Today I spent about 20 minutes trying to figure out why
this worked as expected:
users_stories_dict[a] = s + [b]
but this would have a
users_stories_dict[a] = s.append(b)
Anyone know why the append function does not return the new list? I’m looking for some sort of sensible reason this decision was made; it looks like a Python novice gotcha to me right now.
append works by actually modifying a list, and so all the magic is in side-effects. Accordingly, the result returned by
append is None. In other words, what one wants is:
users_stories_dict[a] = s
But, you’ve already figured that much out. As to why it was done this way, while I don’t really know, my guess is that it might have something to do with a
false) exit value indicating that an operation proceeded normally, and by returning
None for functions whose role is to modify their arguments in-place you report that the modification succeeded.
But I agree that it would be nice if it returned the modified list back. At least, Python’s behavior is consistent across all such functions.
The append() method returns a None, because it modifies the list it self by adding the object appended as an element, while the
+ operator concatenates the two lists and return the resulting list
a = [1,2,3,4,5] b = [6,7,8,9,0] print a+b # returns a list made by concatenating the lists a and b [1, 2, 3, 4, 5, 6, 7, 8, 9, 0] print a.append(b) # Adds the list b as element at the end of the list a and returns None None print a # the list a was modified during the last append call and has the list b as last element [1, 2, 3, 4, 5, [6, 7, 8, 9, 0]]
So as you can see the easiest way is just to add the two lists together as even if you append the list b to a using append() you will not get the result you want without additional work