While developing TestEcho, we modified the echo function to use the package-level variable out when writing its output, so that the test could replace the standard output with an alternative implementation that records the data for later inspection. Using the same technique, we can replace other parts of the production code with easy-to-test “fake” implementations. The advantage of fake implementations is that they can be simpler to configure, more predictable, more reliable, and easier to observe. They can also avoid undesirable side effects such as updating a production database or charging
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