Passing variables to a function

When you pass a variable to a function, what does the function actually receive?

First lets name things to make this a little simpler. The variable we pass to the function will be called the argument whereas the variable used by the function will be called the parameter.

def function(parameter):
	# The variable used by the function is called the parameter
	...
 
argument = ...
 
# The variable we pass to the function is called the argument
 
function(argument)

There are two main choices of what can happen to the argument when it gets passed into the parameter, it can be:

• copied so the function has a new object to play with, this is called Passed by value, or
• given directly to the function so changes made to the parameter in the function will effect the argument that was passed, this is called Passed by reference.

Different approaches can be useful in different contexts and what happens is language specific.

Passed by value

Passing by value means that you make a copy of that argument when it is passed to the function (or something similar - Python example). This reduces the chances of side effects and makes the code simpler to use but can mean slower programs with less efficient use of computer memory.

Passed by reference

Passing by reference means the parameter points to the same object as the argument. This is very efficient for larger data structures both in terms of computer memory and program speed however it complicates your code by increasing the changes of side effects.

In Python Index functionally all arguments are passed by reference - however their mutability allows for something similar to passing by value.