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

def increment(n):
  n += 1

x = 1
increment(x)
print(x)  # Output: 1

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:

def increment(n):
  n[0] += 1

x = [1]
increment(x)
print(x[0])  # Output: 2

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

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Idiomatic Python approach

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

def increment(n):
  return n + 1

x = 1
x = increment(x)
print(x)  # Output: 2