Functions create reusable behavior
A function groups a piece of logic behind a meaningful name. Good functions have a clear purpose, explicit inputs and a predictable return value. They are one of the main tools for preventing duplicated code.
Parameters and return values
Parameters describe what a function needs, while return sends a result back to the caller. A function that reaches the end without a return statement returns None.
Keep responsibilities small
A calculation function is easier to test when it does not also read input, print output, access a database and send an email. Separate those responsibilities when the application grows.
Practice: write a reusable discount calculation function and test normal, zero and invalid discount values.
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