Instance of class "object"

[Carl Banks]

On May 16, 4:46 am, "甜瓜" <littlesweetme... at gmail.com> wrote:
> Howdy,
>     I wonder why below does not work.
>
> a = object()
> a.b = 1        # dynamic bind attribute failed...
>
> To make it correct, we have to create a new class:
> class MyClass(object): pass
> a = MyClass()
> a.b = 1       # OK
>
> Does this strange behavior break the LSP (Liskov substitution principle)?


I suspect it's possible to come up with a property that causes LSP to
be violated for any two given distinct classes.

For example, if the property you're considering is "Raises an
exception when you try to access an attribute", then yes, the subtype
cannot be substituted for the base type and would violate LSP.
However, that's not really such a useful property in most cases.

The reason instances of object don't have dynamic attributes is
because dyanmic attributes are internally represented by a dict
object.  To keep memory footprint low, instances of many built-in
types don't have an associated dict and so can't bind attributes
dynamically.  Instances of object can't have an associated dict
because, if they did, it would mean all objects would have to have an
associated dict, because all types are subclasses of object.

Because of this, direct instances of object really don't have much use
in Python.  It's one main use case is as a sentinel or marker of some
sort.  It's only useful property is that it is itself, and nothing
else.  But, that property is true of all instances in Python;
therefore any instance may be substituted for an object instances,
therefore it satisfies LSP.

(Phew: what a tangle of nomenclature that was.)


Carl Banks