Op maandag 3 februari 2014 23:19:39 UTC+1 schreef Steven D'Aprano: > On Mon, 03 Feb 2014 13:36:24 -0800, Jean Dupont wrote: > > I have a list like this: > > [1,2,3] > > > > The argument of my function should be a repeated version e.g. > > [1,2,3],[1,2,3],[1,2,3],[1,2,3] (could be a different number of times > > repeated also) > > > > what is the prefered method to realize this in Python? > > I don't really understand your question. It could mean any of various > things, so I'm going to try to guess what you mean. If my guesses are > wrong, please ask again, giving more detail, and possibly an example of > what you want to do and the result you expect. > I think you mean that you have some function that needs to take (say) > five arguments, and you want to avoid writing: > result = function([1, 2, 3], [1, 2, 3], [1, 2, 3], [1, 23], [1, 2, 3]) > because it's too easy to make a mistake (as I did, deliberately, above -- > can you see it?). > If my guess is correct, try this: > mylist = [1, 2, 3] > result = function(mylist, mylist, mylist, mylist, mylist) > > That's perfectly reasonable for two or three arguments, but not so much > for five. Instead, here's a trick: first we make five identical > references to the same list: > [mylist]*5 # same as [mylist, mylist, mylist, mylist, mylist] > then expand them as arguments to the function: > mylist = [1, 2, 3] > list_of_lists = [mylist]*5 > result = function(*list_of_lists) > (The * operator means multiplication when used between two arguments, and > inside a function call a leading * also does argument expansion.) > > But wait... there's something slightly weird here. Even though there are > five distinct references to mylist, they're all the same list! Change > one, change all. This may be what you want, or it may be a problem. Hard > to tell from your question. > Think about references as being a little bit like names. A *single* > person could be known as "son", "Dad", "Mr Obama", "Barack", "Mr > President", "POTUS", and more. In this case, we have a single list, [1, > 2, 3], which is known by six references: the name "mylist", and five > additional references list_of_lists index 0, list_of_lists index 1, and > so on up to list_of_lists index 4. > We can prove that they all refer to the same list by running a bit of > code in the interactive interpreter: > > py> mylist = [1, 2, 3] > py> list_of_lists = [mylist]*5 > py> list_of_lists > [[1, 2, 3], [1, 2, 3], [1, 2, 3], [1, 2, 3], [1, 2, 3]] > py> mylist.append(99) > py> list_of_lists > [[1, 2, 3, 99], [1, 2, 3, 99], [1, 2, 3, 99], [1, 2, 3, 99], [1, 2, 3, > 99]] > > So rather than having five references to the same, identical, list, you > might want five *copies*. You can copy a list using slicing: > mylist = [1, 2, 3] > copy = mylist[:] > Instead of using list multiplication to repeat five identical lists, we > make five copies using a list comprehension: > list_of_lists = [mylist[:] for i in range(5)] > then expand it in the function call as before: > result = function(*list_of_lists) > > Hope this helps, Yes it does, thanks a lot to you and all the others who responded, "the missing link" which until now I wasn't aware of but which was essential for the solution was the "*" in result = function(*list_of_lists)
kind regards, jean -- https://mail.python.org/mailman/listinfo/python-list
