On May 16, 4:16 am, Hrvoje Niksic <[EMAIL PROTECTED]> wrote: > "甜瓜" <[EMAIL PROTECTED]> 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'. > > > 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.
Honor and narrow down: > 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. to: mass & volume, probably isolating -same-object- and -state-. No sinking. -- http://mail.python.org/mailman/listinfo/python-list
