stretchr/testify

Allow dynamic returns based on arguments

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#350 opened on Sep 15, 2016

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 (35 comments) (59 reactions) (0 assignees)Go (25,958 stars) (1,704 forks)batch import
help wanted

Description

C# Moq allows to return data based on call arguments.

testify/mock don't. It's possible to do using Run, but it's a big mess of code.

It would be great to have something like this:

myMock.On("Load", mock.AnythingOfType("string")).ReturnFn(func (token string) (*MyObj, error) {
    if isValid(token) {
        return someStuff(), nil
    } else {
        return nil, errors.New("Oh!")
    }
})

I can send a PR if someone like this idea.

Contributor guide

Tech stack
go
Domain
testing
Issue type
feature
DifficultyEstimated implementation difficulty for a new contributor, from 1 for very small changes to 5 for expert-level work.
3
Estimated timeA rough time range for an experienced contributor to investigate, implement, test, and prepare a pull request.
1-2 days
Activity statusHow available the issue appears right now: fresh, active, stale, blocked, or waiting on maintainer input.
stale
ClarityHow clearly the issue explains the expected change, acceptance criteria, and next step.
clear
Prerequisites
Gotestify mockGo interfaces
Newbie friendlinessA 1-100 score estimating how approachable this issue is for first-time contributors.
50
Research direction
The issue proposes adding a `ReturnFn` method to `testify/mock` for dynamic returns based on arguments. Review existing mock methods like `On` and `Return` in `github.com/stretchr/testify/mock` to understand the pattern. Check the 35 comments for implementation discussions, potential pitfalls (e.g., type safety, interface handling), and any prior attempts. The feature is not yet implemented, so focus on designing a clean API that integrates with the existing mock structure.