Testing your code is a great way to check whether you have implemented everything correctly. No matter how much you try, no matter how hard you analyze all the cases, sometimes, just the smallest mistake can lead to a big problem which could have maybe been avoided writing tests.
When you write tests, you are obviously not going to use data from production. We need to use some dummy data that does not belong to anyone who is actually using the project. This is called faking the data.
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