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The code below will not run in its current state. However, if I change sum_vec.extend( vec1[i] + vec2[i] ) to sum_vec.append( vec1[i] + vec2[i] ) it works just fine. I understand the basic different between append and extend, but I don't understand why the code doesn't work if I use extend. 

def addVectors(v1, v2): vec1 = list(v1) vec2 = list(v2) sum_vec = [] vec1_len = len(vec1) vec2_len = len(vec2) min_len = min( vec1_len, vec2_len ) # adding up elements pointwise if vec1_len == 0 and vec2_len == 0: return sum_vec else: for i in xrange(0, min_len): sum_vec.extend( vec1[i] + vec2[i] ) # in case one vector is longer than the other if vec1_len != vec2_len: if vec1_len > vec2_len: sum_vec.extend( vec1[min_len : vec1_len] ) else: sum_vec.extend( vec2[min_len : vec2_len] ) print sum_vec return sum_vec v1 = [1,3,5] v2 = [2,4,6,8,10] addVectors(v1,v2)
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As others have pointed out, extend takes an iterable (such as a list, tuple or string), and adds each element of the iterable to the list one at a time, while append adds its argument to the end of the list as a single item. The key thing to note is that extend is a more efficient version of calling append multiple times.

a = [1,2] b = [1,2] a.extend([3, 4]) for x in [3, 4]: b.append(x) assert a == b

append can take an iterable as its argument, but it treats it as a single object:

a = [1,2] a.append([3,4]) assert a == [1, 2, [3, 4]] # not [1, 2, 3, 4]
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Why does python treat a string as a list when you do a.extend(str) ?
It doesn't treat it as a list; it treats it as an iterable, which is anything you can iterate over. You iterate over a string by getting its individual characters one by one. You iterate over a list or a tuple by getting its elements one by one. You iterate over a dict by getting its keys one by one. extend doesn't care what its argument is, as long as it can be iterated over.
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You can read the documentation on list:

list.append adds a single item to the end of your list:

Add an item to the end of the list; equivalent to a[len(a):] = [x].

list.extend uses an iterable and adds all of it's elements to the end of your list:

Extend the list by appending all the items in the given list; equivalent to a[len(a):] = L.

You need to use:

sum_vec.extend([vec1[i] + vec2[i]]) # note that a list is created

That way an iterable with a single item (vec1[i] + vec2[i]) is passed. But list.append is more suitable when you're always adding a single item.

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When you run your code, you get an exception like this:

Traceback (most recent call last): File ".../stack.py", line 28, in <module> addVectors(v1,v2) File ".../stack.py", line 15, in addVectors sum_vec.extend( vec1[i] + vec2[i] ) TypeError: 'int' object is not iterable

In other words, the extend method expects an iterable as argument. However append method gets an item as argument.

Here is a small example of the difference between extend and append:

l = [1, 2, 3, 4] m = [10, 11] r = list(m) m.append(l) r.extend(l) print(m) print(r)

Output:

[10, 11, [1, 2, 3, 4]] [10, 11, 1, 2, 3, 4]
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The method append adds its parameter as a single element to the list, while extend gets a list and adds its content. 

letters = ['a', 'b'] letters.extend(['c', 'd']) print(letters) # ['a', 'b', 'c', 'd'] letters.append(['e', 'f']) print(letters) # ['a', 'b', 'c', 'd', ['e', 'f']] names = ['Foo', 'Bar'] names.append('Baz') print(names) # ['Foo', 'Bar', 'Baz'] names.extend('Moo') print(names) # ['Foo', 'Bar', 'Baz', 'M', 'o', 'o']
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extend needs an iterable as a parameter if you want to extend list by a single element you need to dress is in a list

a = [] b = 1 a.extend([b]) a.append(b)
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