On Sun, Dec 1, 2013 at 5:04 PM, Steven D'Aprano <st...@pearwood.info> wrote: > On Sun, Dec 01, 2013 at 02:32:38PM +1000, Amit Saha wrote: >> Hello, >> >> I was told by someone (as a comment) that a code snippet such as this >> "would make Pythonistas talk my ear off about how evil the append()" >> function is: > > There is absolutely nothing wrong with append. Either you have > misunderstood, or the person who told you this was smoking crack.
heh, no I literally quote the person above, so I think I understood him alright. > >> >>> mylist = [] >> >>> mylist.append(1) >> # a number of times over > > However, growing a list one item at a time using append like this is too > much hard work. Some better solutions: > > # You have an existing list, and want to append a bunch of things to it > mylist.extend([x, y+1, z*2]) > > # You want a list consisting of the same objects repeated many times > mylist = [0, 1, 2]*100 # like [0, 1, 2, 0, 1, 2, 0, 1, 2, ...] > > # You want to create a list containing known objects > mylist = [1, 2, 4, 8, 16, 32, 64] > > # Like above, but using a list comprehension > mylist = [2**n for n in range(7)] > > # You don't know how many things you want to add. > mylist = [] > x = 1 > while x < 100: > mylist.append(x) > x = 3*x-1 > > and so on. As you can see, append has its job to do. > The last case is the closest to the context in which I used the above code construct. > >> I have some ideas that on an append() the list's internal size >> increases (doubled?) since CPython over-allocates memory, and such. > > Not just on append. Any operation that adds or deletes items from a list > might trigger a re-size. If the list needs to increase, it will double > in size up to some maximum, then it will grow by a smaller amount. The > purpose of this is that *on average* appending to the list will take a > fixed amount of time. > > >> So, the question is: what is the alternative to growing a list when I >> have no idea what items or how many may be there? > > Don't worry about growing the list. That's all handled for you as part > of the list interface. The whole point of using Python is to not have to > worry about internal details like that. Right, that's what I thought. Good that I checked here, now I can write a nice reply to the comment :-) Thanks Steven and Dave. Best, Amit. -- http://echorand.me _______________________________________________ Tutor maillist - Tutor@python.org To unsubscribe or change subscription options: https://mail.python.org/mailman/listinfo/tutor
