mapping the append function to a list

Question

I want to do the following elegantly. I have a list:

list1 = [[1,2],[3,1,4,7],[5],[7,8]]

I'd like to append the number 1 to each element of the list, so that I have

list1 = [[1,2,1],[3,1,4,7,1],[5,1],[7,8,1]]

I'm trying to map this via

map(list.append([1]), vectors)

but this returns the error append() takes exactly one argument (0 given) and if I just try append([1]) (without list.), I get NameError: global name 'append' is not defined. I guess I could do it with a loop, but this seems more elegant, is there a way to map this correctly?

Answer 1

Here is a several ways to implement what you want:

More readable and classic way

for el in list1:
  el.append(1)

List comprehension

list1 = [el + [1] for el in list1]

Generators:

list1 = (el + [1] for el in list1)

Map

list1 = map(lambda el: el + [1], list1)

What to use?

It depends on you own situation and may depends on execution speed optimizations, code readability, place of usage.

• Map is a worst choice in case of readability and execution speed
• For is a fastest and more plain way to do this
• Generators allows you to generate new list only when you really need this
• List comprehension - one liner for classic for and it takes advantage when you need quickly filter the new list using if

i.e. if you need only add element to each item - for loop is a best choice to do this, but if you need add item only if item > 40, then you may consider to use List comprehension.

For example:

Classic For

x = 41

for el in list1:
  if x > 40:
    el.append(x)

List comprehension

   x = 1
   list1 = [el + [x] for el in list1 if x > 40]

as @jmd_dk mentioned, in this sample is one fundamental difference: with simple for you can just append to an existing object of the list which makes much less impact to execution time and memory usage. When you use List comprehension, you will get new list object and in this case new list object for each item.