On Thu, Dec 27, 2018 at 1:56 PM <jf...@ms4.hinet.net> wrote: > > I saw the code below at stackoverflow. I have a little idea about the scope > of a class, and list comprehension and generator expressions, but still can't > figure out why Z4 works and Z5 not. Can someone explain it? (in a > not-too-complicated way:-) > > class Foo(): > XS = [15, 15, 15, 15] > Z4 = sum(val for val in XS) > try: > Z5 = sum(XS[i] for i in range(len(XS))) > except NameError: > Z5 = None > > print(Foo.Z4, Foo.Z5) > >>> 60 None >
Class scope is special, and a generator expression within that class scope is special too. There have been proposals to make these kinds of things less special, but the most important thing to remember is that when you create a generator expression, it is actually a function. Remember that a function inside a class statement becomes a method, and that inside the method, you have to use "self.X" rather than just "X" to reference class attributes. That's what's happening here. ChrisA -- https://mail.python.org/mailman/listinfo/python-list
