Semantics of ==

[David M. Cooke]

At some point, axelboldt at yahoo.com (Axel Boldt) wrote:

> Still trying to understand "=="... It appears as if two equal objects
> can become unequal if you perform the same operations on them:
>
>   >>> l=[1]
>   >>> s=l
>   >>> l.append(s)

You're building a cyclic list: l contains a reference to itself.
General advice: don't do that unless you know what you're doing.

>   >>> w=[1]
>   >>> r=[1,w]
>   >>> w.append(r)
>   >>> s
>   [1, [...]]
>   >>> w
>   [1, [1, [...]]]
>   >>> s==w
>   True
>
> Note that they're equal, yet are printed differently.

Element-by-element, they're equal, yes.

>   >>> s[0]=2
>   >>> w[0]=2
>   >>> s==w
>   False
>
> All of a sudden they have become unequal.

That's b/c s originally looked like this if we expand out the
recursion a few more levels:
[1, [1, [1, [1, [...]]]]]
where each sub-list is also s.

w also looks like
[1, [1, [1, [1, [...]]]]]
where the *second* sub-list is w.

Set the first elements to 2:

s is now:
[2, [2, [2, [2, ...]]]]
w is now:
[2, [1, [2, [1, ...]]]]

See? The second sub-list of w is not w, so it wasn't changed.

-- 
|>|\/|<
/--------------------------------------------------------------------------\
|David M. Cooke
|cookedm(at)physics(dot)mcmaster(dot)ca