What is the difference between the list methods append and extend?
append adds its argument as a single element to the end of a list. The length of the list itself will increase by one.
extend iterates over its argument adding each element to the list, extending the list. The length of the list will increase by however many elements were in the iterable argument.
list.append method appends an object to the end of the list.
Whatever the object is, whether a number, a string, another list, or something else, it gets added onto the end of
my_list as a single entry on the list.
>>> my_list ['foo', 'bar'] >>> my_list.append('baz') >>> my_list ['foo', 'bar', 'baz']
So keep in mind that a list is an object. If you append another list onto a list, the first list will be a single object at the end of the list (which may not be what you want):
>>> another_list = [1, 2, 3] >>> my_list.append(another_list) >>> my_list ['foo', 'bar', 'baz', [1, 2, 3]] #^^^^^^^^^--- single item at the end of the list.
list.extend method extends a list by appending elements from an iterable:
So with extend, each element of the iterable gets appended onto the list. For example:
>>> my_list ['foo', 'bar'] >>> another_list = [1, 2, 3] >>> my_list.extend(another_list) >>> my_list ['foo', 'bar', 1, 2, 3]
Keep in mind that a string is an iterable, so if you extend a list with a string, you'll append each character as you iterate over the string (which may not be what you want):
>>> my_list.extend('baz') >>> my_list ['foo', 'bar', 1, 2, 3, 'b', 'a', 'z']
+= operators are defined for
list. They are semantically similar to extend.
my_list + another_list creates a third list in memory, so you can return the result of it, but it requires that the second iterable be a list.
my_list += another_list modifies the list in-place (it is the in-place operator, and lists are mutable objects, as we've seen) so it does not create a new list. It also works like extend, in that the second iterable can be any kind of iterable.
Don't get confused -
my_list = my_list + another_list is not equivalent to
+= - it gives you a brand new list assigned to my_list.
Append has (amortized) constant time complexity, O(1).
Extend has time complexity, O(k).
Iterating through the multiple calls to
append adds to the complexity, making it equivalent to that of extend, and since extend's iteration is implemented in C, it will always be faster if you intend to append successive items from an iterable onto a list.
Regarding "amortized" - from the list object implementation source:
/* This over-allocates proportional to the list size, making room * for additional growth. The over-allocation is mild, but is * enough to give linear-time amortized behavior over a long * sequence of appends() in the presence of a poorly-performing * system realloc().
This means that we get the benefits of a larger than needed memory reallocation up front, but we may pay for it on the next marginal reallocation with an even larger one. Total time for all appends is linear at O(n), and that time allocated per append, becomes O(1).
You may wonder what is more performant, since append can be used to achieve the same outcome as extend. The following functions do the same thing:
def append(alist, iterable): for item in iterable: alist.append(item) def extend(alist, iterable): alist.extend(iterable)
So let's time them:
import timeit >>> min(timeit.repeat(lambda: append(, "abcdefghijklmnopqrstuvwxyz"))) 2.867846965789795 >>> min(timeit.repeat(lambda: extend(, "abcdefghijklmnopqrstuvwxyz"))) 0.8060121536254883
Addressing a comment on timings
A commenter said:
Perfect answer, I just miss the timing of comparing adding only one element
Do the semantically correct thing. If you want to append all elements in an iterable, use
extend. If you're just adding one element, use
Ok, so let's create an experiment to see how this works out in time:
def append_one(a_list, element): a_list.append(element) def extend_one(a_list, element): """creating a new list is semantically the most direct way to create an iterable to give to extend""" a_list.extend([element]) import timeit
And we see that going out of our way to create an iterable just to use extend is a (minor) waste of time:
>>> min(timeit.repeat(lambda: append_one(, 0))) 0.2082819009956438 >>> min(timeit.repeat(lambda: extend_one(, 0))) 0.2397019260097295
We learn from this that there's nothing gained from using
extend when we have only one element to append.
Also, these timings are not that important. I am just showing them to make the point that, in Python, doing the semantically correct thing is doing things the Right Way™.
It's conceivable that you might test timings on two comparable operations and get an ambiguous or inverse result. Just focus on doing the semantically correct thing.
We see that
extend is semantically clearer, and that it can run much faster than
append, when you intend to append each element in an iterable to a list.
If you only have a single element (not in an iterable) to add to the list, use