Python One Line For Loop Append

Problem: How to append elements to a list using a single line for loop?

Example: You hope to accomplish something like this where you create an initial list (this one is empty) and you append multiple elements to it:

>>> [].append(i) for i in range(5)
[0, 1, 2, 3, 4]

However, this statement doesn’t work! Is there a one-line for loop to append elements to a given list?

Let’s dive into several methods to accomplish this! Here’s a quick overview:

Exercise: Can you modify the code to append elements in a tuple to the given list in a single line of code?

Let’s dive into the three methods in more detail!

Method 1: Use List Comprehension

If you don’t need to add elements to a given list but you’re fine to create a new list, list comprehension is your best shot!

# Method 1
lst = [i for i in range(5)]
# [0, 1, 2, 3, 4]

This one-liner accomplishes the desired result—but it does create a new list. Let’s quickly recap how list comprehension works in this video:

List comprehension is a compact way of creating lists. The simple formula is [expression + context].

  • Expression: What to do with each list element?
  • Context: What elements to select? The context consists of an arbitrary number of for and if statements.

The example [x for x in range(3)] creates the list [0, 1, 2].

But is there another way if you have a list and you just want to append elements to this list?

Method 2: Single-Line For Loop with append()

Sure! You can write blocks in a single line—if the block body itself is not nested!

# Method 2
friends = ['Ann', 'Alice']
new_friends = ['Bob', 'Liz']

# One-Liner:
for f in new_friends: friends.append(f)

# Results
# ['Ann', 'Alice', 'Bob', 'Liz']

You use the list.append() method repeatedly for each element in the iterable new_friends that contains the elements to be appended to the original list friends. The fact that the for loop is compressed in a single line doesn’t make this one-liner ambiguous so Python is okay with it.

Method 3: extend()

list append vs extend

However, a much better option to append all elements in a given iterable to a given list is to use the list.extend() method:

# Method 3
friends = ['Ann', 'Alice']
new_friends = ['Bob', 'Liz']

# One-Liner:

# Results
# ['Ann', 'Alice', 'Bob', 'Liz']

The one-liner is much shorter and even faster. You can find a detailed speed comparison here.

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