On May 17, 1:09 am, Carl Banks <[EMAIL PROTECTED]> wrote: > On May 16, 4:46 am, "甜瓜" <[EMAIL PROTECTED]> 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
Python is the same language on sufficiently many distinct machines that all the people can use it if they want. That means identical outputs. Do their 100%-ers go with the times? -- http://mail.python.org/mailman/listinfo/python-list
