Programming Fundamentals › Collections

Pass by Value vs Reference

Whether a function receives a copy or a reference to the caller's data.

Also known as: call by value, call by reference, pass by reference, call by sharing

When you pass a variable into a function, does the function get its own copy, or access to the caller’s original?

• Pass by value: the function gets a copy. Changing it has no effect outside.
• Pass by reference: the function gets an alias to the caller’s variable. Changing it changes the caller’s data.

Languages differ. C++ has both. Go passes copies. Python, JavaScript and Java do something in between that’s often called call by sharing: the function receives a copy of the reference, so both names point at the same object.

What call by sharing means in practice

def add_item(items):
    items.append("x")      # changes the caller's list

def replace(items):
    items = ["y"]          # rebinds the local name only

my = ["a"]
add_item(my)    # my == ["a", "x"]
replace(my)     # my is still ["a", "x"]

Two rules cover it:

1. Mutating the object through the parameter is visible to the caller.
2. Reassigning the parameter is not.

Immutable values (numbers, strings, tuples) can’t be mutated, so they behave as if they were passed by value.

The bug to avoid

A function that quietly edits its input:

def top_three(scores):
    scores.sort(reverse=True)      # reorders the caller's list!
    return scores[:3]

def top_three_safe(scores):
    return sorted(scores, reverse=True)[:3]   # new list; input untouched

Prefer returning a new value over modifying arguments. If you must keep a private copy, make one explicitly, and remember a plain copy is shallow: nested objects are still shared.

Rule of thumb: check your language’s rule once, then assume a function can change any mutable argument unless it says otherwise.