Everyone that responded, Thanks very much for the excellent explanations! The distinction between a reference to an object and a seperate copy of the object is quite clear now.
--Bill On Apr 18, 2015 1:44 AM, "Alan Gauld" <alan.ga...@btinternet.com> wrote: > On 18/04/15 04:16, Bill Allen wrote: > >> If I have a list defined as my_list = ['a','b','c'], what is the is >> differnce between refering to it as my_list or my_list[:]? These seem >> equivalent to me. Is that the case? Is there any nuance I am missing >> here? Situations where one form should be used as opposed to the other? >> > > Others have already given some good explanations. > I'll add a slightly different take. > > Your confusion starts with your first statement: > > > I have a list defined as my_list = ['a','b','c'] > > What you should be saying is > > I have a list defined as ['a', 'b', 'c'] > > Thats the list object that you are working with. The object is completely > separate from the name that you choose to associate > with it. > > You then bound that list to a name: my_list. > You could bind it to any number of names but > there would still only be one object: > > foo = my_list > bar = foo > baz = my_list > > Now I have 4 names all referring to the same list object. > > The next source of confusion comes from another mist-statement: > > > differnce between refering to it as my_list or my_list[:] > > The [:] at the end is an operator that returns a copy of the list. > So when you use it you are NOT referring to the original list > at all. You are creating a new copy. > > So if we now take one of our previous names, say foo, and do: > > foo = my_list[:] > > foo no longer refers to your original list ['a','b','c'] > but to a completely new copy of that list. > > If you modify my_list the changes will show up when you look > at bar and baz as well. But foo will be unchanged > > my_list[0] = 'z' > print baz -> prints ['z','b','c'] - the same list as my_list > print foo -> prints ['a','b','c'] - a different list object > > Understanding the separation of names from objects in Python is essential > to understanding how it works. It is different to many > other languages in this respect. > > And understanding the difference between identity and value is also > important. Two completely different objects can have the same value > and so appear the same but they are in fact entirely different. > Think about two drivers who both buy the exact same model of car. > They may look identical, but they are two separate cars. > > HTH > -- > Alan G > Author of the Learn to Program web site > http://www.alan-g.me.uk/ > http://www.amazon.com/author/alan_gauld > Follow my photo-blog on Flickr at: > http://www.flickr.com/photos/alangauldphotos > > > _______________________________________________ > Tutor maillist - Tutor@python.org > To unsubscribe or change subscription options: > https://mail.python.org/mailman/listinfo/tutor > _______________________________________________ Tutor maillist - Tutor@python.org To unsubscribe or change subscription options: https://mail.python.org/mailman/listinfo/tutor
