Isolating Dependencies with Test Doubles: Stubs

Welcome to the second lesson in our journey into Test-Driven Development (TDD) using Go and Testify. In the previous lesson, we discussed how to utilize dummies to isolate dependencies. In this lesson, we will shift our focus to another kind of test double — Stubs.

By the end of this lesson, you'll understand what stubs are and how to implement them in your tests using Go, specifically for isolating dependencies like external services.

In testing, test doubles help us isolate parts of our application. We've previously discussed dummies. Stubs, a more useful type of test double, provide predefined answers to method calls during testing. Unlike other test doubles, stubs aren't concerned with tracking usage; they are simple yet powerful for specific scenarios.

Stubs are especially useful when testing functions that rely on external services or complex dependencies. By simulating function outputs, stubs make tests faster and more predictable. Stubs focus on ensuring your application's logic functions as expected without verifying the correctness of external dependencies.

To demonstrate the concept of stubs, we will create a WeatherAlertService using stubs in a test-driven development process with Go.

We'll build a WeatherAlertService that fetches data from a WeatherService. This service will issue alerts based on specific conditions. Relying on the actual data source is impractical for testing, so we'll use stubbed data instead.

Create a new test file named weather_alert_service_test.go with the following test setup:

package main

import (
    "testing"
    "github.com/stretchr/testify/assert"
)

type WeatherServiceStub struct {
    temperature int
    conditions  string
}

func (ws *WeatherServiceStub) SetWeather(temperature int, conditions string) {
    ws.temperature = temperature
    ws.conditions = conditions
}

func (ws *WeatherServiceStub) GetCurrentWeather(location string) (*WeatherData, error) {
    return &WeatherData{
        Temperature: ws.temperature,
        Conditions:  ws.conditions,
    }, nil
}

func TestShouldReturnHeatWarningWhenTemperatureIsAbove35(t *testing.T) {
    // Arrange
    weatherService = &WeatherServiceStub{}
    weatherService.SetWeather(36, "sunny")
    alertService = NewWeatherAlertService(suite.weatherService)

    // Act
    alert := alertService.ShouldSendAlert("London")

    // Assert
    assert.Equal(t, "Extreme heat warning. Stay hydrated!", alert)
}

In this test:

• We create a manual stub WeatherServiceStub that simulates weather data by setting predefined values for Temperature and Conditions via the SetWeather method.
• The stub’s GetCurrentWeather method returns a WeatherData struct with these predefined values, simulating expected weather conditions for the test scenario.
• The Testify library is used to assert whether the WeatherAlertService correctly interprets the weather data and returns the appropriate alert, specifically checking for a heat warning when the temperature exceeds 35 degrees.

Run this test, expecting it to fail initially, as WeatherAlertService has yet to implement the logic for handling these conditions.