Explaining names vs variables in Python

[Marko Rauhamaa]

Chris Angelico <rosuav at gmail.com>:

> On Thu, Mar 3, 2016 at 12:39 AM, Marko Rauhamaa <marko at pacujo.net> wrote:
>> Chris Angelico <rosuav at gmail.com>:
>>
>>> Python defines that every object has an identity, which can be
>>> represented as an integer. Since this is an intrinsic part of the
>>> object, no two distinct objects can truly have identical
>>> characteristics. Python's objects are like rifles - there are many
>>> like it, but this one is mine.
>>
>> How can you be sure Python isn't returning the same id value for two
>> distinct objects?
>
> The same way I can be sure about anything else in Python. It's a
> language guarantee.

Actually, my question is (intentionally) nonsensical.

The sameness or distinctness of two objects is not directly defined in
Python. The definition is simply:

   Two objects X and Y are called identical if

       X is Y

   evaluates to a true value.

Additionally, we have:

   If objects X and Y are identical, it is guaranteed that

      id(X) == id(Y)

   evaluates to a true value.

Even more strongly, we have:

   For any objects X and Y,

      id(X) == id(Y) if X is Y else id(X) != id(Y)

   evaluates to a true value.

What is missing is the rules that are obeyed by the "is" operator.


Marko