Are integers in Python passed by value or reference?

Question

I wrote the following code to check if integers are passed by value or reference.

foo = 1

def f(bar):
    print id(foo) == id(bar)
    bar += 1
    print foo, b         


        

Answer 1

The following happens:

print id(foo) == id(bar)

The identity is the same. print foo is bar would have yielded the same, BTW.

bar += 1

This is translated to:

bar = bar.__iadd__(1)

And only if this does not work or does not exist, it calls:

bar = bar.__add__(1)

(I omit the case that bar = 1.__radd__(bar) could as well be called.)

As bar refers to a number, which is immutable, a different object is returned instead, so that bar refers to 2 now, leaving foo untouched.

If you do any of

print id(foo) == id(bar)
print foo is bar

now, you see that they now point to different objects.