[Zope3-dev] LocationProxy+None

[Jeff Shell]

On 8/14/07, Adam Groszer <agroszer at gmail.com> wrote:
> A strange thing happened today:
> >>> x = LocationProxy(None, "foo", "bar")
> >>> x is None
> False
> >>> x == None
> True
>
> Is this OK or I'm just missing something?

You have to understand how Python's ``is`` differs from ``==``. An
`is` check is an identity check, not an equality check. It's generally
faster than comparing equality. It's basically like this::

   a is b if id(a) == id(b)

The Python `id` function is basically a memory address of that
specific instance. None, True, and False are all basically constants
that have the same identity unless that name is overridden in a local
namespace.

When proxies enter the picture, you're wrapping the object with a
proxy, so it's no longer the same by identity.

    >>> from zope.location import LocationProxy
    >>> located_none = LocationProxy(None, "foo", "bar")
    >>> id(None)
    3120456
    >>> id(located_none)
    404616
    >>> id(None) == id(located_none)
    False
    >>> located_none is None
    False

Since the proxy object is masquerading as its wrapped object, equality
checks works. But identity doesn't. The `zope.proxy` package includes
some functions that you can use for help.

    >>> from zope.proxy import removeAllProxies, sameProxiedObjects, isProxy
    >>> isProxy(None)
    False
    >>> isProxy(located_none)
    True

You have the option of removing all proxies, which would return the
object wrapped up by the proxy. Since we wrapped up None, it would
return the actual None object and identity checks would pass:

    >>> removeAllProxies(located_none) is None
    True

Or you can use `sameProxiedObjects`, which does the identity check on
the internal objects of one or two proxies:

    >>> sameProxiedObjects(located_none, None)
    True

You can see this when wrapping another instance of None with a location proxy.

    >>> singing_nun = LocationProxy(None, 'austria', 'jane')
    >>> id(singing_nun)
    7378424
    >>> singing_nun is located_none
    False
    >>> sameProxiedObjects(singing_nun, located_none)
    True

-- 
Jeff Shell