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Package assert

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Published: Nov 11, 2018 | License: Apache-2.0 | Module:


Package assert provides assertions for comparing expected values to actual values. When an assertion fails a helpful error message is printed.

Assert and Check

Assert() and Check() both accept a Comparison, and fail the test when the comparison fails. The one difference is that Assert() will end the test execution immediately (using t.FailNow()) whereas Check() will fail the test (using t.Fail()), return the value of the comparison, then proceed with the rest of the test case.

Example usage

The example below shows assert used with some common types.

import (

    is ""

func TestEverything(t *testing.T) {
    // booleans
    assert.Assert(t, ok)
    assert.Assert(t, !missing)

    // primitives
    assert.Equal(t, count, 1)
    assert.Equal(t, msg, "the message")
    assert.Assert(t, total != 10) // NotEqual

    // errors
    assert.NilError(t, closer.Close())
    assert.Error(t, err, "the exact error message")
    assert.ErrorContains(t, err, "includes this")
    assert.ErrorType(t, err, os.IsNotExist)

    // complex types
    assert.DeepEqual(t, result, myStruct{Name: "title"})
    assert.Assert(t, is.Len(items, 3))
    assert.Assert(t, len(sequence) != 0) // NotEmpty
    assert.Assert(t, is.Contains(mapping, "key"))

    // pointers and interface
    assert.Assert(t, is.Nil(ref))
    assert.Assert(t, ref != nil) // NotNil


Package provides many common comparisons. Additional comparisons can be written to compare values in other ways. See the example Assert (CustomComparison).

Automated migration from testify

gty-migrate-from-testify is a binary which can update source code which uses testify assertions to use the assertions provided by this package.




Package Files

func Assert

func Assert(t TestingT, comparison BoolOrComparison, msgAndArgs ...interface{})

Assert performs a comparison. If the comparison fails the test is marked as failed, a failure message is logged, and execution is stopped immediately.

The comparison argument may be one of three types: bool, cmp.Comparison or error. When called with a bool the failure message will contain the literal source code of the expression. When called with a cmp.Comparison the comparison is responsible for producing a helpful failure message. When called with an error a nil value is considered success. A non-nil error is a failure, and Error() is used as the failure message.

Example (CustomComparison)


package assert_test

import (


var t = &testing.T{}

func ExampleAssert_customComparison() {
	regexPattern := func(value string, pattern string) cmp.Comparison {
		return func() cmp.Result {
			re := regexp.MustCompile(pattern)
			if re.MatchString(value) {
				return cmp.ResultSuccess
			return cmp.ResultFailure(
				fmt.Sprintf("%q did not match pattern %q", value, pattern))
	assert.Assert(t, regexPattern("12345.34", `\d+.\d\d`))

func Check

func Check(t TestingT, comparison BoolOrComparison, msgAndArgs ...interface{}) bool

Check performs a comparison. If the comparison fails the test is marked as failed, a failure message is logged, and Check returns false. Otherwise returns true.

See Assert for details about the comparison arg and failure messages.

func DeepEqual

func DeepEqual(t TestingT, x, y interface{}, opts ...gocmp.Option)

DeepEqual uses google/go-cmp ( to assert two values are equal and fails the test if they are not equal.

Package provides some additional commonly used Options.

This is equivalent to Assert(t, cmp.DeepEqual(x, y)).

func Equal

func Equal(t TestingT, x, y interface{}, msgAndArgs ...interface{})

Equal uses the == operator to assert two values are equal and fails the test if they are not equal.

If the comparison fails Equal will use the variable names for x and y as part of the failure message to identify the actual and expected values.

If either x or y are a multi-line string the failure message will include a unified diff of the two values. If the values only differ by whitespace the unified diff will be augmented by replacing whitespace characters with visible characters to identify the whitespace difference.

This is equivalent to Assert(t, cmp.Equal(x, y)).

func Error

func Error(t TestingT, err error, message string, msgAndArgs ...interface{})

Error fails the test if err is nil, or the error message is not the expected message. Equivalent to Assert(t, cmp.Error(err, message)).

func ErrorContains

func ErrorContains(t TestingT, err error, substring string, msgAndArgs ...interface{})

ErrorContains fails the test if err is nil, or the error message does not contain the expected substring. Equivalent to Assert(t, cmp.ErrorContains(err, substring)).

func ErrorType

func ErrorType(t TestingT, err error, expected interface{}, msgAndArgs ...interface{})

ErrorType fails the test if err is nil, or err is not the expected type.

Expected can be one of: a func(error) bool which returns true if the error is the expected type, an instance of (or a pointer to) a struct of the expected type, a pointer to an interface the error is expected to implement, a reflect.Type of the expected struct or interface.

Equivalent to Assert(t, cmp.ErrorType(err, expected)).

func NilError

func NilError(t TestingT, err error, msgAndArgs ...interface{})

NilError fails the test immediately if err is not nil. This is equivalent to Assert(t, err)

type BoolOrComparison

type BoolOrComparison interface{}

BoolOrComparison can be a bool, or cmp.Comparison. See Assert() for usage.

type TestingT

type TestingT interface {
	Log(args ...interface{})

TestingT is the subset of testing.T used by the assert package.

Documentation was rendered with GOOS=linux and GOARCH=amd64.

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