go – How do you mock an error return value in mockery for GoLang?-ThrowExceptions

Exception or error:

I’m using https://github.com/vektra/mockery to generate my mock files.

Assume I have this test for function GetQualifyingListOfBooks. The signature for the (mocked) client method GetListOfBooks is:

func GetListOfBooks(ctx context.Context, in *GetListOfBooksRequest, opts ...grpc.CallOption) (*GetListOfBooksResponse, error)

Goal: I want to mock the situation where GetListOfBooks returns an error, so I’ve written the following:

package whatever

import (
    "context"
    "testing"
    "github.com/stretchr/testify/assert"
    mock "github.com/stretchr/testify/mock"
    mockClients "github.com/MyOrg/MyApp/internal/app/myservice/mocks"
)

func TestGetQualifyingListOfBooks (t *testing.T) {
    client := &mockClients.BookClient{}
    client.On("GetListOfBooks", mock.Anything, mock.Anything).Return(nil, mock.AnythingOfType("error"))
    realInstance.bookClient = client
    b, err := GetQualifyingListOfBooks(realInstance, 10) // this calls `GetListOfBooks`
    assert.Nil(t, b)
    assert.NotNil(t, err)
}

but it panics with this:

--- FAIL: TestGetQualifyingListOfBooks (0.00s)
panic: assert: arguments: Error(1) failed because object wasn't correct type: Error(1) [recovered]
    panic: assert: arguments: Error(1) failed because object wasn't correct type: Error(1)

How do you mock a return value of an error?

How to solve:

Changing this line

client.On("GetListOfBooks", mock.Anything, mock.Anything).Return(nil, mock.AnythingOfType("error"))

to this

client.On("GetListOfBooks", mock.Anything, mock.Anything).Return(nil, errors.New("hello world"))

did it for me.

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