Python calling convention

Python is neither pass by value nor pass by reference. Python is "pass by assignment".

Supporting reference, the Python FAQ.

IOW:

If you pass an immutable value, changes to it do not change its value in the caller - because you are rebinding the name to a new object. This is because there is no way of mutating an immutable object.
If you pass a mutable value, changes made in the called function, also change the value in the caller, so long as you do not rebind that name to a new object.
- If you assign the name to a new value, that change and subsequent changes to the name are not seen in the caller.
- So if you pass a list, and change its 0th value, that change is seen in both the called and the caller.
- But if you reassign the name to a new list, this change is lost.
- But if you slice the list and replace that with a new list, that change is seen in both the called and the caller.

EG:

    def change_it(list_):
        # This change would be seen in the caller if we left it alone
        list_[0] = 28

        # This change is also seen in the caller, and replaces the above
        # change
        list_[:] = [1, 2]

        # This change is not seen in the caller.
        # If this were pass by reference, this change too would be seen in
        # caller.
        list_ = [3, 4]

    thing = [10, 20]
    change_it(thing)
    # here, thing is [1, 2]

If you're a C fan, you can think of this as passing a pointer by value - not a pointer to a pointer to a value, just a pointer to a value. And sometimes the thing you are pointing at is const, and sometimes it is not.

HTH.

Here's a presentation on the topic by Ned Batchelder.

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Timestamp: 2025-11-01 19:03:10 PDT

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