km wrote: > Hi all, > > I have extended a prototype idea from Alex Martelli's resource on > metaclasses regarding time stamping of instances. > > <code> > import time > class Meta(type): > start = time.time() > def __call__(cls, *args, **kw): > print 'Meta start time %e'%cls.start > x = super(Meta, cls).__call__(*args, **kw) > current_time = time.time() > x._created = current_time - Meta.start > Meta.start = time.time() > return x > > class X(object): > __metaclass__ = Meta > class Y(X): > __metaclass__ = Meta > pass > a = X() > print 'a time stamp %e'%a._created > b = Y() > print 'b time stamp %e'%b._created > print abs(a._created - b._created) > </code> > > I donot understand the difference between > 1) setting __metaclass__ to 'Meta' in class Y > 2) not setting __metaclass__ to Meta in class Y > > Why is the difference in behaviour of time stamping between 1 & 2 ? > > kindly enlighten > I don't see and difference. The rules for establishing the metaclass of a class are fairly well defined: see
http://docs.python.org/ref/metaclasses.html which says * If dict['__metaclass__'] exists, it is used. * Otherwise, if there is at least one base class, its metaclass is used (this looks for a __class__ attribute first and if not found, uses its type). * Otherwise, if a global variable named __metaclass__ exists, it is used. * Otherwise, the old-style, classic metaclass (types.ClassType) is used. By the second rule it would appear that the metaclass of X is the same as that of Y even if the __metaclass__ assignment is commented out, and informal testing appears to show that this is the case. Debugging with Wing IDE and examining the classes at a breakpoint shows this to be true (even after Y's __metaclass__ assignment is commented out): >>> X.__metaclass__ <class '__main__.Meta'> >>> Y.__metaclass__ <class '__main__.Meta'> >>> regards Steve -- Steve Holden +1 571 484 6266 +1 800 494 3119 Holden Web LLC/Ltd http://www.holdenweb.com Skype: holdenweb http://del.icio.us/steve.holden --------------- Asciimercial ------------------ Get on the web: Blog, lens and tag the Internet Many services currently offer free registration ----------- Thank You for Reading ------------- -- http://mail.python.org/mailman/listinfo/python-list
