> int, float

Immutable? Could you explain this a bit more? (As far as I know, "x += 1" will modify the block of memory that x points to)

No, I had to check that, but it doesn't appear to be true. `id` uses the ptr of the object in CPython, and:

    >>> a = 1
    >>> id(a)
    32855992
    >>> a += 1
    >>> id(a)
    32855968

So it's actually creating a new Integer object, which `a` then points to, or in other words:

    a += 1 is equivalent to a = a + 1

This behavior makes sense, if you wrote:

    a = 10
    b = a
    a += 20

You wouldn't expect b to be 30.