python - Why can a function modify some arguments as perceived by the caller, but not others?

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I'm trying to understand Python's
approach to variable scope. In this example, why is f() able to
alter the value of x, as perceived within
main(), but not the value of
n?

def
            f(n, x):
 n = 2

 x.append(4)
 print('In f():',
            n, x)

def main():
 n = 1
 x =
            [0,1,2,3]
 print('Before:', n, x)
 f(n, x)
 print('After:
            ', n,
            x)


main()

Output:

Before:
            1 [0, 1, 2, 3]
In f(): 2 [0, 1, 2, 3, 4]
After: 1 [0, 1, 2, 3,
            4]

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class="normal">Answer

Some answers contain the word
"copy" in a context of a function call. I find it
confusing.

Python doesn't copy
objects you pass during a function call
ever.

Function
parameters are names. When you call a function Python binds these
parameters to whatever objects you pass (via names in a caller
scope).

Objects can be mutable (like lists) or
immutable (like integers, strings in Python). Mutable object you can change. You can't
change a name, you just can bind it to another
object.

Your example is not about href="https://docs.python.org/2/tutorial/classes.html#python-scopes-and-namespaces"
rel="noreferrer">scopes or namespaces, it is about href="http://docs.python.org/reference/executionmodel.html#naming-and-binding"
rel="noreferrer">naming and binding and href="http://docs.python.org/reference/datamodel.html#objects-values-and-types"
rel="noreferrer">mutability of an object in Python.

def f(n, x): # these
            `n`, `x` have nothing to do with `n` and `x` from main()
 n = 2 # put `n`
            label on `2` balloon
 x.append(4) # call `append` method of whatever object
            `x` is referring to.
 print('In f():', n, x)
 x = [] # put `x` label
            on `[]` ballon
 # x = [] has no effect on the original list that is passed
            into the function

Here
are nice pictures on href="https://web.archive.org/web/20180121150727/http://python.net/~goodger/projects/pycon/2007/idiomatic/handout.html#other-languages-have-variables"
rel="noreferrer">the difference between variables in other languages and names in
Python.