On Tue, Apr 17, 2018 at 3:34 AM, Peter Otten <__pete...@web.de> wrote: > Irv Kalb wrote: > >> I have been writing OOP code for many years in other languages and for the >> past few years in Python. I am writing new curriculum for a course on OOP >> in Python. In order to see how others are explaining OOP concepts, I have >> been reading as many books and watching as many videos as I can. I've >> been watching some videos created by Dr. Chuck Severance in a series >> called "Python For Everyone". I think "Dr. Chuck" is an excellent teacher >> and I think his videos are outstanding. >> >> Today I watched this video: https://www.youtube.com/watch?v=b2vc5uzUfoE >> <https://www.youtube.com/watch?v=b2vc5uzUfoE> which is about 10 minutes >> long. In that video he gives a very basic overview of OOP and classes. >> He gives a demonstration using the following example: >> >> class PartyAnimal(): >> x = 0 >> >> def party(self): >> self.x = self.x + 1 >> print('So far', self.x) >> >> an = PartyAnimal() >> an.party() >> an.party() >> an.party() > > This style is rather brittle. Consider the following variant: > >>>> class A: > ... x = "" > ... >>>> a = A() >>>> b = A() >>>> a.x += "a" >>>> a.x += "a" >>>> b.x += "b" >>>> a.x > 'aa' >>>> b.x > 'b' >>>> A.x > '' > > Seems to work. Now let's change x to something mutable: > >>>> A.x = [] >>>> a = A() >>>> b = A() >>>> a.x += "a" >>>> a.x += "a" >>>> b.x += "b" >>>> a.x > ['a', 'a', 'b'] >>>> b.x > ['a', 'a', 'b'] >>>> A.x > ['a', 'a', 'b'] > > Conclusion: don't do this except to learn how attributes work in Python.
Be careful: Your example looks nice and tidy because you're adding single-character strings onto a list. They happen to work as you'd expect. Normally, though, if you're adding onto a list, you probably want to use another list: a.x += ["a"] But you've successfully - if partly unwittingly - shown how hairy this can be :) ChrisA -- https://mail.python.org/mailman/listinfo/python-list
