nice explanation .. thanks this cleared my doubt >>> k = [20] >>> for i in [[1,2,3],[3,4,5]]: ... k += i ... >>> k [20, 1, 2, 3, 3, 4, 5]
On Tue, Aug 1, 2017 at 1:35 AM, Alan Gauld via Tutor <tutor@python.org> wrote: > On 01/08/17 07:06, ramakrishna reddy wrote: > > >> sum([1,2,3]) returns 6. > > > > But sum with [] 'empty list' as second parameter returns as below. > > > >> sum([[1,2,3], [3,4,5]], []) returns [1, 2, 3, 3, 4, 5] > > An interesting question, I wasn't aware that you could > add lists with sum. > > However, the results seem clear enough. Let's start with the help() > description: > ################# > Help on built-in function sum in module builtins: > > sum(...) > sum(iterable[, start]) -> value > > Return the sum of an iterable of numbers (NOT strings) plus > the value of parameter 'start' (which defaults to 0). > When the iterable is empty, return start. > ################# > > And lets see how it behaves with integers first: > >>> sum([1,2,3]) # use start default of 0 > 6 > >>> sum([1,2,3],0) # use explicit start=0 > 6 > >>> sum([1,2,3],1) # use start=1 > 7 > >>> sum([1,2,3],9) # start = 9 > 15 > > So in each case we get the sum of the items in the iterable > plus the value of start. sum() is effectively doing > > result = start > for n in iterable: result += n > return result > > Now lets try using a list of lists: > > >>> sum([ [1,2,3],[3,4,5] ]) > Traceback (most recent call last): > File "<stdin>", line 1, in <module> > TypeError: unsupported operand type(s) for +: 'int' and 'list' > > We get a type error because the start uses 0 and we can't > add an int and a list. So let's provide a list as the > start value: > > >>> sum([ [1,2,3],[3,4,5] ], []) > [1, 2, 3, 3, 4, 5] > >>> > > We now get the behaviour you saw because it adds the > three lists together using the same algorithm as above. > > And if we use a non-empty start it becomes even more clear: > > >>> sum([ [1,2,3],[3,4,5] ], [42]) > [42, 1, 2, 3, 3, 4, 5] > > We could use tuples instead of lists and get the same result. > > What is interesting is that if we try the same thing with > other iterables, such as a string, it doesn't work, even > though string addition is defined. There is obviously > some type checking taking place in there. > > 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
