Bryan wrote: > in the python cookbook 2nd edition, section 6.7 (page 250-251), there a > problem > for implementing tuples with named items. i'm having trouble > understanding how one of commands work and hope someone here can explain > what exactly is going on. > without copying all the code here, here is the gist of the problem: > > from operator import itemgetter > > class supertup(tuple): > def __new__(cls, *args): > return tuple.__new__(cls, args) > > setattr(supertup, 'x', property(itemgetter(0))) > > >>> t = supertup(2, 4, 6) > >>> t.x > >>> 2 > > > i understand what itemgetter does, > > >>> i = itemgetter(0) > >>> i((2, 3, 4)) > >>> 2 > >>> i((4, 8, 12)) > >>> 4 > > i understand what property does, and i understand what setattr does. i > tested this problem myself and it works, but i can't understand how t.x > evaluates to 2 in this case. how does itemgetter (and property) know what > tuple to use? in my itemgetter sample, the tuple is passed to itemgetter > so it's obvious to see what's going on. but in the supertup example, it > isn't obvious to me.
Perhaps it helps to see the "intermediate" steps between a standard property definition and your setattr() example: >>> class supertup(tuple): ... def getx(self): return self[0] ... x = property(getx) ... gety = itemgetter(1) ... y = property(gety) ... z = property(itemgetter(2)) ... >>> supertup.t = property(itemgetter(3)) >>> setattr(supertup, "u", property(itemgetter(4))) >>> t = supertup(range(5)) >>> t.u, t.t, t.z, t.y, t.x (4, 3, 2, 1, 0) class T: def method(self): pass and class T: pass def method(self): pass T.method = method are both creating a class 'T' with a method 'method'. setattr() is only needed if you don't know the attribute's name at compile time -- a method is just and attribute of a class object. Peter -- http://mail.python.org/mailman/listinfo/python-list
