Python: What is the difference between Call-by-Value and Call-by-Object?

Question

Many people say that in Python arguments to functions are passed using a call-by-value model. As I understand it, it is not actually a call-by-value language, but a call-by-object or call-by-sharing model.

What are the differences between a call-by-value model and a call-by-object model? What is an example in Python that shows how these models are different?

Answer 1

Variables in Python aren't values, they're object references. When you call a Python function the arguments are copies of the references to the original object. I don't know how this relates to the terminology you posed in the question.

For example consider the following Python code:

def foo(bar, baz):
    bar = 3
    baz[0] = 4

a = 1
b = [2]
foo(a, b)
print a, b

a is assigned to the object 1, and b is assigned to the list object containing a reference to the object 2. Inside of the function foo, bar is also assigned to the same object 1 and baz is assigned to the same list object. Since 1 is immutable you can't change the object, but you can reassign bar to refer to a different object such as 3. A list is modifiable, so by setting baz[0] to 4 you are also changing the list object that b refers to. The output of the above will be 1 [4].