In general, functions in Python may also have side effects rather than just turning an input into an output. The print() function is an example of this: it returns None while having the side effect of outputting something to the console. However, to understand decorators, it's enough to think about functions as tools that turn given arguments into values.
In functional programming, you work almost entirely with pure functions that don't have side effects. While not a purely functional language, Python supports many functional programming concepts, including treating functions as first-class objects.
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