How are arguments passed to Python functions?

Explains Python calls through object sharing, local parameter binding, mutation, reassignment, and explicit ownership.

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When Python calls a function, each parameter name is bound to the object supplied by the caller. The function does not receive access to rebind the caller's name. If the object is mutable, the function can mutate that shared object and the caller can observe the change. If the function assigns a different object to its parameter, only the local parameter is rebound. This is often called call by sharing or passing an object reference by value.

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The useful explanation avoids a pass-by-value versus pass-by-reference slogan and shows the observable behavior of shared objects and local names.
When Python calls a function, each parameter name is bound to the object supplied by the caller. The function does not receive access to rebind the caller's name. If the object is mutable, the function can mutate that shared object and the caller can observe the change. If the function assigns a different object to its parameter, only the local parameter is rebound. This is often called call by sharing or passing an object reference by value.
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For example, a function that calls items.append(value) changes the list both caller and callee can reach. A function that executes items = [value] only points its local name at another list. The same rule applies to user-defined objects. The important API question is therefore not just how Python passes arguments, but whether the callee is allowed to mutate or retain the object. Returning a new value can make ownership clearer when mutation would surprise the caller.

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Saying Python is simply pass-by-reference.
Saying every argument is copied as a new object.
Using visible mutation as proof that the caller's name was passed.
Ignoring nested sharing after making a shallow copy.

How are arguments passed to Python functions?

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