Instance of class "object"

[甜瓜]

2008/5/16 Hrvoje Niksic <hniksic at xemacs.org>:
> "甜瓜" <littlesweetmelon at gmail.com> writes:
>
>> Howdy,
>>     I wonder why below does not work.
>>
>> a = object()
>> a.b = 1        # dynamic bind attribute failed...
>
> Because the default object class doesn't have a dict or other
> indication of state.  It's a "pure" Python object whose only visible
> properties are its type and its identity.  (On the implementation
> level it also has a refcount.)
>
> This is necessary because all other Python objects (both new-style
> classes and C extensions) inherit from 'object', on the C level.
> Having state in 'object' would mean having that same in *all* other
> Python objects.  The current design, on the other hand, allows
> creation of very lightweight python objects that don't even have a
> dict.
>
> Subclassing object instructs Python as to what kind of state you want
> your class to have.  The default is to have a dict to store
> properties:
>
> # a class with dict -- any property goes through dict, and creating a
> # Foo object actually creates two objects, one Foo and one dict
> class Foo(object):
>    pass
>
> But you can also specify:
>
> # an efficient 'Pair' class holding two objects
> class Pair(object):
>    __slots__ = 'first', 'second'
>
> Instances of Pair take up even less room that 2-element tuples
> because they don't carry the size information in the object.
>
> Now, if the object class carried a dict with it, it would be
> impossible to create a class like 'Pair'.
>
Really interesting. When the tuple ('first', 'second') is assigning to
__slot__, a special operation is done which makes __slot__ pointing
to a magic structure rather than a normal tuple. Am I right?


>> 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)?
>
> It follows from LSP that what the subclass may not introduce new
> preconditions.  In this case the subclass accepts a case that the
> original class didn't, so it doesn't introduce a new precondition, it
> simply weakens (removes) an existing one.

Indeed.

Thank you very much.

--
ShenLei