe, keep
• Panics(t, f) -> useful, keep
• EqualError(t, a, b) -> useful, keep
• Nil(t, value) -> frequently used, keep

Not keeping, replace with ...

• ElementsMatch(t, a, b) - use peterrk/slices or stdlib sort support once it lands.
• IsType(t, a, b) -> require.Equal(t, reflect.TypeOf(a).String(), reflect.TypeOf(b).String())
• FileExists() -> very little use, drop
• JSONEq() -> very little use, drop
• PanicsWithValue() -> very little use, drop
• Eventually() -> very little use, drop
• Contains(t, haystack []T, needle T) - very little use, replace with
• Contains(t, haystack map[K]V, needle K) - very little use, drop
• Len(t, v, n) -> cannot be implemented as a single function with genericsEqual(t, len(v), n)
• EqualValues() - Equal(t, TYPE(a), TYPE(b))
• Fail() -> t.Fatal()