Problem

First problem

We have a problem. We want to add credit card processing to the project application so that we can make money. Testing the credit card functionality presents immediate difficulties. For one, we don’t want to make a credit card purchase during testing accidentally because that would be bad. But even if the purchase gateway provides a test sandbox, we still don’t want to depend on it for the unit tests to run. That network call is slow, and we don’t want the passing tests to depend on a remote server’s status.

Second problem

Or we might ...