How do I pass a variable by reference?

In Python, arguments are passed by assignment. This means the function receives a reference to the object, but you cannot rebind the original variable from inside the function. Whether a change is visible outside depends on whether the object is mutable or immutable.

Calling a function with an immutable variable

Here, n += 1 creates a new integer and rebinds the local name n. The original x remains unchanged.

Modifying a mutable object

To simulate pass-by-reference behavior, you can pass a mutable container like a list:

This works because lists are mutable. The function modifies the object in place, so the change is visible outside.

Idiomatic Python approach

The standard Python practice is to return the new value instead of trying to modify the original variable: