How to Effectively Stub Out `time.Now()` for Golang Unit Tests?

How to Globally Stub Out time.Now() for Unit Tests

In time-sensitive applications, accurately mocking the current time can be crucial for unit testing. However, time.Now() is often invoked at multiple locations, making it challenging to track elapsed time accurately.

Using a Custom Interface

The solution suggested in the golang-nuts thread you referenced provides a solid approach by defining a custom Clock interface that wraps the time package's functionality:

type Clock interface {
  Now() time.Time
  After(d time.Duration) <-chan time.Time
}

You can implement this interface with a concrete implementation like this:

type realClock struct{}
func (realClock) Now() time.Time { return time.Now() }
func (realClock) After(d time.Duration) <-chan time.Time { return time.After(d) }

For testing, create a mock implementation of the Clock interface.

Handling System Time Changes

While it may seem tempting to change the system time globally for testing, this approach is highly discouraged. It can introduce unpredictable dependencies and side effects.

Custom Time Package

As an alternative, consider creating a custom time package that wraps the standard library's time package. This custom package could provide a function to switch to a mock time implementation during testing.

Best Practices for Time-Sensitive Code

To improve testability and reduce side effects, strive to design your code with stateless components as much as possible. By structuring your code into separate, testable parts, you simplify the unit testing process and make it easier to ensure the correct behavior under different time conditions.

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