Reference or Value?

Andrew Koenig ark at
Sun Feb 22 13:39:01 EST 2009

"andrew cooke" <andrew at> wrote in message 
news:mailman.464.1235320654.11746.python-list at

> as far as i understand things, the best model is:
> 1 - everything is an object
> 2 - everything is passed by reference
> 3 - some objects are immutable
> 4 - some (immutable?) objects are cached/reused by the system

0 - Assignment rebinds the reference on its left-hand side; it does not 
change the object to which that reference refers.


    x = 42
    y = x

Now x and y are bound to the same object

    x = x + 1

This statement computes the value of x + 1, which is a new object with value 
43.  It then rebinds x to refer to this object, so x and y now refer to 
different objects.  Therefore:

    def f(a):
        a = a + 1
    x = 42

This example behaves analogously to the previous one: The assignment a = a + 
1 binds a to a new object, so it does not affect the object to which x is 

    z = [3]
    y = z
    z[0] = z[0] + 1

The assignment rebinds z[0] to refer to a new object that has the value 4. 
This rebinding does not affect the object formerly bound to z[0].  It does, 
however, affect the value of the object to which z is bound, because it 
changes the value of its list element.  By analogy:

    def g(b):
        b[0] = b[0] + 1
    w = [42]

Now w[0] will be 43.

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