Mock go function that modifies and returns the value passed as argument

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05-10-2024

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Mocking Go Functions: Modifying and Returning Arguments

Mocking is a crucial technique in software development, particularly when testing. It allows us to simulate the behavior of external dependencies or complex components without actually executing them. In Go, mocking functions that modify and return their arguments can seem tricky, but with the right approach, it becomes a straightforward process.

The Scenario:

Imagine you have a function processString that takes a string as input, modifies it by adding a prefix, and returns the modified string:

func processString(str string) string {
  return "Prefix_" + str
}

Now, you want to write a test for a function that uses processString, but you don't want to rely on the actual implementation of processString. This is where mocking comes in.

The Solution:

To mock processString, we can use the testify/mock package. It provides a convenient way to create mock objects with customizable behavior. Here's a basic example:

import (
	"testing"

	"github.com/stretchr/testify/mock"
)

type MockProcessString struct {
	mock.Mock
}

func (m *MockProcessString) ProcessString(str string) string {
	args := m.Called(str)
	return args.String(0)
}

func TestWithMockedFunction(t *testing.T) {
	// Create a mock object
	mockProcessString := new(MockProcessString)

	// Define the expected behavior
	mockProcessString.On("ProcessString", "test").Return("Mocked_test")

	// Use the mock object in your test
	result := mockProcessString.ProcessString("test")

	// Assert the expected result
	if result != "Mocked_test" {
		t.Errorf("Expected 'Mocked_test', got '%s'", result)
	}
}

Explanation:

1. Create a mock struct: We define a struct MockProcessString that embeds the mock.Mock interface.
2. Define the mock function: We define a method ProcessString in the mock struct, which internally calls m.Called(str) to record the call and return the desired result.
3. Set expectations: We use mockProcessString.On("ProcessString", "test").Return("Mocked_test") to define the expected behavior for a specific input. This means when ProcessString is called with "test", it should return "Mocked_test".
4. Use the mock: We call the mock function mockProcessString.ProcessString("test") and assert that the result matches the expected value.

Key Insights:

• Flexibility: You can define different expectations for different inputs, allowing you to simulate various scenarios.
• Testability: Mocks isolate your tests from the actual implementation, making them more reliable and easier to maintain.
• Control: You have full control over the mock's behavior, ensuring your tests are focused and predictable.

Additional Value:

• Error Handling: You can also define mock behavior for error scenarios by returning an error object in Return().
• Advanced Mocking: The testify/mock package offers more advanced features, such as mocking multiple methods and defining complex return values.

Conclusion:

Mocking functions that modify and return their arguments in Go is straightforward with the testify/mock package. By creating mock objects and defining expectations, you can isolate your tests, achieve higher code coverage, and build more robust software.

References:

• Testify/mock package
• Mocking in Go