Lists in Python Lists as Arguments/Parameters

Lists as arguments to functions Just like other data types, lists can be sent to functions as arguments The function must be aware of the fact that a parameter is a list, so it can treat it as a list (use subscripts) Lists are passed to functions in a different way than other arguments you have seen so far

Passed by value The process y ou have seen so far is that the values of arguments are copied into the spaces reserved for parameters The function can do anything you want with the copies The copies are deleted when the function finishes and returns This is called ‘passing arguments by value’

Passing by reference Lists are handled differently Their address is sent to the function in the place of a parameter Using the address (also called the reference) the function can change the elements of the list – in other words, the function is changing the original contents of the original list In other words, the list contents can be changed permanently

Python versus other languages In languages like C, C++ and Java, passing by reference is fairly common A main reason for this is that those languages can return no more than one value via a return statement If a function needs to return two things, it must use pass by reference to get one of the things back to the calling code

Python vs. other languages Python on the other hand can return many different values through the return statement Typically functions do not need to pass things by reference because of this fact The main thing to be aware of is that a function can change a list, which will affect the calling function’s variable. Most Python programmers would consider doing this a mistake by the programmer. If you do use passing by reference, document it WELL in the header, so that anyone using the function knows that a list argument can be changed!