In python, is a function return a shallow or deep copy?

In python everything is a reference. Nothing gets copied unless you explicitly copy it.

In your example, x and y reference the same object.

It will be a shallow copy, as nothing has been explicitly copied.

def foo(list):
    list[1] = 5
    return list

For example:

>>> listOne = [1, 2]
>>> listTwo = [3, 4]
>>> listTwo = listOne
>>> foo(listTwo)
[1, 5]
>>> listOne
[1, 5]