multiple times from many different places in program parameters allow functions to have changeable parts the parameters appearing in the function definition are called formal parametersand the expressions appearing in function call are known as actual parameters call to function initiates four step process the calling program is suspended the values of actual parameters are assigned to the formal parameters the body of the function is executed control returns immediately following the function call in the calling program the scope of variable is the area of the program where it may be referenced formal parameters and other variables inside function definitions are local to the function local variables are distinct from variables of the same name that may be used elsewhere in the program functions can communicate information back to the caller through return values python functions may return multiple values value returning functions should generally be called from inside of an expression functions that don' explicitly return value return the special object none python passes parameters by value if the value being passed is mutable objectthen changes made to the object may be visible to the caller exercises review questions true/false programmers rarely define their own functions function may only be called at one place in program information can be passed into function through parameters every python function returns some value in pythonsome parameters are passed by reference in pythona function can return only one value python functions can never modify parameter one reason to use functions is to reduce code duplication variables defined in function are local to that function it' bad idea to define new functions if it makes program longer