The Python Apprentice

Modifying external objects in a function

To demonstrate Python's argument passing semantics, we'll define a function at the REPL which appends a value to a list and prints the modified list. First we'll create a list and give it the name m:

>>> m = [9, 15, 24]

Then we'll define a function modify() which appends to, and prints, the list passed to it. The function accepts a single formal argument named k:

>>> def modify(k):...     k.append(39)...     print("k =", k)...

We then call modify(), passing our list m as the actual argument:

>>> modify(m)k = [9, 15, 24, 39]

This indeed prints the modified list with four elements. But what does our list reference m outside the function now refer to?

>>> m[9, 15, 24, 39]

The list referred to by m has been modified ...