User-defined functions are reusable code blocks. They only need to be written once, then they can be used multiple times. They can even be used in other applications, too.
In Python a function is defined using the def Keyword.
To call a function, use the function name():
With the Return command, you can return values of a function:
Depending on the function definition, one function can be called with zero, one, two or more parameters.