fluentassert

Extensible, type-safe, fluent assertion Go library.

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Description

The fluent API makes the assertion code easier to read and write (more).

The generics (type parameters) make the usage type-safe.

The library is extensible by design.

In general, avoid using assertion libraries. Use this library if you still have a preference to use one. Consider using github.com/google/go-cmp and writing custom helpers instead.

Quick start

package test

import (
	"testing"

	"github.com/fluentassert/verify"
)

func Foo() (string, error) {
	return "wrong", nil
}

func TestFoo(t *testing.T) {
	got, err := Foo()

	verify.NoError(err).Require(t)           // Require(f) uses t.Fatal(f), stops execution if fails
	verify.String(got).Equal("ok").Assert(t) // Assert(f) uses t.Error(f), continues execution if fails
}

$ go test
--- FAIL: TestFoo (0.00s)
    basic_test.go:17:
        the objects are not equal
        got: "wrong"
        want: "ok"

⚠ Do not forget calling Assert(t) or Require(t) which executes the actual assertion.

Supported types

Out-of-the-box the package provides fluent assertions for the following types. The more specific function you use, the more assertions you get.

Go type	Assertion entry point
interface{} (any)	verify.Any()
comparable	verify.Obj()
constraints.Ordered	verify.Ordered()
constraints.Number	verify.Number()
string	verify.String()
error	verify.Error()
[]T (slice)	verify.Slice()
map[K]V (map)	verify.Map()

Below you can find some convenience functions.

• verify.NoError()
• verify.IsError()
• verify.Nil()
• verify.NotNil()
• verify.True()
• verify.False()

Deep equality

For testing deep equality use DeepEqual() or NotDeepEqual().

package test

import (
	"testing"

	"github.com/fluentassert/verify"
)

type A struct {
	Str   string
	Bool  bool
	Slice []int
}

func TestDeepEqual(t *testing.T) {
	got := A{Str: "wrong", Slice: []int{1, 4}}

	verify.Any(got).DeepEqual(
		A{Str: "string", Bool: true, Slice: []int{1, 2}},
	).Assert(t)
}

$ go test
--- FAIL: TestDeepEqual (0.00s)
    deepeq_test.go:20:
        mismatch (-want +got):
          test.A{
        -       Str:  "string",
        +       Str:  "wrong",
        -       Bool: true,
        +       Bool: false,
                Slice: []int{
                        1,
        -               2,
        +               4,
                },
          }

Collection assertions

The library contains many collection assertion. Below is an example of checking unordered equality.

package test

import (
	"testing"

	"github.com/fluentassert/verify"
)

func TestSlice(t *testing.T) {
	got := []int { 3, 1, 2 }

	verify.Slice(got).Equivalent([]int { 2, 3, 4 }).Assert(t)
}

$ go test
--- FAIL: TestSlice (0.00s)
    slice_test.go:12:
        not equivalent
        got: [3 1 2]
        want: [2 3 4]
        extra got: [1]
        extra want: [4]

Periodic polling

For asynchronous testing you can use verify.Eventually() or verify.EventuallyChan().

package test

import (
	"net/http"
	"testing"
	"time"

	"github.com/fluentassert/verify"
)

func TestPeriodic(t *testing.T) {
	verify.Eventually(10*time.Second, time.Second, func() verify.FailureMessage {
		client := http.Client{Timeout: time.Second}
		resp, err := client.Get("http://not-existing:1234")
		if err != nil {
			return verify.NoError(err)
		}
		return verify.Number(resp.StatusCode).Lesser(300)
	}).Assert(t)
}

$ go test
--- FAIL: TestPeriodic (10.00s)
    async_test.go:19:
        function never passed, last failure message:
        Get "http://not-existing:1234": context deadline exceeded (Client.Timeout exceeded while awaiting headers)

Custom predicates

For the most basic scenarios, you can use one of the Check(), Should(), ShouldNot() assertions.

package test

import (
	"strings"
	"testing"

	"github.com/fluentassert/verify"
)

func TestShould(t *testing.T) {
	got := "wrong"

	chars := "abc"
	verify.Any(got).Should(func(got string) bool {
		return strings.ContainsAny(got, chars)
	}).Assertf(t, "does not contain any of: %s", chars)
}

$ go test
--- FAIL: TestShould (0.00s)
    should_test.go:16: does not contain any of: abc
        object does not meet the predicate criteria
        got: "wrong"

Panics

For testing panics use verify.Panics() and verify.NotPanics().

Custom assertion function

You can create a function that returns FailureMessage. Use verify.And() and verify.Or() functions together with Prefix() method to create complex assertions.

package test

import (
	"testing"

	"github.com/fluentassert/verify"
)

type A struct {
	Str string
	Ok  bool
}

func TestCustom(t *testing.T) {
	got := A{Str: "something was wrong"}

	verifyA(got).Assert(t)
}

func verifyA(got A) verify.FailureMessage {
	return verify.And(
		verify.String(got.Str).Contain("ok").Prefix("got.String: "),
		verify.True(got.Ok).Prefix("got.Ok: "),
	)
}

$ go test
--- FAIL: TestCustom (0.00s)
    custom_test.go:17:
        got.String: the value does not contain the substring
        got: "something was wrong"
        substr: "ok"

        got.Ok: the value is false

Extensibility

You can take advantage of the FailureMessage and Fluent* types to create your own fluent assertions for a given type.

For reference, take a look at the implementation of existing fluent assertions in this repository (for example comparable.go).

Supported Go versions

Minimal supported Go version is 1.18.

Contributing

See CONTRIBUTING.md if you want to help.

License

fluentassert is licensed under the terms of the MIT license.

github.com/google/go-cmp (license: BSD-3-Clause) is the only third-party dependency.