Learning Python

Argument Passing

Let’s expand on the notion of argument passing in Python. Earlier, we noted that arguments are passed by assignment ; this has a few ramifications that aren’t always obvious to beginners:

Arguments are passed by assigning objects to local names

Function arguments should be familiar territory by now: they’re just another instance of Python assignment at work. Function arguments are references to (possibly) shared objects referenced by the caller.

Assigning to argument names inside a function doesn’t affect the caller

Argument names in the function header become new, local names when the function runs, in the scope of the function. There is no aliasing between function argument names and names in the caller.

Changing a mutable object argument in a function may impact the caller

On the other hand, since arguments are simply assigned to objects, functions can change passed-in mutable objects, and the result may affect the caller.

Here’s an example that illustrates some of these properties at work:

>>> def changer(x, y):
...    x = 2             # changes local name's value only
...    y[0] = 'spam'     # changes shared object in place
...
>>> X = 1
>>> L = [1, 2]
>>> changer(X, L)        # pass immutable and mutable
>>> X, L                 # X unchanged, L is different
(1, ['spam', 2])

In this code, the changer function assigns to argument name x and a component in the object referenced by argument y. Since x is a local name in the function’s scope, the first assignment has no effect on the caller; it doesn’t change ...