Does Python pass by reference or value?

One topic which comes up from time to time, usually generating a lot of heat and not much light, is the question of how Python passes values to functions and methods. Usually the question is posed as "Does Python use pass-by-reference or pass-by-value?".

The answer is, neither. Python, like most modern object-oriented languages, uses an argument passing strategy first named by one of the pioneers of object-oriented programming, Barbara Liskov, in 1974 for the language CLU. Liskov named it pass-by-object-sharing, or just pass-by-sharing, but the actual strategy is a lot older, being the same as how Lisp passes arguments. Despite this august background, most people outside of Python circles have never heard of pass-by-sharing, and consequently there is a lot of confusion about argument passing terminology.

Let's start by looking at how people get confused. Let's start by "proving" that Python is pass-by-value (also know as call-by-value), then we'll "prove" that Python is pass-by-reference (call-by-reference). It's actually neither, but if you think that there are only two ways to pass arguments to a function, you might be fooled into thinking Python uses both. ( Read more... )